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Versions: 00 01 02 03 04 05 RFC 7868

Internet Engineering Task Force                               D. Savage
Internet-Draft                                                    J. Ng
Intended status: Informational                                 S. Moore
Expires: August, 2016                                     Cisco Systems
                                                               D. Slice
                                                       Cumulus Networks
                                                              P. Paluch
                                                   University of Zilina
                                                               R. White
                                                               LinkedIn
                                                            23 Feb 2016


                                   Cisco
                 Enhanced Interior Gateway Routing Protocol
                        draft-savage-eigrp-05.txt

Abstract

This document describes the protocol design and architecture for
Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP is a routing
protocol based on Distance Vector technology. The specific algorithm
used is called DUAL, a Diffusing Update Algorithm as reference in
"Loop-Free Routing using Diffusing Computations". The algorithm and
procedures were researched, developed, and simulated by SRI
International.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF). Note that other groups may also distribute working
documents as Internet-Drafts. The list of current Internet-Drafts is
at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

This Internet-Draft will expire on August 16, 2016.




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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents carefully,
as they describe your rights and restrictions with respect to this
document. Code Components extracted from this document must include
Simplified BSD License text as described in Section 4.e of the Trust
Legal Provisions and are provided without warranty as described in the
Simplified BSD License.

This document may not be modified, and derivative works of it may not
be created, except to format it for publication as an RFC or to
translate it into languages other than English.




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Table of Contents
1 Introduction..................................................... 6
2 Terminology...................................................... 6
3 The DUAL Diffusing Update Algorithm.............................. 10
  3.1 Algorithm Description ....................................... 10
  3.2 Route States ................................................ 11
  3.3 Feasibility Condition ....................................... 12
  3.4 DUAL Message Types .......................................... 14
  3.5 DUAL Finite State Machine (FSM) ............................. 15
  3.6 DUAL Operation - Example Topology ........................... 19
4 EIGRP Packets.................................................... 21
  4.1 UPDATE Packets .............................................. 21
  4.2 QUERY Packets ............................................... 22
  4.3 REPLY Packets ............................................... 22
  4.4 Exception Handling .......................................... 23
    4.4.1 Active Duration (Stuck-in-Active) ....................... 23
      4.4.1.1 SIA-QUERY............................................ 23
      4.4.1.2 SIA-REPLY............................................ 24
5 EIGRP Protocol Operation......................................... 25
  5.1 Finite State Machine ........................................ 25
  5.2 Reliable Transport Protocol ................................. 25
    5.2.1 Bandwidth on Low-Speed Links ............................ 34
  5.3 Neighbor Discovery/Recovery ................................. 34
    5.3.1 Neighbor Hold Time ...................................... 34
    5.3.2 HELLO Packets ........................................... 34
    5.3.3 UPDATE Packets .......................................... 35
    5.3.4 Initialization Sequence ................................. 36
    5.3.5 Neighbor Formation ...................................... 37
    5.3.6 QUERY Packets During Neighbor Formation ................. 37
  5.4 Topology Table .............................................. 38
    5.4.1 Route Management ........................................ 38
      5.4.1.1 Internal Routes...................................... 38
      5.4.1.2 External routes...................................... 39
    5.4.2 Split Horizon and Poison Reverse ........................ 40
      5.4.2.1 Startup Mode......................................... 40
      5.4.2.2 Advertising Topology Table Change.................... 40
      5.4.2.3 Sending a QUERY/UPDATE............................... 40
  5.5 EIGRP Metric Coefficients ................................... 41
    5.5.1 Coefficients K1 and K2 .................................. 41
    5.5.2 Coefficient K3 .......................................... 41
    5.5.3 Coefficients K4 and K5 .................................. 42
    5.5.4 Coefficient K6 .......................................... 42
      5.5.4.1 Jitter............................................... 42
      5.5.4.2 Energy............................................... 42
  5.6 EIGRP Metric Calculations ................................... 43
    5.6.1 Classic Metrics ......................................... 43
      5.6.1.1 Classic Composite Formulation........................ 43
    5.6.2 Wide Metrics ............................................ 45
      5.6.2.1 Wide Metric Vectors.................................. 45

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      5.6.2.2 Wide Metric Conversion Constants..................... 46
      5.6.2.3 Throughput Calculation............................... 46
      5.6.2.4 Latency Calculation.................................. 47
      5.6.2.5 Composite Calculation................................ 47
6 EIGRP Packet Formats............................................  48
  6.1 Protocol Number ............................................. 48
  6.2 Protocol Assignment Encoding ................................ 48
  6.3 Destination Assignment Encoding ............................. 49
  6.4 EIGRP Communities Attribute ................................. 49
  6.5 EIGRP Packet Header ......................................... 50
  6.6 EIGRP TLV Encoding Format ................................... 52
    6.6.1 Type Field Encoding ..................................... 52
    6.6.2 Length Field Encoding ................................... 52
    6.6.3 Value Field Encoding .................................... 53
  6.7 EIGRP Generic TLV Definitions ............................... 53
    6.7.1 0x0001 - PARAMETER_TYPE ................................. 53
    6.7.2 0x0002 - AUTHENTICATION_TYPE ............................ 54
      6.7.2.1 0x02 - MD5 Authentication Type....................... 54
      6.7.2.2 0x03 - SHA2 Authentication Type...................... 54
    6.7.3 0x0003 - SEQUENCE_TYPE .................................. 54
    6.7.4 0x0004 - SOFTWARE_VERSION_TYPE .......................... 55
    6.7.5 0x0005 - MULTICAST_SEQUENCE_TYPE ........................ 55
    6.7.6 0x0006 - PEER_INFORMATION_TYPE .......................... 55
    6.7.7 0x0007 - PEER_ TERMINATION_TYPE ......................... 55
    6.7.8 0x0008 - TID_LIST_TYPE .................................. 56
  6.8 Classic Route Information TLV Types ......................... 57
    6.8.1 Classic Flag Field Encoding ............................. 57
    6.8.2 Classic Metric Encoding ................................. 57
    6.8.3 Classic Exterior Encoding ............................... 58
    6.8.4 Classic Destination Encoding ............................ 59
    6.8.5 IPv4 Specific TLVs ...................................... 60
      6.8.5.1 IPv4 INTERNAL_TYPE................................... 60
      6.8.5.2 IPv4 EXTERNAL_TYPE................................... 61
      6.8.5.3 IPv4 COMMUNITY_TYPE.................................. 62
    6.8.6 IPv6 Specific TLVs ...................................... 63
      6.8.6.1 IPv6 INTERNAL_TYPE................................... 63
      6.8.6.2 IPv6 EXTERNAL_TYPE................................... 64
      6.8.6.3 IPv6 COMMUNITY_TYPE.................................. 65
  6.9 Multi-Protocol Route Information TLV Types .................. 66
    6.9.1 TLV Header Encoding ..................................... 66
    6.9.2 Wide Metric Encoding .................................... 67
    6.9.3 Extended Metrics ........................................ 68
      6.9.3.1 0x00 - NoOp.......................................... 69
      6.9.3.2 0x01 - Scaled Metric................................. 69
      6.9.3.3 0x02 - Administrator Tag............................. 70
      6.9.3.4 0x03 - Community List................................ 70
      6.9.3.5 0x04 - Jitter........................................ 70
      6.9.3.6 0x05 - Quiescent Energy.............................. 71
      6.9.3.7 0x06 - Energy........................................ 71

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      6.9.3.8 0x07 - AddPath....................................... 72
      6.9.3.8.1 AddPath with IPv4 Next-hop......................... 72
      6.9.3.8.2 AddPath with IPv6 Next-hop........................  73
    6.9.4 Exterior Encoding ....................................... 74
    6.9.5 Destination Encoding .................................... 74
    6.9.6 Route Information ....................................... 75
      6.9.6.1 INTERNAL TYPE........................................ 75
      6.9.6.2 EXTERNAL TYPE........................................ 75
7 Security Considerations.......................................... 76
8 IANA Considerations.............................................. 76
9 References....................................................... 77
  9.1 Normative References ........................................ 77
  9.2 Informative References ...................................... 77
10 Acknowledgments................................................. 78




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1 Introduction
This document describes the Enhanced Interior Gateway Routing Protocol
(EIGRP), a routing protocol designed and developed by Cisco Systems,
Inc. DUAL, the algorithm used to converge the control plane to a
single set of loop free paths is based on research conducted at SRI
International [3]. The Diffusing Update Algorithm (DUAL) is the
algorithm used to obtain loop-freedom at every instant throughout a
route computation [2]. This allows all routers involved in a topology
change to synchronize at the same time; the routers not affected by
topology changes are not involved in the recalculation. This document
describes the protocol that implements these functions.

2 Terminology
The following list describes acronyms and definitions for terms used
throughout this document:

ACTIVE State
    The local state of a route on a router triggered by any event that
causes all neighbors providing the current least cost path to fail the
Feasibility Condition check. A route in Active state is considered
unusable. During Active state, the router is actively attempting to
compute the least cost loop-free path by explicit coordination with
its neighbors using Query and Reply messages.

Address Family Identifier (AFI)
    Identity of the network layer protocol reachability information
being advertised [12].

Autonomous System (AS)
    A collection of routers exchanging routes under the control of one
or more network administrators on behalf of a single administrative
entity.

Base Topology
    A routing domain representing a physical (non-virtual) view of the
network topology consisting of attached devices and network segments
EIGRP uses to form neighbor relationships. Destinations exchanged
within the Base Topology are identified with a Topology Identifier
value of zero (0).

Computed Distance (CD)
    Total distance (metric) along a path from the current router to
a destination network through a particular neighbor computed using
that neighbor's Reported Distance and the cost of the link between the
two routers. Exactly one Computed Distance is computed and maintained
per the [Destination, Advertising Neighbor] pair.




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Diffusing Computation
    A distributed computation in which a single starting node
commences the computation by delegating subtasks of the computation to
its neighbors that may in turn recursively delegate sub-subtasks
further, including a signaling scheme allowing the starting node to
detect that the computation has finished while avoiding false
terminations. In DUAL, the task of coordinated updates of routing
tables and resulting best path computation is performed as a diffusing
computation.

Diffusing Update Algorithm (DUAL)
    A loop-free routing algorithm used with distance vectors or link
states that provides a diffused computation of a routing table. It
works very well in the presence of multiple topology changes with low
overhead. The technology was researched and developed at SRI
International [3].

Downstream Router
    A router that is one or more hops away from the router in question
in the direction of the destination.

EIGRP
    Enhanced Interior Gateway Routing Protocol.

Feasibility Condition
    The Feasibility Condition is a sufficient condition used by a
router to verify whether a neighboring router provides a loop-free
path to a destination. EIGRP uses the Source Node Condition stating
that a neighboring router meets the Feasibility Condition if the
neighbor's Reported Distance is less than this router's Feasible
Distance.

Feasible Distance (FD)
    Defined as the lowest known total metric to a destination from the
current router since the last transition from ACTIVE to PASSIVE state.
Being effectively a record of the smallest known metric since the last
time the network entered the PASSIVE state, the FD is not necessarily
a metric of the current best path. Exactly one Feasible Distance is
computed per destination network.

Feasible Successor
    A neighboring router that meets the Feasibility Condition for a
particular destination, hence providing a guaranteed loop-free path.

Neighbor / Peer
    For a particular router, another router toward which an EIGRP
session, also known as adjacency, is established. The ability of two
routers to become neighbors depends on their mutual connectivity and
compatibility of selected EIGRP configuration parameters. Two

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neighbors with interfaces connected to a common subnet are known as
adjacent neighbors. Two neighbors that are multiple hops apart are
known as remote neighbors.

PASSIVE state
    The local state of a route in which at least one neighbor
providing the current least cost path passes Feasibility Condition
check. A route in PASSIVE state is considered usable and not in need
of a coordinated re-computation.

Reachability Information (NLRI)
    Information a router uses to calculate the global routing table to
make routing and forwarding decisions.

Reported Distance (RD)
    For a particular destination, the value representing the router's
distance to the destination as advertised in all messages carrying
routing information. Reported Distance is not equivalent to the
current distance of the router to the destination and may be different
from it during the process of path re-computation. Exactly one
Reported Distance is computed and maintained per destination network.

Sub-Topology
    For a given Base Topology, a sub-topology is characterized by an
independent set of routers and links in a network for which EIGRP
performs an independent path calculation. This allows each sub-
topology to implement class-specific topologies to carry class
specific traffic.

Successor
    For a particular destination, a neighboring router that meets the
Feasibility Condition and, at the same time, provides the least cost
path.

Stuck In Active (SIA)
    A destination that has remained in the ACTIVE State in excess of a
predefined time period at the local router (Cisco implements this as 3
minutes).

Successor Directed Acyclic Graph (SDAG)
    For a particular destination, a graph defined by routing table
contents of individual routers in the topology, such that nodes of
this graph are the routers themselves, and a directed edge from router
X to router Y exists if and only if router Y is router X's successor.
After the network has converged, in the absence of topological
changes, SDAG is a tree.




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Topology Change / Topology Change Event
    Any event that causes the Computed Distance for a destination
through a neighbor to be added, modified, or removed. As an example,
detecting a link cost change, receiving any EIGRP message from a
neighbor advertising an updated neighbor's Reported Distance

Topology Identifier (TID)
    A number that is used to mark prefixes as belonging to a specific
sub-topology.

Topology Table
    A data structure used by EIGRP to store information about every
known destination including, but not limited to, network prefix/prefix
length, Feasible Distance, Reported Distance of each neighbor
advertising the destination, Computed Distance over the corresponding
neighbor, and route state.

Type, Length, Value (TLV)
    An encoding format for information elements used in EIGRP messages
to exchange information. Each TLV-formatted information element
consists of three generic fields: Type identifying the nature of
information carried in this element; Length describing the length of
the entire TLV triplet; and Value carrying the actual information. The
Value field may itself be internally structured; this depends on the
actual type of the information element. This format allows for
extensibility and backward compatibility.

Upstream Router
    A router that is one or more hops away from the router in
question, in the direction of the source of the information.

Virtual Routing and Forwarding (VRF)
    Independent Virtual Private Network (VPN) routing/forwarding
tables which co-exist within the same router at the same time.




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3 The DUAL Diffusing Update Algorithm
The Diffusing Update Algorithm (DUAL) constructs least cost paths to
all reachable destinations in a network consisting of nodes and edges
(routers and links). DUAL guarantees that each constructed path is
loop-free at every instant including periods of topology changes and
network re-convergence. This is accomplished by all routers, which are
affected by a topology change, computing the new best path in a
coordinated (diffusing) way and using the Feasibility Condition to
verify prospective paths for loop freedom. Routers that are not
affected by topology changes are not involved in the recalculation.
The convergence time with DUAL rivals that of any other existing
routing protocol.


3.1 Algorithm Description
DUAL is used by EIGRP to achieve fast loop-free convergence with
little overhead, allowing EIGRP to provide convergence rates
comparable, and in some cases better than, most common link state
protocols [10]. Only nodes that are affected by a topology change need
to propagate and act on information about the topology change,
allowing EIGRP to have good scaling properties, reduced overhead, and
lower complexity than many other interior gateway protocols.

Distributed routing algorithms are required to propagate information
as well as coordinate information among all nodes in the network.
Unlike basic Bellman-Ford distance vector protocols that rely on
uncoordinated updates when a topology change occurs, DUAL uses a
coordinated procedure to involve the affected part of the network into
computing a new least cost path, known as a diffusing computation.
A diffusing computation grows by querying additional routers for their
current Reported Distance to the affected destination, and shrinks by
receiving replies from them. Unaffected routers send replies
immediately, terminating the growth of the diffusing computation over
them. These intrinsic properties cause the diffusing computation to
self-adjust in scope and terminate as soon as possible.

One attribute of DUAL is its ability to control the point at which the
diffusion of a route calculation terminates by managing the
distribution of reachability information through the network.
Controlling the scope of the diffusing process is accomplished by
hiding reachability information through aggregation (summarization),
filtering, or other means. This provides the ability to create
effective failure domains within a single AS, and allows the network
administrator to manage the convergence and processing characteristics
of the network.




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3.2 Route States
A route to a destination can be in one of two states, PASSIVE or
ACTIVE. These states describe whether the route is guaranteed to be
both loop-free and the shortest available (the PASSIVE state), or
whether such guarantee cannot be given (the ACTIVE state).
Consequently, in PASSIVE state, the router does not perform any route
recalculation in coordination with its neighbors because no such
recalculation is needed.

In ACTIVE state, the router is actively involved in re-computing the
least cost loop-free path in coordination with its neighbors. The
state is reevaluated and possibly changed every time a topology change
is detected. A topology change is any event that causes the Computed
Distance to the destination over any neighbor to be added, changed, or
removed from EIGRP's topology table.

More exactly, the two states are defined as follows:

   o Passive
     A route is considered in the Passive state when at least one
neighbor that provides the current least total cost path passes the
Feasibility Condition check that guarantees loop freedom. A route in
the PASSIVE is usable and its next hop is perceived to be a downstream
router.

   o Active
     A route is considered in the ACTIVE state if neighbors that do
not pass the Feasibility Condition check provide lowest cost path, and
therefore the path cannot be guaranteed loop free. A route in the
ACTIVE state is considered unusable and this router must coordinate
with its neighbors in the search for the new loop-free least total
cost path.

In other words, for a route to be in PASSIVE state, at least one
neighbor that provides the least total cost path must be a Feasible
Successor. Feasible Successors providing the least total cost path are
also called Successors. For a route to be in PASSIVE state, at least
one Successor must exist.

Conversely, if the path with the least total cost is provided by
routers that are not Feasible Successors (and thus not Successors),
the route is in the ACTIVE state, requiring re-computation.

Notably, for the definition of PASSIVE and ACTIVE states it does not
matter if there are Feasible Successors providing a worse-than-least
total cost path. While these neighbors are guaranteed to provide a
loop free path, that path is potentially not the shortest available.



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The fact that the least total cost path can be provided by a neighbor
that fails the Feasibility Condition check may not be intuitive.
However, such situation can occur during topology changes when the
current least total cost path fails, and the next least total cost
path traverses a neighbor that is not a Feasible Successor.

While a router has a route in the ACTIVE state, it must not change its
Successor (i.e. modify the current SDAG), nor modify its own Feasible
Distance or Reported Distance until the route enters the PASSIVE state
again. Any updated information about this route received during ACTIVE
state is reflected only in Computed Distances. Any updates to the
Successor, Feasible Distance and Reported Distance are postponed until
the route returns to PASSIVE state. The state transitions from PASSIVE
to ACTIVE and from ACTIVE to PASSIVE are controlled by the DUAL FSM
and are described in detail in Section 3.5.


3.3 Feasibility Condition
The Feasibility Condition is a criterion used to verify loop freedom
of a particular path. The Feasibility Condition is a sufficient but
not a necessary condition, meaning that every path meeting the
Feasibility Condition is guaranteed to be loop-free; however, not all
loop-free paths meet the Feasibility condition.

The Feasibility Condition is used as an integral part of DUAL
operation: Every path selection in DUAL is subject to the Feasibility
Condition check. Based on the result of the Feasibility Condition
check after a topology change is detected, the route may either remain
PASSIVE (if, after the topology change, the neighbor providing the
least cost path meets the Feasibility Condition) or it needs to enter
the ACTIVE state (if the topology change resulted in none of the
neighbors providing the least cost path to meet the Feasibility
Condition).

The Feasibility Condition is a part of DUAL that allows the diffused
computation to terminate as early as possible. Nodes that are not
affected by the topology change are not required to perform a DUAL
computation and may not be aware a topology change occurred. This can
occur in two cases;

First, if informed about a topology change, a router may keep a route
in PASSIVE State if it is aware of other paths that are downstream
towards the destination (routes meeting the Feasibility Condition). A
route that meets the Feasibility Condition is determined to be loop-
free and downstream along the path between the router and the
destination.




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Second, if informed about a topology change for which it does not
currently have reachability information, a router is not required to
enter into the ACTIVE state, nor is it required to participate in the
DUAL process.

In order to facilitate describing the Feasibility Condition, a few
definitions are in order.

    o A Successor for a given route is the next-hop used to forward
data traffic for a destination. Typically, the successor is chosen
based on the least cost path to reach the destination.

    o A Feasible Successor is a neighbor that meets the Feasibility
Condition. A Feasible Successor is regarded as a downstream neighbor
towards the destination but it may not be the least cost path, but
could still be used for forwarding data packets in the event equal or
unequal cost load sharing was active. A Feasible Successor can become
a successor when the current successor becomes unreachable.

    o The Feasibility Condition is met when a neighbor's advertised
cost, (RD) to a destination is less than the Feasible Distance for
that destination, or in other words, the Feasibility Condition is met
when the neighbor is closer to the destination than the router itself
has ever been since the destination has entered the PASSIVE state for
the last time.

    o The Feasible Distance is the lowest distance to the destination
since the last time the route went from ACTIVE to PASSIVE state. It
should be noted it is not necessarily the current best distance -
rather, it is a historical record of the best distance known since the
last diffusing computation for the destination has finished. Thus, the
value of the Feasible Distance can either be the same as the current
best distance, or it can be lower.

A neighbor that advertises a route with a cost that does not meet the
Feasibility Condition may be upstream and thus cannot be guaranteed to
be the next hop for a loop free path. Routes advertised by upstream
neighbors are not recorded in the routing table but saved in the
topology table.




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3.4 DUAL Message Types
The DUAL algorithm operates with three basic message types, QUERY,
UPDATE, and REPLY:

    o UPDATE - sent to indicate a change in metric or an addition of a
destination.

    o QUERY - sent when Feasibility Condition fails which can happen
for reasons like a destination becoming unreachable, or the metric
increasing to a value greater than its current Feasible Distance.

    o REPLY - sent in response to a QUERY or SIA-QUERY

In addition to these 3 basic types, two additional sub-types have been
added to EIGRP:

    o SIA-QUERY - sent when a REPLY has not been received within one
half the SIA interval (90 seconds as implemented by Cisco)

    o SIA-REPLY - sent in response to an SIA-QUERY indicating the
route is still in ACTIVE state. This response does not stratify the
original QUERY, but is only used to indicate the sending neighbor is
still in the ACTIVE State for the given destination.

When in the PASSIVE State, a received QUERY may be propagated if there
is no Feasible Successor found. If a Feasible Successor is found, the
QUERY is not propagated and a REPLY is sent for the destination with a
metric equal to the current routing table metric. When a QUERY is
received from a non-successor in ACTIVE State a REPLY is sent and the
QUERY is not propagated. The REPLY for the destination contains a
metric equal to the current routing table metric.




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3.5 DUAL Finite State Machine (FSM)
The DUAL finite state machine embodies the decision process for all
route computations. It tracks all routes advertised by all neighbors.
The distance information, known as a metric, is used by DUAL to select
efficient loop free paths. DUAL selects routes to be inserted into a
routing table based on Feasible Successors. A successor is a
neighboring router used for packet forwarding that has least cost path
to a destination that is guaranteed not to be part of a routing loop.

When there are no Feasible Successors but there are neighbors
advertising the destination, a recalculation must occur to determine a
new successor.

The amount of time it takes to calculate the route impacts the
convergence time. Even though the recalculation is not processor-
intensive, it is advantageous to avoid recalculation if it is not
necessary. When a topology change occurs, DUAL will test for Feasible
Successors. If there are Feasible Successors, it will use any it finds
in order to avoid any unnecessary recalculation.

The finite state machine, which applies per destination in the
topology table, operates independently for each destination. It is
true that if a single link goes down, multiple routes may go into
ACTIVE State. However, a separate Successor Directed Acyclic Graph
(SDAG) is computed for each destination, so loop-free topologies can
be maintained for each reachable destination.




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Figure 1 illustrates the FSM:

 i   Node that is computing route.
 j   Destination node or network.
 K   Any neighbor of node i.
 oij QUERY origin flag
   0 = metric increase during ACTIVE State
   1 = node i originated
   2 = QUERY from, or link increase to, successor during ACTIVE State
   3 = QUERY originated from successor.
 rijk REPLY status flag for each neighbor k for destination j,
   1 = awaiting REPLY,
   0 = received REPLY.
 lik = the link connecting node i to neighbor k.


           +------------+                +-----------+
           |             \              /            |
           |              \            /             |
           |   +=================================+   |
           |   |                                 |   |
           |(1)|             Passive             |(2)|
           +-->|                                 |<--+
               +=================================+
                   ^     |    ^    ^    ^    |
               (14)|     |(15)|    |(13)|    |
                   |  (4)|    |(16)|    | (3)|
                   |     |    |    |    |    +------------+
                   |     |    |    |    |                  \
          +-------+      +    +    |    +-------------+     \
         /              /    /     |                   \     \
        /              /    /      +----+               \     \
       |               |   |            |                |     |
       |               v   |            |                |     v
   +==========+(11) +==========+     +==========+(12) +==========+
   |  Active  |---->|  Active  |(5)  |  Active  |---->|  Active  |
   |          |  (9)|          |---->|          | (10)|          |
   |  Oij=0   |<----|  Oij=1   |     |  Oij=2   |<----|  Oij=3   |
+--|          |  +--|          |  +--|          |  +--|          |
|  +==========+  |  +==========+  |  +==========+  |  +==========+
|      ^   |(5)  |      ^         |    ^    ^      |         ^
|      |   +-----|------|---------|----+    |      |         |
+------+         +------+         +---------+      +---------+
(6,7,8)          (6,7,8)            (6,7,8)          (6,7,8)

                    Figure 1 - DUAL Finite State Machine



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The following describes in detail the state/event/action transitions
of the DUAL FSM. For all steps, the topology table is updated with the
new metric information from either; QUERY, REPLY, or UPDATE received.

(1) A QUERY is received from a neighbor that is not the current
successor. The route is currently in PASSIVE State. As the Successor
is not affected by the QUERY, and a Feasible Successor exists, the
route remains in PASSIVE State. Since a Feasible Successor exists, a
REPLY MUST be sent back to the originator of the QUERY. Any metric
received in the QUERY from that neighbor is recorded in the topology
table and FC is run to check for any change to current successor.

(2) A directly connected interface changes state (connects,
disconnects, or changes metric), or similarly an UPDATE or QUERY has
been received with a metric change for an existing destination, the
route will stay in the PASSIVE State if the current successor is not
affected by the change, or it is no longer reachable and there is a
Feasible Successor. In either case, an UPDATE is sent with the new
metric information if it has changed.

(3) A QUERY was received from a neighbor who is the current successor
and no Feasible Successors exist. The route for the destination goes
into ACTIVE State. A QUERY is sent to all neighbors on all interfaces
that are not split horizon. Split horizon takes effect for a query or
update from the successor it is using for the destination in the
query. The QUERY origin flag is set to indicate the QUERY originated
from a neighbor marked as successor for route. The REPLY status flag
is set for all neighbors to indicate outstanding replies.

(4) A directly connected link has gone down or its cost has increased,
or an UPDATE has been received with a metric increase. The route to
the destination goes to ACTIVE State if there are no Feasible
Successors found. A QUERY is sent to all neighbors on all interfaces.
The QUERY origin flag is to indicate that the router originated the
QUERY. The REPLY status flag is set to 1 for all neighbors to indicate
outstanding replies.

(5) While a route for a destination is in ACTIVE State, and a QUERY is
received from the current successor, the route remains in ACTIVE
State. The QUERY origin flag is set to indicate that there was another
topology change while in ACTIVE State. This indication is used so new
Feasible Successors are compared to the metric which made the route go
to ACTIVE State with the current successor.

(6) While a route for a destination is in ACTIVE State and a QUERY is
received from a neighbor that is not the current successor, a REPLY
should be sent to the neighbor. The metric received in the QUERY
should be recorded.


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(7) If a link cost changes, or an UPDATE with a metric change is
received in ACTIVE State from a non-successor, the router stays in
ACTIVE State for the destination. The metric information in the UPDATE
is recorded. When a route is in the ACTIVE State, neither a QUERY nor
UPDATE are ever sent.

(8) If a REPLY for a destination, in ACTIVE State, is received from a
neighbor or the link between a router and the neighbor fails, the
router records that the neighbor replied to the QUERY. The REPLY
status flag is set to 0 to indicate this. The route stays in ACTIVE
State if there are more replies pending because the router has not
heard from all neighbors.

(9) If a route for a destination is in ACTIVE State, and a link fails
or a cost increase occurred between a router and its successor, the
router treats this case like it has received a REPLY from its
successor. When this occurs after the router originates a QUERY, it
sets the QUERY origin flag to indicate that another topology change
occurred in ACTIVE State.

(10) If a route for a destination is in ACTIVE State, and a link fails
or a cost increase occurred between a router and its successor, the
router treats this case like it has received a REPLY from its
successor. When this occurs after a successor originated a QUERY, the
router sets the QUERY origin flag to indicate that another topology
change occurred in ACTIVE State.

(11) If a route for a destination is in ACTIVE State, the cost of the
link through which the successor increases, and the last REPLY was
received from all neighbors, but there is no Feasible Successor, the
route should stay in ACTIVE State. A QUERY is sent to all neighbors.
The QUERY origin flag is set to 1.

(12) If a route for a destination is in ACTIVE State because of a
QUERY received from the current successor, and the last REPLY was
received from all neighbors, but there is no Feasible Successor, the
route should stay in ACTIVE State. A QUERY is sent to all neighbors.
The QUERY origin flag is set to 3.

(13) Received replies from all neighbors. Since the QUERY origin flag
indicates the successor originated the QUERY, it transitions to
PASSIVE State and sends a REPLY to the old successor.

(14) Received replies from all neighbors. Since the QUERY origin flag
indicates a topology change to the successor while in ACTIVE State, it
need not send a REPLY to the old successor. When the Feasibility
Condition is met, the route state transitions to PASSIVE.



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(15) Received replies from all neighbors. Since the QUERY origin flag
indicates either the router itself originated the QUERY or FC was not
satisfied with the replies received in ACTIVE state, FD is reset to
infinite value and the minimum of all the reported metrics is chosen
as FD and route transitions back to PASSIVE state. A REPLY is sent to
the old-successor if Oij flags indicate that there was a QUERY from
successor.

(16) If a route for a destination is in ACTIVE State because of a
QUERY received from the current successor or there was an increase in
Distance while in ACTIVE state, the last REPLY was received from all
neighbors, and a Feasible Successor exists for the destination, the
route can go into PASSIVE State and a REPLY is sent to successor if
Oij indicates that QUERY was received from the successor.

3.6 DUAL Operation - Example Topology
The following topology (Figure 2) will be used to provide an example
of how DUAL is used to reroute after a link failure. Each node is
labeled with its costs to destination N. The arrows indicate the
successor (next-hop) used to reach destination N. The least cost path
is selected.
                                N
                                |
                             (1)A ---<--- B(2)
                                |         |
                                ^         |
                                |         |
                             (2)D ---<--- C(3)

                        Figure 2 - Stable Topology

In the case where the link between A and D fails (Figure 3);

          N                                   N
          |                                   |
          A ---<--- B                         A ---<--- B
          |         |                         |          |
          X         |                         ^          |
          |         |                         |          |
          D ---<--- C                         D ---<--- C
            Q->                                      <-R

                             N
                             |
                          (1)A ---<--- B(2)
                                       |
                                       ^
                                       |
                          (4)D --->--- C(3)

                  Figure 3 - Link between A and D fails

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Only observing destination provided by node N, D enters the ACTIVE
State and sends a QUERY to all its neighbors, in this case node C.
   C determines that it has a Feasible Successor and replies
immediately with metric 3.
   C changes its old successor of D to its new single successor B and
the route to N stays in PASSIVE State.
   D receives the REPLY and can transition out of ACTIVE State since
it received replies from all its neighbors.
   D now has a viable path to N through C.
   D selects C as its successor to reach node N with a cost of 4.

Notice that node A and B were not involved in the recalculation since
they were not affected by the change.

Let's consider the situation in Figure 4, where Feasible Successors
may not exist. If the link between node A and B fails, B goes into
ACTIVE State for destination N since it has no Feasible Successors.
Node B sends a QUERY to node C. C has no Feasible Successors, so it
goes active for destination N and since C has no neighbors, it replies
to the QUERY, deletes the destination, and returns to the PASSIVE
State for the unreachable route. As C removes the (now unreachable)
destination from its table, C sends REPLY to its old successor. B
receives this REPLY from C, and determines this is the last REPLY it
is waiting on before determining what the new state of the route
should be; on receiving this REPLY, B deletes the route to N from its
routing table.

Since B was the originator of the initial QUERY it does not have to
send a REPLY to its old successor (it would not be able to any ways,
because the link to its old successor is down). Note that nodes A and
D were not involved in the recalculation since their successors were
not affected.

          N                                N
          |                                |
       (1)A ---<--- B(2)                   A ------- B   Q
          |         |                      |         |   |^      ^
          ^         ^                      ^         |   v|      |
          |         |                      |         |      |    |
       (2)D         C(3)                   D         C     ACK   R

                               Figure 4
        No Feasible Successors when link between A and B fails




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4 EIGRP Packets
EIGRP uses 5 different packet types to handle session management and
pass DUAL Message types:

    HELLO Packets (includes ACK)
    QUERY Packets (includes SIA-Query)
    REPLY Packets (includes SIA-Reply)
    REQUEST Packets
    UPDATE Packets

EIGRP packets are directly encapsulated into a network layer protocol,
such as IPv4 or IPv6. While EIGRP is capable of using additional
encapsulation (such as AppleTalk, IPX, etc.) no further encapsulation
is specified in this document.

Support for network layer protocol fragmentation is not supported, and
EIGRP will attempt to avoid a maximum size packets that exceed the
interface MTU by sending multiple packets which are less than or equal
to MTU sized packets.

Each packet transmitted will use either multicast or unicast network
layer destination addresses. When multicast addresses are used a
mapping for the data link multicast address (when available) must be
provided. The source address will be set to the address of the sending
interface, if applicable.

The following network layer multicast addresses; and associated data
link multicast addresses;
        IPv4 "IGRP Routers" [13]
        IPv6 "EIGRP Routers" [14]

Will be used on multicast capable media, and will be media independent
for unicast addresses. Network layer addresses will be used and the
mapping to media addresses will be achieved by the native protocol
mechanisms.


4.1 UPDATE Packets
UPDATE packets carry the DUAL UPDATE message type, and are used to
convey information about destinations and the reachability of those
destinations. When a new neighbor is discovered, unicast UPDATE
packets are used to transmit a full table to the new neighbor, so the
neighbor can build up its topology table. In normal operation (other
than neighbor startup such as a link cost changes), UPDATE packets are
multicast. UPDATE packets are always transmitted reliably. Each TLV
destination will be processed individually through the DUAL state
machine.

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4.2 QUERY Packets
A QUERY packet carries the DUAL QUERY message type and is sent by a
router to advertise that a route is in ACTIVE State and the originator
is requesting alternate path information from its neighbors. An
infinite metric is encoded by setting the Delay part of the metric to
its maximum value.

If there is a topology change that causes multiple destinations to be
marked ACTIVE, EIGRP will build one or more QUERY packet for all
destinations present. The state of each route is recorded
individually, so a responding QUERY or REPLY need not contain all the
same destinations in a single packet. Since EIGRP uses a reliable
transport mechanism, route QUERY packets are also guaranteed be
reliably delivered.

When a QUERY packet is received, each destination will trigger a DUAL
event and the state machine will run individually for each route. Once
the entire original QUERY packet is processed, then a REPLY or SIA-
REPLY will be sent with the latest information.


4.3 REPLY Packets
A REPLY packet carries the DUAL REPLY message type and will be sent in
response to a QUERY or SIA-QUERY packet. The REPLY packet will include
a TLV for each destination and the associated vector metric in its own
topology table.

The REPLY packet is sent after the entire received QUERY packet is
processed. When a REPLY packet is received, there is no reason to
process the packet before an acknowledgment is sent. Therefore, an
acknowledgment is sent immediately and then the packet is processed.
The sending of the acknowledgement is accomplished either by sending
an ACK packet, or piggybacked the acknowledgment onto another packet
already being transmitted.

Each TLV destination will be processed individually through the DUAL
state machine. When a QUERY is received for a route that doesn't exist
in our topology table, a REPLY with infinite metric is sent and an
entry in the topology table is added with the metric in the QUERY if
the metric is not an infinite value.

If a REPLY for a designation not in the Active State, or not in the
topology table, EIGRP will acknowledge the packet, and discard the
REPLY.




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4.4 Exception Handling
4.4.1 Active Duration (Stuck-in-Active)
When an EIGRP router transitions to ACTIVE state for a particular
destination a QUERY is sent to a neighbor and the ACTIVE timer is
started to limit the amount of time a destination may remain in an
ACTIVE State.

A route is regarded as Stuck-In-Active (SIA) when it does not receive
a REPLY within a preset time. This time interval is broken into two
equal periods following the QUERY, and up to 3 additional "busy"
periods in which an SIA-QUERY packet is sent for the destination.

This process is begun when a router sends a QUERY to its neighbor.
After one half the SIA time interval (default implementation is 90
seconds), the router will send an SIA-QUERY; this must be replied to
with either a REPLY or SIA-REPLY. Any neighbor which fails to send
either a REPLY or SIA-REPLY with-in one-half the SIA interval will
result in the neighbor being deemed to be "stuck" in the active state.

Cisco also limits the number of SIA-REPLY messages allowed to three.
Once the timeout occurs after the third SIA-REPLY with the neighbor
remaining in an ACTIVE State (as noted in the SIA-Reply message), the
neighbor being deemed to be "stuck" in the active state.

If the SIA state is declared, DUAL may take one of two actions;
    a) Delete the route from that neighbor, acting as if the neighbor
had responded with an unreachable REPLY message from the neighbor.

    b) Delete all routes from that neighbor and reset the adjacency
with that neighbor, acting as if the neighbor had responded with an
unreachable message for all routes.

Implementation note: Cisco currently implements option (b).




4.4.1.1 SIA-QUERY
When a QUERY is still outstanding and awaiting a REPLY from a
neighbor, there is insufficient information to determine why a REPLY
has not been received. A lost packet, congestion on the link, or a
slow neighbor could cause a lack of REPLY from a downstream neighbor.

In order to attempt to ascertain if the neighboring device is still
attempting to converge on the active route, EIGRP may send an SIA-
QUERY packet to the active neighbor(s). This enables an EIGRP router
to determine if there is a communication issue with the neighbor, or
it is simply still attempting to converge with downstream routers.


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By sending an SIA-QUERY, the originating router may extend the
effective active time by resetting the ACTIVE timer which has been
previously set, thus allowing convergence to continue so long as
neighbor devices successfully communicate that convergence is still
underway.

The SIA-QUERY packet SHOULD be sent on a per-destination basis at one-
half of the ACTIVE timeout period. Up to three SIA-QUERY packets for a
specific destination may be sent, each at a value of one-half the
ACTIVE time, so long as each are successfully acknowledged and met
with an SIA-REPLY.

Upon receipt of an SIA-QUERY packet, an EIGRP router should first send
an ACK and then continue to process the SIA-QUERY information. The
QUERY is sent on a per-destination basis at approximately one-half the
active time.

If the EIGRP router is still active for the destination specified in
the SIA-QUERY, the router should respond to the originator with the
SIA-REPLY indicating that active processing for this destination is
still underway by setting the ACTIVE flag in the packet upon response.

If the router receives an SIA-QUERY referencing a destination for
which it has not received the original QUERY, the router should treat
the packet as though it was a standard QUERY:

    1) Acknowledge the receipt of the packet
    2) Send a REPLY if a Successor exists
    3) If the SIA-QUERY is from the successor, transition to the
       ACTIVE state if and only if feasibility-condition fails
       and send an SIA-REPLY with the ACTIVE bit set



4.4.1.2 SIA-REPLY
An SIA-REPLY packet is the corresponding response upon receipt of an
SIA-QUERY from an EIGRP neighbor. The SIA-REPLY packet will include a
TLV for each destination and the associated metric for which is stored
in its own routing table. The SIA-REPLY packet is sent after the
entire received SIA-QUERY packet is processed.

If the EIGRP router is still ACTIVE for a destination, the SIA-REPLY
packet will be sent with the ACTIVE bit set. This confirms for the
neighbor device that the SIA-QUERY packet has been processed by DUAL
and that the router is still attempting to resolve a loop-free path
(likely awaiting responses to its own QUERY to downstream neighbors).



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The SIA-REPLY informs the recipient that convergence is complete or
still ongoing; it is an explicit notification that the router is still
actively engaged in the convergence process. This allows the device
that sent the SIA-QUERY to determine whether it should continue to
allow the routes that are not converged to be in the ACTIVE state, or
if it should reset the neighbor relationship and flush all routes
through this neighbor.


5 EIGRP Protocol Operation
EIGRP has four basic components:
     o Finite State Machine
     o Reliable Transport Protocol
     o Neighbor Discovery/Recovery
     o Route Management


5.1 Finite State Machine
The detail of DUAL, the State Machine used by EIGRP, is covered in
Section 0

5.2 Reliable Transport Protocol
The reliable transport is responsible for guaranteed, ordered delivery
of EIGRP packets to all neighbors. It supports intermixed transmission
of multicast and unicast packets. Some EIGRP packets must be
transmitted reliably and others need not. For efficiency, reliability
is provided only when necessary.

For example, on a multi-access network that has multicast
capabilities, such as Ethernet, it is not necessary to send HELLOs
reliably to all neighbors individually. EIGRP sends a single multicast
HELLO with an indication in the packet informing the receivers that
the packet need not be acknowledged. Other types of packets, such as
UPDATE packets, require acknowledgment and this is indicated in the
packet. The reliable transport has a provision to send multicast
packets quickly when there are unacknowledged packets pending. This
helps ensure that convergence time remains low in the presence of
varying speed links.

The DUAL Algorithm assumes there is lossless communication between
devices and thus must rely upon the transport protocol to guarantee
that messages are transmitted reliably. EIGRP implements the Reliable
Transport Protocol to ensure ordered delivery and acknowledgement of
any messages requiring reliable transmission. State variables such as
a received sequence number, acknowledgment number, and transmission
queues MUST be maintained on a per neighbor basis.

The following sequence number rules must be met for the reliable EIGRP
protocol to work correctly:

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    o A sender of a packet includes its global sequence number
      in the sequence number field of the fixed header. The sequence
      number wraps around to one when the maximum value is exceeded
      (sequence number zero is reserved for unreliable transmission).
      The sender includes the receivers sequence number in the
      acknowledgment number field of the fixed header.
    o Any packets that do not require acknowledgment must be
      sent with a sequence number of 0.
    o Any packet that has an acknowledgment number of zero (0)
      indicates that sender is not expecting to explicitly
      acknowledging delivery. Otherwise, it is acknowledging
      a single packet.
    o Packets that are network layer multicast must contain
      acknowledgment number of 0.

When a router transmits a packet, it increments its sequence number
and marks the packet as requiring acknowledgment by all neighbors on
the interface for which the packet is sent. When individual
acknowledgments are unicast addressed by the receivers to the sender
with the acknowledgment number equal to the packets sequence number,
the sender SHALL clear the pending acknowledgement requirement for the
packet from the respective neighbor.

If the required acknowledgement is not received for the packet, it
MUST be retransmitted. Retransmissions will occur for a maximum of 5
seconds. This retransmission for each packet is tried 16 times after
which if there is no ACK, the neighbor relationship is reset with that
peer which didn't send the ACK.

The protocol has no explicit windowing support. A receiver will
acknowledge each packet individually and will drop packets that are
received out of order.

Implementers note: The exception to this occurs if a duplicate packet
is received, and the acknowledgement for the original packet has been
scheduled for transmission, but not yet sent. In this case EIGRP will
not send an acknowledgement for the duplicate packet, and the queued
acknowledgement will acknowledge both the original and duplicate
packet

Duplicate packets are also discarded upon receipt. Acknowledgments are
not accumulative. Therefore, an ACK with a non-zero sequence number
acknowledges a single packet.

There are situations when multicast and unicast packets are
transmitted close together on multi-access broadcast capable networks.
The reliable transport mechanism MUST assure that all multicasts are
transmitted in order as well as not mixing the order among unicasts
and multicast packets. The reliable transport provides a mechanism to

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deliver multicast packets in order to some receivers quickly, while
some receivers have not yet received all unicast or previously sent
multicast packets. The SEQUENCE_TYPE TLV in HELLO packets achieves
this. This will be explained in more detail in this section.

Figure 5 illustrates the reliable transport protocol on point-to-point
links. There are two scenarios that may occur, an UPDATE initiated
packet exchange, or a QUERY initiated packet exchange.




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This example will assume no packet loss.

Router A                          Router B

                An Example UPDATE Exchange
                                 <----------------
                                 UPDATE (multicast)
A receives packet                SEQ=100, ACK=0
                                 Add Packet to A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=100                   Receives ACK
Process UPDATE                   Delete Packet from A's retransmit list


                An Example QUERY Exchange
                                 <----------------
                                 QUERY (multicast)
A receives packet                SEQ=101, ACK=0
Process QUERY                    Add Packet to A's retransmit list

---------------->
REPLY (unicast)
SEQ=201, ACK=101                 Process ACK
                                 Delete Packet from A's retransmit
list
                                 Process REPLY pkt
                                 <----------------
                                 ACK (unicast)
A receives packet                SEQ=0, ACK=201

       Figure 5 - Reliable Transfer on point-to-point links

The UPDATE exchange sequence requires UPDATE packets sent to be
delivered reliably. The UPDATE packet transmitted contains a sequence
number that is acknowledged by a receipt of an ACK packet. If the
UPDATE or the ACK packet is lost on the network, the UPDATE packet
will be retransmitted.




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This example will assume there is heavy packet loss on a network.

Router A                           Router B
                                 <----------------
                                 UPDATE (multicast)
A receives packet                SEQ=100, ACK=0
                                 Add Packet to A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=100                   Receives ACK
Process Update                   Delete Packet from A's retransmit list

                                 <--/LOST/--------------
                                 UPDATE (multicast)
                                 SEQ=101, ACK=0
                                 Add Packet to A's retransmit list

                                 Retransmit Timer Expires
                                 <----------------
                                 Retransmit UPDATE (unicast)
                                 SEQ=101, ACK=0
                                 Keeps packet on A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=101                   Receives ACK
Process Update                   Delete Packet from A's retransmit list

                                Figure 6
           Reliable Transfer on lossy point-to-point links




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Reliable delivery on multi-access LANs works in a similar fashion to
point-to-point links. The initial packet is always multicast and
subsequent retransmissions are unicast addressed. The acknowledgments
sent are always unicast addressed. Figure 7 shows an example with 4
routers on an Ethernet.

        Router B -----------+
                            |
        Router C -----------+------------ Router A
                            |
        Router D -----------+


                        An Example UPDATE Exchange
                                  <----------------
                                  A send UPDATE (multicast)
                                  SEQ=100, ACK=0
                                  Add Packet to B's retransmit list
                                  Add Packet to C's retransmit list
                                  Add Packet to D's retransmit list

---------------->
B sends ACK (unicast)
SEQ=0, ACK=100                    Receives ACK
Process Update                    Delete Packet from B's retransmit list

---------------->
C sends ACK (unicast)
SEQ=0, ACK=100                    Receives ACK
Process Update                    Delete Packet from C's retransmit list

---------------->
D sends ACK (unicast)
SEQ=0, ACK=100                    Receives ACK
Process Update                    Delete Packet from D's retransmit list




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                         An Example QUERY Exchange


                                  <----------------
                                  A send UPDATE (multicast)
                                  SEQ=101, ACK=0
                                  Add Packet to B's retransmit list
                                  Add Packet to C's retransmit list
                                  Add Packet to D's retransmit list

---------------->
B send REPLY (unicast)            <----------------
SEQ=511, ACK=101                  A sends ACK (unicast to B)
Process Update                    SEQ=0, ACK=511
                                  Delete Packet from B's retransmit list

---------------->
C send REPLY (unicast)            <----------------
SEQ=200, ACK=101                  A sends ACK (unicast to C)
Process Update                    SEQ=0, ACK=200
                                  Delete Packet from C's retransmit list

---------------->
D send REPLY (unicast)            <----------------
SEQ=11, ACK=101                   A sends ACK (unicast to D)
Process Update                    SEQ=0, ACK=11
                                  Delete Packet from D's retransmit list

                           Figure 7
             Reliable Transfer on Multi-Access Links




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And finally, a situation where numerous multicast and unicast packets
are sent close together in a multi-access environment is illustrated
in Figure 9.

        Router B -----------+
                            |
        Router C -----------+------------ Router A
                            |
        Router D -----------+


                                <----------------
                                A send UPDATE (multicast)
                                SEQ=100, ACK=0
---------------/LOST/->         Add Packet to B's retransmit list
B send ACK (unicast)            Add Packet to C's retransmit list
SEQ=0, ACK=100                  Add Packet to D's retransmit list

---------------->
C sends ACK (unicast)
SEQ=0, ACK=100                  Delete Packet from C's retransmit list

---------------->
D sends ACK (unicast)
SEQ=0, ACK=100                  Delete Packet from D's retransmit list
                                <----------------
                                A send HELLO (multicast)
                                SEQ=0, ACK=0, SEQ_TLV listing B

B receives Hello, does not set CR-Mode
C receives Hello, sets CR-Mode
D receives Hello, sets CR-Mode

                                <----------------
                                A send UPDATE (multicast)
                                SEQ=101, ACK=0, CR-Flag=1
---------------/LOST/->         Add Packet to B's retransmit list
B send ACK (unicast)            Add Packet to C's retransmit list
SEQ=0, ACK=100                  Add Packet to D's retransmit list

B ignores UPDATE 101 because CR-Flag
is set and it is not in CR-Mode

---------------->
C sends ACK (unicast)
SEQ=0, ACK=101

---------------->
D sends ACK (unicast)

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SEQ=0, ACK=101
                                <----------------
                                A resends UPDATE (unicast to B)
                                SEQ=100, ACK=0
B Packet duplicate

--------------->
B sends ACK (unicast)           A removes pkt from retransmit list
SEQ=0, ACK=100
                                <----------------
                                A resends UPDATE (unicast to B)
                                SEQ=101, ACK=0

--------------->
B sends ACK (unicast)            A removes pkt from retransmit list
SEQ=0, ACK=101

                           Figure 9

Initially Router-A sends a multicast addressed UPDATE packet on the
LAN. B and C receive it and send acknowledgments. Router-B receives
the UPDATE but the acknowledgment sent is lost on the network. Before
the retransmission timer for Router-B's packet expires, there is an
event that causes a new multicast addressed UPDATE to be sent.

Router-A detects that there is at least one neighbor on the interface
with a full queue. Therefore, it MUST signal that neighbor to not
receive the next packet or it would receive the retransmitted packet
out of order. If all neighbors on the interface have a full queue,
then EIGRP should reschedule the transmission of the UPDATE once the
queues are no longer full.

Router-A builds a HELLO packet with a SEQUENCE_TYPE TLV indicating all
the neighbors that have full queues. In this case, the only neighbor
address in the list is Router-B. The HELLO packet is sent via
multicast unreliably out the interface.

Router-C and Router-D process the SEQUENCE_TYPE TLV by looking for its
own address in the list. If not found, they put themselves in
Conditionally Received (CR-mode) mode.

Router-B does not find its address in the SEQUENCE TLV peer list, so
it enters CR-mode. Packets received by Router-B with the CR-flag MUST
be discarded and not acknowledged.

Later, Router-A will unicast transmit both packets 100 and 101
directly to Router-B. Router-B already has 100 so it discards and
acknowledges it.


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Router-B then accepts and acknowledges packet 101. Once an
acknowledgement is received, Router-A can remove both packets from
Router-B's transmission list.


5.2.1 Bandwidth on Low-Speed Links
By default, EIGRP limits itself to using no more than 50% of the
bandwidth reported by an interface when determining packet-pacing
intervals. If the bandwidth does not match the physical bandwidth (the
network architect may have put in an artificially low or high
bandwidth value to influence routing decisions), EIGRP may:

   1. Generate more traffic than the interface can handle, possibly
causing drops, thereby impairing EIGRP performance.

   2. Generate a lot of EIGRP traffic that could result in little
bandwidth remaining for user data. To control such transmissions an
interface-pacing timer is defined for the interfaces on which EIGRP is
enabled. When a pacing timer expires, a packet is transmitted out on
that interface.


5.3 Neighbor Discovery/Recovery
Neighbor Discovery/Recovery is the process that routers use to
dynamically learn of other routers on their directly attached
networks. Routers MUST also discover when their neighbors become
unreachable or inoperative. This process is achieved with low overhead
by periodically sending small HELLO packets. As long as any packets
are received from a neighbor, the router can determine that neighbor
is alive and functioning. Only after a neighbor router is considered
operational can the neighboring routers exchange routing information.


5.3.1 Neighbor Hold Time
Each router keeps state information about adjacent neighbors. When
newly discovered neighbors are learned the address, interface, and
hold time of the neighbor is noted. When a neighbor sends a HELLO, it
advertises its Hold Time. The Hold Time is the amount of time a router
treats a neighbor as reachable and operational. In addition to the
HELLO packet, if any packet is received within the hold time period,
then the Hold Time period will be reset. When the Hold Time expires,
DUAL is informed of the topology change.


5.3.2 HELLO Packets
When an EIGRP router is initialized, it will start sending HELLO
packets out any interface on which EIGRP is enabled. HELLO packets,
when used for neighbor discovery, are normally sent multicast
addressed. The HELLO packet will include the configured EIGRP metric
K-values. Two routers become neighbors only if the K-values are the

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same. This enforces that the metric usage is consistent throughout the
Internet. Also included in the HELLO packet, is a Hold Time value.
This value indicates to all receivers the length of time in seconds
that the neighbor is valid. The default Hold Time will be 3 times the
HELLO interval. HELLO packets will be transmitted every 5 seconds (by
default). There may be a configuration command that controls this
value and therefore changes the Hold Time. HELLO packets are not
transmitted reliably so the sequence number should be set to 0.


5.3.3 UPDATE Packets
When a router detects a new neighbor by receiving a HELLO packet from
a neighbor not presently known, it will send a unicast UPDATE packet
to the neighbor with no routing information (the NULL UPDATE packet).
The initial NULL UPDATE packet sent MUST have the INIT-flag set and
containing no topology information.

Implementation note: The use of the NULL UPDATE is used to ensure bi-
directional UNICAST packet delivery as the NULL UPDATE and the ACK are
both sent unicast. Additional UPDATE packets cannot be sent until the
initial NULL UPDATE packet is acknowledged.

The INIT-flag instructs the neighbor to advertise its routes, and is
also useful when a neighbor goes down and comes back up before the
router detects it went down. In this case, the neighbor needs new
routing information. The INIT-Flag informs the router to send it.

Implementation note: When a router sends an UPDATE with the INIT-Flag
set, and without the Restart Flag (RS) set in the header, the
receiving neighbor must also send an UPDATE with the INIT-Flag.
Failure to do so will result in a Cisco device posting a  stuck in
INIT state" error and discards subsequent.




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5.3.4 Initialization Sequence
         Router A                                 Router B
     (just booted)                           (up and running)

     (1)---------------->
         HELLO (multicast)            <----------------     (2)
         SEQ=0, ACK=0                  HELLO (multicast)
                                       SEQ=0, ACK=0

                                      <----------------    (3)
                                       UPDATE (unicast)
                                       SEQ=10, ACK=0, INIT
     (4)---------------->              UPDATE 11 is queued
          UPDATE (unicast)
          SEQ=100, ACK=10, INIT       <----------------     (5)
                                      UPDATE (unicast)
                                      SEQ=11, ACK=100
                                      All UPDATES sent
     (6)--------------/lost/->
          ACK (unicast)
          SEQ=0, ACK=11
                                      (5 seconds later)
                                      <----------------     (7)
          Duplicate received,         UPDATE (unicast)
          Packet discarded            SEQ=11, ACK=100
     (8)--------------->
          ACK (unicast)
          SEQ=0, ACK=11

                 Figure 9 - Initialization Sequence

(1) Router A sends multicast HELLO and Router B discovers it.

(2) Router B sends an expedited HELLO and starts the process of
sending its topology table to Router A. In addition, Router B sends
the NULL UPDATE packet with the INIT-Flag. The second packet is
queued, but cannot be sent until the first is acknowledged.

(3) Router A receives first UPDATE packet and processes it as a DUAL
event. If the UPDATE contains topology information, the packet will be
process and stored in topology table. Sends its first and only UPDATE
packet with an accompanied ACK.

(4) Router B receives UPDATE packet 100 from Router A. Router B can
dequeue packet 10 from A's transmission list since the UPDATE
acknowledged 10. It can now send UPDATE packet 11 and with an
acknowledgment of Router A's UPDATE.

(5) Router A receives the last UPDATE packet from Router B and

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acknowledges it. The acknowledgment gets lost.

(6) Router B later retransmits the UPDATE packet to Router A.

(7) Router A detects the duplicate and simply acknowledges the packet.
Router B dequeues packet 11 from A's transmission list and both
routers are up and synchronized.


5.3.5 Neighbor Formation
To prevent packets from being sent to a neighbor prior to verifying
multicast and unicast packet delivery is reliable, a 3-way handshake
is utilized.

During normal adjacency formation, multicast HELLOs cause the EIGRP
process to place new neighbors into the neighbor table. Unicast
packets are then used to exchange known routing information, and
complete the neighbor relationship (section 5.2)

To prevent EIGRP from sending sequenced packets to neighbor which fail
to have bidirectional unicast/multicast, or one neighbor restarts
while building the relationship, EIGRP MUST place the newly discovered
neighbor in a "pending" state as follows:

When Router-A receives the first multicast HELLO from Router-B, it
places Router-B in the pending state, and transmits a unicast UPDATE
containing no topology information and SHALL set the initialization
bit. While Router-B is in this state, A will not send it any a QUERY
or UPDATE. When Router-A receives the unicast acknowledgement from
Router-B, it will check the state from "pending" to "up".


5.3.6 QUERY Packets During Neighbor Formation
As described above, during the initial formation of the neighbor
relationship, EIGRP uses a form of three-way handshake to verify both
unicast and multicast connectivity are working successfully. During
this period of neighbor creation, the new neighbor is considered to be
the pending state, and is not eligible to be included in the
convergence process.

Because of this, any QUERY received by an EIGRP router would not cause
a QUERY to be sent to the new (and pending) neighbor. It would perform
the DUAL process without the new peer in the conversation.
To do this, when a router in the process of establishing a new
neighbor receives a QUERY from a fully established neighbor, it
performs the normal DUAL Feasible Successor check to determine whether
it needs to REPLY with a valid path or whether it needs to enter the
ACTIVE process on the prefix.



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If it determines that it must go active, each fully established
neighbor that participates in the convergence process will be sent a
QUERY packet and REPLY packets are expected from each. Any pending
neighbor will not be expected to REPLY and will not be sent a QUERY
directly. If it resides on an interface containing a mix of fully
established neighbors and pending neighbors, it might receive the
QUERY but will not be expected to REPLY to it.


5.4 Topology Table
The Topology Table is populated by the protocol dependent modules
(IPv4/IPv6 PDM), and is acted upon by the DUAL finite state machine.
Associated with each entry are the destination address and a list of
neighbors that have advertised this destination, and the metric
associated with the destination. The metric is referred to as the
Computed Distance.

The Computed Distance is the best-advertised Reported Distance from
all neighbors, plus the link cost between the receiving router and the
neighbor.

The Reported Distance is the Computed Distance as advertised by the
Feasible Successor for the destination. Said another way, the Computed
distance, when sent by a neighbor, is referred to as the Reported
Distance and is the metric that the neighboring router uses to reach
the destination (Its Computed Distance as described above).

If the router is advertising a destination route, it MUST be using the
route to forward packets; this is an important rule that distance
vector protocols MUST follow.


5.4.1 Route Management
Within the topology table, EIGRP has the notion of internal and
external routes. Internal routes MUST be preferred over external
routes independent of the metric. In practical terms, if an internal
route is received, the Diffusion computation will be run considering
only the internal routes. Only when no internal routes for a given
destination exist, will EIGRP choose the Successor from the available
external routes.


5.4.1.1 Internal Routes
Internal routes are destinations that have been originated within the
same EIGRP Autonomous System. Therefore, a directly attached network
that is configured to run EIGRP is considered an internal route and is
propagated with this information throughout the network topology.




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Internal routes are tagged with the following information:
    o Router ID of the EIGRP router that originated the route.
    o Configurable administrator tag.



5.4.1.2 External routes
External routes are destinations that have been learned from another
source, such as a different routing protocol or static route. These
routes are marked individually with the identity of their origination.
External routes are tagged with the following information:
    o Router ID of the EIGRP router that redistributed the route.
    o AS number where the destination resides.
    o Configurable administrator tag.
    o Protocol ID of the external protocol.
    o Metric from the external protocol.
    o Bit flags for default routing.

As an example, suppose there is an AS with three border routers (BR1,
BR2, and BR3). A border router is one that runs more than one routing
protocol. The AS uses EIGRP as the routing protocol. Two of the border
routers, BR1 and BR2, also use Open Shortest Path First (OSPF) [10]
and the other, BR3, also uses Routing Information Protocol (RIP).

Routes learned by one of the OSPF border routers, BR1, can be
conditionally redistributed into EIGRP. This means that EIGRP running
in BR1 advertises the OSPF routes within its own AS. When it does so,
it advertises the route and tags it as an OSPF learned route with a
metric equal to the routing table metric of the OSPF route. The
router-id is set to BR1. The EIGRP route propagates to the other
border routers.

Let's say that BR3, the RIP border router, also advertises the same
destinations as BR1. Therefore, BR3, redistributes the RIP routes into
the EIGRP AS. BR2, then, has enough information to determine the AS
entry point for the route, the original routing protocol used, and the
metric.

Further, the network administrator could assign tag values to specific
destinations when redistributing the route. BR2 can use any of this
information to use the route or re-advertise it back out into OSPF.

Using EIGRP route tagging can give a network administrator flexible
policy controls and help customize routing. Route tagging is
particularly useful in transit AS's where EIGRP would typically
interact with an inter-domain routing protocol that implements global
policies.




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5.4.2 Split Horizon and Poison Reverse
In some circumstances, EIGRP will suppress or poison QUERY and UPDATE
information to prevent routing loops as changes propagate though the
network.

The split horizon rule states: "Never advertise a route out of the
interface through which it was learned". EIGRP implements this to
mean if you have a Successor route to a destination, never advertise
the route out the interface on which it was learned.

The poison reverse rule states: "A route learned through an interface
will be advertised as unreachable through that same interface". As
with the case of Split Horizon, EIGRP applies this rule only to
interfaces its is using for reaching the destination. Routes learned
though interfaces which EIGRP is NOT using to reach the destination
may have the route advertised out those interfaces.

In EIGRP, split horizon suppresses a QUERY, where as Reverse Poison
advertises a destination as unreachable. This can occur for a
destination under any of the following conditions:

    o two routers are in startup or restart mode
    o advertising a topology table change
    o sending a query


5.4.2.1 Startup Mode
When two routers first become neighbors, they exchange topology tables
during startup mode. For each destination a router receives during
startup mode, it advertises the same destination back to its new
neighbor with a maximum metric (Poison Route).


5.4.2.2 Advertising Topology Table Change
If a router uses a neighbor as the Successor for a given destination,
it will send an UPDATE for the destination with a metric of infinity.


5.4.2.3 Sending a QUERY/UPDATE
In most cases EIGRP follows normal split-horizon rules. When a metric
change is received from the Successor via QUERY or UPDATE that causes
the route to go ACTIVE, the router will send a QUERY to neighbors on
all interfaces except the interface toward the Successor.

In other words, the router does not send the QUERY out of the inbound
interface through which the information causing the route to go ACTIVE
was received.

An exception to this can occur if a router receives a QUERY from its
successor while already reacting to an event that did not cause it to

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go ACTIVE. For example, a metric change from the Successor that did
not cause an ACTIVE transition, but was followed by the UPDATE/QUERY
that does result the router to transition to ACTIVE.


5.5 EIGRP Metric Coefficients
EIGRP allows for modification of the default composite metric
calculation (See Section 5.6) through the use of coefficients (K-
values). This adjustment allows for per-deployment tuning of network
behavior. Setting K-values up to 254 scales the impact of the scalar
metric on the final composite metric.

EIGRP default coefficients have been carefully selected to provide
optimal performance in most networks. The default K-values are

            K1 == K3 == 1
            K2 == K4 == K5 == 0
            K6 == 0

If K5 is equal to 0 then reliability quotient is defined to be 1.


5.5.1 Coefficients K1 and K2
K1 is used to allow path selection to be based on the bandwidth
available along the path. EIGRP can use one of two variations of
Throughput based path selection.
  o Maximum Theoretical Bandwidth: paths chosen based on the highest
reported bandwidth
  o Network Throughput: paths chosen based on the highest "available"
bandwidth adjusted by congestion-based effects (interface reported
load)

By default, EIGRP computes the Throughput using the maximum
theoretical throughput expressed in picoseconds per kilobyte of data
sent. This inversion results in a larger number (more time) ultimately
generating a worse metric.

If K2 is used, the effect of congestion as a measure of load reported
by the interface will be used to simulate the "available throughput"
by adjusting the maximum throughput.


5.5.2 Coefficient K3
K3 is used to allow delay or latency-based path selection. Latency and
Delay are similar terms that refer to the amount of time it takes a
bit to be transmitted to an adjacent neighbor. EIGRP uses one-way
based values either provided by the interface, or computed as a factor
of the link s bandwidth.



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5.5.3 Coefficients K4 and K5
K4 and K5 are used to allow for path selection based on link quality
and packet loss. Packet loss caused by network problems result in
highly noticeable performance issues or jitter with streaming
technologies, voice over IP, online gaming and videoconferencing, and
will affect all other network applications to one degree or another.

Critical services should pass with less than 1% packet loss. Lower
priority packet types might pass with less than 5% and then 10% for
the lowest of priority of services. The final metric can be weighted
based on the reported link quality.

The handling of K5 is conditional. If K5 is equal to 0 then
reliability quotient is defined to be 1.


5.5.4 Coefficient K6
K6 has been introduced with Wide Metric support and is used to allow
for Extended Attributes, which can be used to reflect in a higher
aggregate metric than those having lower energy usage.
Currently there are two Extended Attributes, jitter and energy,
defined in the scope of this document.


5.5.4.1 Jitter
Use of Jitter-based Path Selection results in a path calculation with
the lowest reported jitter. Jitter is reported as the interval between
the longest and shortest packet delivery and is expressed in
microseconds. Higher values result in a higher aggregate metric when
compared to those having lower jitter calculations.

Jitter is measured in microseconds and is accumulated along the path,
with each hop using an averaged 3-second period to smooth out the
metric change rate.

Presently, EIGRP does not currently have the ability to measure
jitter, and as such the default value will be zero (0). Performance
based solutions such as PfR could be used to populate this field.


5.5.4.2 Energy
Use of Energy-based Path Selection results in paths with the lowest
energy usage being selected in a loop free and deterministic manner.
The amount of energy used is accumulative and has results in a higher
aggregate metric than those having lower energy.

Presently, EIGRP does not report energy usage, and as such the default
value will be zero (0).



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5.6 EIGRP Metric Calculations
5.6.1 Classic Metrics
The composite metric is based on bandwidth, delay, load, and
reliability. MTU is not an attribute for calculating the composite
metric, but carried in the vector metrics.

One of the original goals of EIGRP was to offer and enhance routing
solutions for IGRP. To achieve this, EIGRP used the same composite
metric as IGRP, with the terms multiplied by 256 to change the metric
from 24 bits to 32 bits.


5.6.1.1 Classic Composite Formulation
EIGRP calculates the composite metric with the following formula:

metric = 256*({(K1*BW)+[(K2*BW)/(256-LOAD)]+(K3*DELAY)}*(K5/(REL+K4)))

In this formula, Bandwidth (BW) is the lowest interface bandwidth
along the path, and delay (DELAY) is the sum of all outbound interface
delays along the path. Load (LOAD) and reliability (REL) values are
expressed percentages with a value of 1 to 255.

Implementation note: Cisco IOS routers display reliability as a
fraction of 255. That is, 255/255 is 100 percent reliability or a
perfectly stable link; a value of 229/255 represents a 90 percent
reliable link. Load is a value between 1 and 255. A load of 255/255
indicates a completely saturated link. A load of 127/255 represents a
50 percent saturated link. These values are not dynamically measured
they are only measured at the time a link changes.

Bandwidth is the inverse minimum bandwidth (in kbps) of the path in
bits per second scaled by a factor of 10^7. The formula for bandwidth
is
                  (10^7)/BWmin

Implementation note: When converting the real bandwidth to the
composite bandwidth, truncate before applying the scaling factor.
When converting the composite bandwidth to the real bandwidth, apply
the scaling factor before the division and only then truncate.

The delay is the sum of the outgoing interface delay (in tens of
microseconds) to the destination. A delay set to it maximum value
(hexadecimal 0xFFFFFFFF) indicates that the network is unreachable.
The formula for delay is

                  [sum of delays]




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The default composite metric, adjusted for scaling factors, for EIGRP
is:

          metric = 256 x { [(10^7)/ BWmin] + [sum of delays]}

Minimum Bandwidth (BWmin) is represented in kbps, and the "sum of
delays" is represented in 10s of microseconds. The bandwidth and delay
for an Ethernet interface are 10Mbps and 1ms, respectively.

The calculated EIGRP bandwidth (BW) metric is then:

            256 x (10^7)/BW = 256 x {(10^7)/10,000}
                            = 256 x 1000
                            = 256,000

And the calculated EIGRP delay metric is then:

         256 x sum of delay = 256 x 100 x 10 microseconds
                            = 25,600 (in tens of microseconds)

5.5.1.2 Cisco Interface Delay Compatibility
For compatibility with Cisco products, the following table shows the
times in nanoseconds EIGRP uses for bandwidth and delay
    Bandwidth        Classic     Wide Metrics     Interface
    (Kbps)           Delay       Delay            Type
    ---------------------------------------------------------
    9               500000000   500000000         Tunnel
    56               20000000    20000000         56Kb/s
    64               20000000    20000000         DS0
    1544             20000000    20000000         T1
    2048             20000000    20000000         E1
    10000             1000000     1000000         Ethernet
    16000              630000      630000         TokRing16
    45045            20000000    20000000         HSSI
    100000             100000      100000         FDDI
    100000             100000      100000         FastEthernet
    155000             100000      100000         ATM 155Mb/s
    1000000             10000       10000         GigaEthernet
    2000000             10000        5000         2 Gig
    5000000             10000        2000         5 Gig
    10000000            10000        1000         10 Gig
    20000000            10000          500        20 Gig
    50000000            10000          200        50 Gig
    100000000           10000          100        100 Gig
    200000000           10000           50        200 Gig
    500000000           10000           20        500 Gig



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5.6.2 Wide Metrics
To accommodate interfaces with high bandwidths, and to allow EIGRP to
perform the path selection; the EIGRP packet and composite metric
formula has been modified to choose paths based on the computed time,
measured in picoseconds, information takes to travel though the links.


5.6.2.1 Wide Metric Vectors
EIGRP uses five "vector metrics": minimum throughput, latency, load,
reliability, and maximum transmission unit (MTU). These values are
calculated from destination to source as follows:
           o Throughput - Minimum value
           o Latency       - accumulative
           o Load          - maximum
           o Reliability   - minimum
           o MTU           - minimum
           o Hop count     - accumulative

To this there are two additional values: jitter and energy. These two
values are accumulated from destination to source:
        o Jitter - accumulative
        o Energy - accumulative

These Extended Attributes, as well as any future ones, will be
controlled via K6. If K6 is non-zero, these will be additive to the
path's composite metric. Higher jitter or energy usage will result in
paths that are worse than those which either does not monitor these
attributes, or which have lower values.

EIGRP will not send these attributes if the router does not provide
them. If the attributes are received, then EIGRP will use them in the
metric calculation (based on K6) and will forward them with those
routers values assumed to be "zero" and the accumulative values are
forwarded unchanged.

The use of the vector metrics allows EIGRP to compute paths based on
any of four (bandwidth, delay, reliability, and load) path selection
schemes. The schemes are distinguished based on the choice of the key
measured network performance metric.

Of these vector metric components, by default, only minimum throughput
and latency are traditionally used to compute best path. Unlike most
metrics, minimum throughput is set to the minimum value of the entire
path, and it does not reflect how many hops or low throughput links
are in the path, nor does it reflect the availability of parallel
links. Latency is calculated based on one-way delays, and is a
cumulative value, which increases with each segment in the path.




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Network Designers Note: when trying to manually influence EIGRP path
selection though interface bandwidth/delay configuration, the
modification of bandwidth is discouraged for following reasons:

The change will only effect the path selection if the configured value
is the lowest bandwidth over the entire path.
Changing the bandwidth can have impact beyond affecting the EIGRP
metrics. For example, Quality of Service (QoS) also looks at the
bandwidth on an interface.

EIGRP throttles its packet transmissions so it will only use 50
percent of the configured bandwidth. Lowering the bandwidth can cause
EIGRP to starve an adjacency, causing slow or failed convergence and
control plane operation.

Changing the delay does not impact other protocols nor does it cause
EIGRP to throttle back; changing the delay configured on a link only
impacts metric calculation.


5.6.2.2 Wide Metric Conversion Constants
EIGRP uses a number of defined constants for conversion and
calculation of metric values. These numbers are provided here for
reference

        EIGRP_BANDWIDTH                    10,000,000
        EIGRP_DELAY_PICO                    1,000,000
        EIGRP_INACCESSIBLE       0xFFFFFFFFFFFFFFFFLL
        EIGRP_MAX_HOPS                            100
        EIGRP_CLASSIC_SCALE                       256
        EIGRP_WIDE_SCALE                        65536

When computing the metric using the above units, all capacity
information will be normalized to kilobytes and picoseconds before
being used. For example, delay is expressed in microseconds per
kilobyte, and would be converted to kilobytes per second; likewise,
energy would be expressed in power per kilobytes per second of usage.


5.6.2.3 Throughput Calculation
The formula for the conversion for Max-Throughput value directly from
the interface without consideration of congestion-based effects is as
follows:

                               (EIGRP_BANDWIDTH * EIGRP_WIDE_SCALE)
     Max-Throughput = K1 *     ------------------------------------
                                    Interface Bandwidth (kbps)



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If K2 is used, the effect of congestion as a measure of load reported
by the interface will be used to simulate the "available throughput"
by adjusting the maximum throughput according to the formula:

                                        K2 * Max-Throughput
     Net-Throughput = Max-Throughput + ---------------------
                                           256 - Load
K2 has the greatest effect on the metric occurs when the load
increases beyond 90%.


5.6.2.4 Latency Calculation
Transmission times derived from physical interfaces MUST be n units of
picoseconds, or converted to picoseconds prior to being exchanged
between neighbors, or used in the composite metric determination.

This includes delay values present in configuration-based commands
(i.e. interface delay, redistribute, default-metric, route-map, etc.)

The delay value is then converted to a "latency" using the formula:

                       Delay * EIGRP_WIDE_SCALE
     Latency = K3 *   --------------------------
                          EIGRP_DELAY_PICO




5.6.2.5 Composite Calculation
                                                             K5
   metric =[(K1*Net-Throughput) + Latency)+(K6*ExtAttr)] * ------
                                                           K4+Rel

By default, the path selection scheme used by EIGRP is a combination
of Throughput and Latency where the selection is a product of total
latency and minimum throughput of all links along the path:

   metric = (K1 * min(Throughput)) + (K3 * sum(Latency)) }




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6 EIGRP Packet Formats

6.1 Protocol Number
The IPv6 and IPv4 protocol identifier number spaces are common and
will both use protocol identifier 88 [8] [9].

EIGRP IPv4 will transmit HELLO packets using either the unicast
destination of a neighbor or using a multicast host group address [7]
with a source address EIGRP IPv4 multicast address [13].

EIGRP IPv6 will transmit HELLO packets with a source address being the
link-local address of the transmitting interface. Multicast HELLO
packets will have a destination address of EIGRP IPv6 multicast
address [14]. Unicast packets directed to a specific neighbor
will contain the destination link-local address of the neighbor.

There is no requirement that two EIGRP IPv6 neighbors share a common
prefix on their connecting interface. EIGRP IPv6 will check that a
received HELLO contains a valid IPv6 link-local source address. Other
HELLO processing will follow common EIGRP checks, including matching
Autonomous system number and matching K-values.


6.2 Protocol Assignment Encoding
External Protocol Field is an informational assignment to identify the
originating routing protocol that this route was learned by. The
following values are assigned:

        Protocols             Value
        IGRP                    1
        EIGRP                   2
        Static                  3
        RIP                     4
        HELLO                   5
        OSPF                    6
        ISIS                    7
        EGP                     8
        BGP                     9
        IDRP                   10
        Connected              11



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6.3 Destination Assignment Encoding
Destinations types are encoded according to the IANA address family
number assignments. Currently only the following types are used:

      AFI Designation            AFI Value
     --------------------------------------
      IPv4 Address                   1
      IPv6 Address                   2
      Service Family Common      16384
      Service Family IPv4        16385
      Service Family IPv6        16386


6.4 EIGRP Communities Attribute
EIGRP supports communities similar to the BGP Extended Communities RFC
4360 [4] extended type with Type Field composed of 2 octets and Value
Field composed of 6 octets. Each Community is encoded as an 8-octet
quantity, as follows:
       - Type Field: 2 octets
       - Value Field: Remaining octets

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high     | Type low      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+          Value                |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In addition to well-known communities supported by BGP (such as Site
of Origin), EIGRP defines a number of additional Community values in
the "Experimental Use" [5] range as follows:

  Type high: 0x88
  Type low:

    Value       Name               Description
    ---------------------------------------------------------------
      00        EXTCOMM_EIGRP      EIGRP route information appended
      01        EXTCOMM_DAD        Data: AS + Delay
      02        EXTCOMM_VRHB       Vector: Reliability + Hop + BW
      03        EXTCOMM_SRLM       System: Reserve + Load + MTU
      04        EXTCOMM_SAR        System: Remote AS + Remote ID
      05        EXTCOMM_RPM        Remote: Protocol + Metric
      06        EXTCOMM_VRR        Vecmet: Rsvd + RouterID


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6.5 EIGRP Packet Header
The basic EIGRP packet payload format is identical for both IPv4 and
IPv6, although there are some protocol-specific variations. Packets
consist of a header, followed by a set of variable-length fields
consisting of Type/Length/Value (TLV) triplets.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Version | Opcode        |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Flags                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgement number                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Virtual Router ID             |   Autonomous system number    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Header Version - EIGRP Packet Header Format version. Current Version
is 2. This field is not the same as the TLV Version field.

Opcode - Indicates the type of the message. It will be one of the
following values:

        EIGRP_OPC_UPDATE              1
        EIGRP_OPC_REQUEST             2
        EIGRP_OPC_QUERY               3
        EIGRP_OPC_REPLY               4
        EIGRP_OPC_HELLO               5
        Reserved                      6      (EIGRP_OPC_IPXSAP)
        Reserved                      7      (EIGRP_OPC_PROBE)
        Reserved                      8      (EIGRP_OPC_ACK)
        Reserved                      9
        EIGRP_OPC_SIAQUERY           10
        EIGRP_OPC_SIAREPLY           11

Checksum - Each packet will include a checksum for the entire contents
of the packet. The check-sum will be the standard ones complement of
the ones complement sum. For purposes of computing the checksum, the
value of the checksum field is zero. The packet is discarded if the
packet checksum fails.

Flags - Defines special handling of the packet. There are currently
four defined flag bits.

Init Flag (0x01) - This bit is set in the initial UPDATE sent to a
newly discovered neighbor. It instructs the neighbor to advertise its
full set of routes.

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CR Flag (0x02) - This bit indicates that receivers should only accept
the packet if they are in Conditionally Received mode. A router enters
conditionally received mode when it receives and processes a HELLO
packet with a Sequence TLV present.

RS (0x04) - The Restart flag is set in the HELLO and the UPDATE
packets during the restart period. The router looks at the RS flag to
detect if a neighbor is restarting. From the restarting routers
perspective, if a neighboring router detects the RS flag set, it will
maintain the adjacency, and will set the RS flag in its UPDATE packet
to indicated it is doing a soft restart.

EOT (0x08) - The End-of-Table flag marks the end of the startup
process with a neighbor. If the flag is set, it indicates the neighbor
has completed sending all UPDATEs. At this point the router will
remove any stale routes learned from the neighbor prior to the restart
event. A stale route is any route, which existed before the restart,
and was not refreshed by the neighbor via and UPDATE.

Sequence - Each packet that is transmitted will have a 32-bit sequence
number that is unique with respect to a sending router. A value of 0
means that an acknowledgment is not required.

ACK - The 32-bit sequence number that is being acknowledged with
respect to the receiver of the packet. If the value is 0, there is no
acknowledgment present. A non-zero value can only be present in
unicast-addressed packets. A HELLO packet with a nonzero ACK field
should be decoded as an ACK packet rather than a HELLO packet.

Virtual Router ID (VRID) - A 16-bit number which identifies the
virtual router this packet is associated with. Packets received with
an unknown, or unsupported, value will be discarded.

       Value Range       Usage
         0x0000            Unicast Address Family
         0x0001            Multicast Address Family
         0x0002-0x7FFF     Reserved
         0x8000            Unicast Service Family
         0x8001-0xFFFF     Reserved

Autonomous System (AS) - 16-bit unsigned number of the sending system.
This field is indirectly used as an authentication value. That is, a
router that receives and accepts a packet from a neighbor must have
the same AS number or the packet is ignored. The range of valid AS
numbers is 1 through 65,535.



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6.6 EIGRP TLV Encoding Format
The contents of each packet can contain a variable number of fields.
Each field will be tagged and include a length field. This allows for
newer versions of software to add capabilities and coexist with old
versions of software in the same configuration. Fields that are tagged
and not recognized can be skipped over. Another advantage of this
encoding scheme allows multiple network layer protocols to carry
independent information. Therefore, later if it is decided to
implement a single "integrated" protocol this can be done.

The format of a {type, length, value} (TLV) is encoded as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high     | Type low      |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Value (variable length)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The type values are the ones defined below. The length value specifies
the length in octets of the type, length and value fields. TLVs can
appear in a packet in any order and there are no inter-dependencies
among them.

Malformed TLVs contained in EIGRP messages are handled by silently
discarding the containing message. A TLV is malformed if the TLV
Length is invalid or if the TLV extends beyond the end of the
containing message.


6.6.1 Type Field Encoding
The type field is structured as follows:
Type High: 1 octet that defines the protocol classification:
         Protocol            ID   VERSION
         General            0x00    1.2
         IPv4               0x01    1.2
         IPv6               0x04    1.2
         SAF                0x05    3.0
         Multi-Protocol     0x06    2.0

Type Low: 1 octet that defines the TLV Opcode; See TLV Definitions in
Section 3.


6.6.2 Length Field Encoding
The Length field is a 2 octet unsigned number, which indicates the
length of the TLV. The value includes the Type and Length fields



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6.6.3 Value Field Encoding
The Value field is a multi-octet field containing the payload for the
TLV.

6.7 EIGRP Generic TLV Definitions
                                    Ver 1.2   Ver 2.0
      PARAMETER_TYPE                0x0001    0x0001
      AUTHENTICATION_TYPE           0x0002    0x0002
      SEQUENCE_TYPE                 0x0003    0x0003
      SOFTWARE_VERSION_TYPE         0x0004    0x0004
      MULTICAST_SEQUENCE_TYPE       0x0005    0x0005
      PEER_INFORMATION_TYPE         0x0006    0x0006
      PEER_TERMINATION_TYPE         0x0007    0x0007
      PEER_TID_LIST_TYPE             ---      0x0008


6.7.1 0x0001 - PARAMETER_TYPE
This TLV is used in HELLO packets to convey the EIGRP metric
coefficient values - noted as "K-values" as well as the Hold Time
values. This TLV is also used in an initial UPDATE packet when a
neighbor is discovered.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0001             |            0x000C             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       K1      |       K2      |       K3      |       K4      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       K5      |       K6      |           Hold Time           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

K-values - The K-values associated with the EIGRP composite metric
equation. The default values for weights are:
          K1 - 1
          K2 - 0
          K3 - 1
          K4 - 0
          K5 - 0
          K6 - 0

Hold Time - The amount of time in seconds that a receiving router
should consider the sending neighbor valid. A valid neighbor is one
that is able to forward packets and participates in EIGRP. A router
that considers a neighbor valid will store all routing information
advertised by the neighbor.


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6.7.2 0x0002 - AUTHENTICATION_TYPE
This TLV may be used in any EIGRP packet and conveys the
authentication type and data used. Routers receiving a mismatch in
authentication shall discard the packet.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             0x0002            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Auth Type    | Auth Length  |      Auth Data (Variable)     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication Type - The type of authentication used.
Authentication Length - The length, measured in octets, of the
individual authentication.

Authentication Data - Variable length field reflected by "Auth Length"
which is dependent on the type of authentication used. Multiple
authentication types can be present in a single AUTHENTICATION_TYPE
TLV.


6.7.2.1 0x02 - MD5 Authentication Type
MD5 Authentication will use Auth Type code 0x02, and the Auth Data
will be the MD5 Hash value.


6.7.2.2 0x03 - SHA2 Authentication Type
SHA2-256 Authentication will use Type code 0x03, and the Auth Data
will be the 256 bit SHA2 [6] Hash value


6.7.3 0x0003 - SEQUENCE_TYPE
This TLV is used for a sender to tell receivers to not accept packets
with the CR-flag set. This is used to order multicast and unicast
addressed packets.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0003             |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Address Length |                 Protocol Address              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Address Length and Protocol Address will be repeated one or more
times based on the Length Field.

Address Length - Number of octets for the address that follows. For

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IPv4, the value is 4. For IPv6 it is 16. While no longer in use, for
AppleTalk, the value is 4 and for Novell IPX, the value is 10,

Protocol Address - Neighbor address on interface in which the HELLO
with SEQUENCE TLV is sent. Each address listed in the HELLO packet is
a neighbor that should not enter Conditionally Received mode.


6.7.4 0x0004 - SOFTWARE_VERSION_TYPE
        Field                        Length
        Vender OS major version        1
        Vender OS minor version        1
        EIGRP major revision           1
        EIGRP minor revision           1

The EIGRP TLV Version fields are used to determine TLV format
versions. Routers using Version 1.2 TLVs do not understand version 2.0
TLVs, therefore Version 2.0 routers must send the packet with both TLV
formats in a mixed network.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0004             |            0x000C             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vendor Major V.|Vendor Minor V.| EIGRP Major V.| EIGRP Minor V.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6.7.5 0x0005 - MULTICAST_SEQUENCE_TYPE
The next multicast sequence TLV.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0005             |             0x0008            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Sequence Number                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6.7.6 0x0006 - PEER_INFORMATION_TYPE
This TLV is reserved, and not part of this IETF document.


6.7.7 0x0007 - PEER_ TERMINATION_TYPE
This TLV is used in HELLO Packets to notify the list of neighbor(s)
the router has reset the adjacency. This is used anytime a router
need to expedite a  doing down process so the neighbors can process
the down neighbor without waiting for a HELLO timeout.

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 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0007             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Address List (variable)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Implementation note: Older Cisco routers implement this using the
"Parameters TLV" with all K-values set to 255 (except K6).


6.7.8 0x0008 - TID_LIST_TYPE
List of sub-topology identifiers, including the base topology,
supported by the router.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0x0008             |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Topology Identification List (variable)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If this information changes from the last state, it means either a new
topology was added, or an existing topology was removed. This TLV is
ignored until the three-way handshake has finished

When the TID list is received, it compares the list to the previous
list sent. If a TID is found which does not previously exist, the TID
is added to the neighbor's topology list, and the existing sub-
topology is sent to the peer.

If a TID, which was in a previous list, is not found, the TID is
removed from the neighbor's topology list and all routes learned
though that neighbor for that sub-topology are removed from the
topology table.


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6.8 Classic Route Information TLV Types
6.8.1 Classic Flag Field Encoding
EIGRP transports a number of flags with in the TLVs to indicate
addition route state information. These bits are defined as follows:

Flags Field
-----------
Source Withdraw (Bit 0) - Indicates if the router which is the
original source of the destination is withdrawing the route from the
network, or if the destination is lost due as a result of a network
failure.

Candidate Default (CD) (Bit 1) - Set to indicate the destination
should be regarded as a candidate for the default route. An EIGRP
default route is selected from all the advertised candidate default
routes with the smallest metric.

ACTIVE (Bit 2) - Indicates if the route is in the ACTIVE State.
6.8.2 Classic Metric Encoding
The handling of bandwidth and delay for Classic TLVs are encoded in
the packet "scaled" form relative to how they are represented on the
physical link.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Scaled Delay                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Scaled Bandwidth                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   MTU                         | Hop-Count     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reliability   |       Load    | Internal Tag  | Flags Field   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Scaled Delay - An administrative parameter assigned statically on a
per interface type basis to represent the time it takes along an
unloaded path. This is expressed in units of 10s of microseconds
divvied by 256. A delay of 0xFFFFFFFF indicates an unreachable route.

Scaled Bandwidth - The path bandwidth measured in bits per second. In
units of 2,560,000,000/kbps

MTU - The minimum maximum transmission unit size for the path to the
destination.

Hop Count - The number of router traversals to the destination.


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Reliability - The current error rate for the path. Measured as an
error percentage. A value of 255 indicates 100% reliability

Load - The load utilization of the path to the destination, measured
as a percentage. A value of 255 indicates 100% load.

Internal-Tag - A tag assigned by the network administrator that is
untouched by EIGRP. This allows a network administrator to filter
routes in other EIGRP border routers based on this value.

Flag Field - See Section 6.8.1


6.8.3 Classic Exterior Encoding
Additional routing information so provided for destinations outside of
the EIGRP autonomous system as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Router Identification (RID)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               External Autonomous System Number (AS)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Administrative Tag                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    External Protocol Metric                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Reserved            |Extern Protocol|  Flags Field  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router Identifier (RID) - A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.

External Autonomous System (AS) - 32-bit number indicating the
external autonomous system the sending router is a member of. If the
source protocol is EIGRP, this field will be the [VRID|AS] pair. If
the external protocol does not have an AS, other information can be
used (example Cisco uses process-id for OSPF).

Administrator Tag - A tag assigned by the network administrator that
is untouched by EIGRP. This allows a network administrator to filter
routes in other EIGRP border routers based on this value.

External Protocol Metric   32-bit value of the composite metric that
resides in the routing table as learned by the foreign protocol. If
the External Protocol is IGRP or another EIGRP routing process, the
value can optionally be the composite metric or 0, and the metric


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information is stored in the metric section.

External Protocol - Defines the external protocol that this route was
learned. See Section 6.2

Flag Field - See Section 6.8.1


6.8.4 Classic Destination Encoding
EIGRP carries destination in a compressed form, where the number of
bits significant in the variable length address field are indicated in
a counter:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subnet Mask   |    Destination Address (variable length)      |
| Bit Count     |         ((Bit Count - 1) / 8) + 1             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Subnet Mask Bit Count - 8-bit value used to indicate the number of
bits in the subnet mask. A value of 0 indicates the default network
and no address is present.

Destination Address - A variable length field used to carry the
destination address. The length is determined by the number of
consecutive bits in the destination address. The formula to calculate
the length is address-family dependent:

    IPV4: ((Bit Count - 1) / 8) + 1
    IPv6: (Bit Count == 128) ? 16 : ((x / 8) + 1)


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6.8.5 IPv4 Specific TLVs

       INTERNAL_TYPE       0x0102
       EXTERNAL_TYPE       0x0103
       COMMUNITY_TYPE      0x0104


6.8.5.1 IPv4 INTERNAL_TYPE
This TLV conveys IPv4 destination and associated metric information
for IPv4 networks. Routes advertised in this TLV are network
interfaces that EIGRP is configured on as well as networks that are
learned via other routers running EIGRP.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |       0x02    |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Next Hop Forwarding Address                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Vector Metric Section (See Section 6.8.2)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                       Destination Section                     |
|                 IPv4 Address (variable length)                |
|                       (See Section 6.8.4)                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Forwarding Address - IPv4 address is represented by 4 8-bit
values (total 4 octets). If the value is zero (0), the IPv4 address
from the received IPv4 header is used as the next-hop for the route.
Otherwise, the specified IPv4 address will be used.

Metric Section - vector metrics for destinations contained in this
TLV. See description of metric encoding in section 6.8.2

Destination Section - The network/subnet/host destination address
being requested. See description of destination in section 6.8.4




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6.8.5.2 IPv4 EXTERNAL_TYPE
This TLV conveys IPv4 destination and metric information for routes
learned by other routing protocols that EIGRP injects into the AS.
Available with this information is the identity of the routing
protocol that created the route, the external metric, the AS number,
an indicator if it should be marked as part of the EIGRP AS, and a
network administrator tag used for route filtering at EIGRP AS
boundaries.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |       0x03    |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Next Hop Forwarding Address                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Exterior Section (See Section6.8.3)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Vector Metric Section (See Section 6.8.2)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                       Destination Section                     |
|                 IPv4 Address (variable length)                |
|                       (See Section 6.8.4)                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Forwarding Address - IPv4 address is represented by 4 8-bit
values (total 4 octets). If the value is zero (0), the IPv4 address
from the received IPv4 header is used as the next-hop for the route.
Otherwise, the specified IPv4 address will be used

Exterior Section - Additional routing information provides for a
destination outside of the autonomous system and that has been
redistributed into the EIGRP. See Section 6.8.3

Metric Section - vector metrics for destinations contained in this
TLV. See description of metric encoding in Section 6.8.2

Destination Section - The network/subnet/host destination address
being requested. See description of destination in Section 6.8.4




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6.8.5.3 IPv4 COMMUNITY_TYPE
This TLV is used to provide community tags for specific IPv4
destinations.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |       0x04    |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          IPv4 Destination                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Reserved           |       Community Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Community List                        |
|                        (variable length)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination - The IPv4 address the community information should be
stored with.

Community Length - 2 octet unsigned number that indicates the length
of the Community List. The length does not include the IPv4 Address,
reserved, or Length fields

Community List - One or more 8 octet EIGRP community as defined in
section 6.4




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6.8.6 IPv6 Specific TLVs

       INTERNAL_TYPE                 0x0402
       EXTERNAL_TYPE                 0x0403
       COMMUNITY_TYPE                0x0404


6.8.6.1 IPv6 INTERNAL_TYPE
This TLV conveys IPv6 destination and associated metric information
for IPv6 networks. Routes advertised in this TLV are network
interfaces that EIGRP is configured on as well as networks that are
learned via other routers running EIGRP.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x04     |       0x02    |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Next Hop Forwarding Address                 |
|                            (16 octets)                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Vector Metric Section (See Section 6.8.2)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                       Destination Section                     |
|                 IPv6 Address (variable length)                |
|                       (See Section 6.8.4)                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Forwarding Address - IPv6 address is represented by 8 groups
of 16-bit values (total 16 octets). If the value is zero (0), the IPv6
address from the received IPv6 header is used as the next-hop for the
route.

Metric Section - vector metrics for destinations contained in this
TLV. See description of metric encoding in section 6.8.2

Destination Section - The network/subnet/host destination address
being requested. See description of destination in section 6.8.4


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6.8.6.2 IPv6 EXTERNAL_TYPE
This TLV conveys IPv6 destination and metric information for routes
learned by other routing protocols that EIGRP injects into the
topology. Available with this information is the identity of the
routing protocol that created the route, the external metric, the AS
number, an indicator if it should be marked as part of the EIGRP AS,
and a network administrator tag used for route filtering at EIGRP AS
boundaries.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x04     |        0x03   |           Length              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Next Hop Forwarding Address                 |
|                             (16 octets)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Exterior Section (See Section6.8.3)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Vector Metric Section (See Section 6.8.2)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                        Destination Section                    |
|                 IPv6 Address (variable length)                |
|                       (See Section 6.8.4)                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Forwarding Address - IPv6 address is represented by 8 groups
of 16-bit values (total 16 octets). If the value is zero (0), the IPv6
address from the received IPv6 header is used as the next-hop for the
route. Otherwise, the specified IPv6 address will be used.

Exterior Section - Additional routing information provided for a
destination outside of the autonomous system and that has been
redistributed into the EIGRP. See description of exterior encoding in
Section 6.8.3

Metric Section - vector metrics for destinations contained in this
TLV. See description of metric encoding in section 6.8.2

Destination Section - The network/subnet/host destination address
being requested. See description of destination in section 6.8.4




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6.8.6.3 IPv6 COMMUNITY_TYPE
This TLV is used to provide community tags for specific IPv4
destinations.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x04     |       0x04    |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Destination                        |
|                            (16 octets)                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Reserved           |       Community Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Community List                        |
|                        (variable length)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination - The IPv6 address the community information should be
stored with.

Community Length - 2 octet unsigned number that indicates the length
of the Community List. The length does not include the IPv6 Address,
Reserved or Length fields

Community List - One or more 8 octet EIGRP community as defined in
section 6.4




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6.9 Multi-Protocol Route Information TLV Types
This TLV conveys topology and associated metric information

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Version |    Opcode     |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              Flags                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgement number                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Virtual Router ID             |   Autonomous system number    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            TLV Header Encoding                |
|                           (See Section 6.9.1)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Wide Metric Encoding                |
|                           (See Section 6.9.2)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Destination Descriptor                |
|                             (variable length)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6.9.1 TLV Header Encoding
There has been a long-standing requirement for EIGRP to support
routing technologies such as multi-topologies and provide the ability
to carry destination information independent of the transport. To
accomplish this, a Vector has been extended to have a new "Header
Extension Header" section. This is a variable length field and, at a
minimum, will support the following fields:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high     | Type low      |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               AFI             |             TID               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Router Identifier (RID)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Value (variable length)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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The available fields are:

TYPE - Topology TLVs have the following TYPE codes:
    Type High: 0x06
    Type Low:
        REQUEST_TYPE                 0x01
        INTERNAL_TYPE                0x02
        EXTERNAL_TYPE                0x03

Router Identifier (RID) - A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.


6.9.2 Wide Metric Encoding
Multi-Protocol TLV's will provide an extendable section of metric
information, which is not used for the primary routing compilation.
Additional per path information is included to enable per-path cost
calculations in the future. Use of the per-path costing along with the
VID/TID will prove a complete solution for multidimensional routing.

 0                   1                     2                 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Offset     |   Priority    | Reliability   |        Load   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               MTU                             |   Hop-Count   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               Delay                           |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                             Bandwidth                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Reserved        |         Opaque Flags          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Attributes                      |
|                        (variable length)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields are:
Offset - Number of 16-bit words in the Extended Attribute section,
used to determine the start of the destination information. A value of
zero indicates no Extended Attributes are attached.

Priority - Priority of the prefix when processing route. In an AS
using priority values, a destination with a higher priority receives
preferential treatment and is serviced before a destination with a
lower priority. A value of zero indicates no priority is set.

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Reliability - The current error rate for the path. Measured as an
error percentage. A value of 255 indicates 100% reliability

Load - The load utilization of the path to the destination, measured
as a percentage. A value of 255 indicates 100% load.

MTU - The minimum maximum transmission unit size for the path to the
destination. Not used in metric calculation, but available to
underlying protocols

Hop Count - The number of router traversals to the destination.

Delay - The one-way latency along an unloaded path to the destination
expressed in units of picoseconds per kilobit. This number is not
scaled, a value of 0xFFFFFFFFFFFF indicates an unreachable route.

Bandwidth - The path bandwidth measured in kilobit per second as
presented by the interface. This number is not scaled, a value of
0xFFFFFFFFFFFF indicates an unreachable route.

Reserved - Transmitted as 0x0000

Opaque Flags - 16-bit protocol specific flags. Values currently
defined by Cisco are:
    OPAQUE_SRCWD    0x01   Route Source Withdraw
    OPAQUE_CD       0x02   Candidate default route
    OPAQUE_ACTIVE   0x04   Route is currently in active state
    OPAQUE_REPL     0x08   Route is replicated from another VRF

Extended Attributes - (Optional) When present, defines extendable per
destination attributes. This field is not normally transmitted.


6.9.3 Extended Metrics
Extended metrics allows for extensibility of the vector metrics in a
manner similar to RFC 6390 [11]. Each Extended metric shall consist of
a standard Type-Length header followed by application-specific
information. Extended metrics values not understood must be treated as
opaque and passed along with the associated route.

The general formats for the Extended Metric fields are:
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Opcode    |      Offset   |              Data             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Opcode - Indicates the type of Extended Metric

Offset - Number of 16-bit words in the sub-field. Offset does not


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include the length of the Opcode or offset fields)

Data - Zero or more octets of data as defined by Opcode


6.9.3.1 0x00 - NoOp
This is used to pad the attribute section to ensure 32-bit alignment
of the metric encoding section.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     0x00      |      0x00     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields are:
Opcode - Transmitted as zero (0)

Offset - Transmitted as zero (0) indicating no data is present

Data - No data is present with this attribute.


6.9.3.2 0x01 - Scaled Metric
If a route is received from a back-rev neighbor, and the route is
selected as the best path, the scaled metric received in the older
UPDATE, may be attached to the packet. If received, the value is for
informational purposes, and is not affected by K6

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x01    |       0x04    |          Reserved             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Scaled Bandwidth                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Scaled Delay                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved - Transmitted as 0x0000

Scaled Delay - An administrative parameter assigned statically on a
per interface type basis to represent the time it takes along an
unloaded path. This is expressed in units of 10s of microseconds
divvied by 256. A delay of 0xFFFFFFFF indicates an unreachable route.

Scaled Bandwidth - The minimum bandwidth along a path expressed in
units of 2,560,000,000/kbps. A bandwidth of 0xFFFFFFFF indicates an
unreachable route.


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6.9.3.3 0x02 - Administrator Tag
This is used to provide and administrative tags for specific topology
entries. It is not affected by K6

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x02    |       0x02    |       Administrator Tag       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Administrator Tag (cont)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Administrator Tag - A tag assigned by the network administrator that
is untouched by EIGRP. This allows a network administrator to filter
routes in other EIGRP border routers based on this value.


6.9.3.4 0x03 - Community List
This is used to provide communities for specific topology entries. It
is not affected by K6

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x03    |      Offset   |          Community List       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                          (variable length)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Offset - Number of 16-bit words in the sub-field. Currently
transmitted as 4

Community List - One or more community values as defined in section
6.4


6.9.3.5 0x04 - Jitter
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        0x04    |      0x03    |             Jitter            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Jitter - The measure of the variability over time of the latency
across a network measured in measured in microseconds.


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6.9.3.6 0x05 - Quiescent Energy
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        0x05    |        0x02  |        Q-Energy (high)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Q-Energy (low)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Q-Energy - Paths with higher idle (standby) energy usage will be
reflected in a higher aggregate metric than those having lower energy
usage. If present, this number will represent the idle power
consumption expressed in milliwatts per kilobit.


6.9.3.7 0x06 - Energy
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        0x06    |      0x02    |          Energy (high)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Energy (low)         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Energy - Paths with higher active energy usage will be reflected in a
higher aggregate metric than those having lower energy usage. If
present, this number will represent the power consumption expressed in
milliwatts per kilobit.




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6.9.3.8 0x07 - AddPath
The Add Path enables EIGRP to advertise multiple best paths to
adjacencies. There will be up to a maximum of 4 AddPath supported,
where the format of the field will be as follows;

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x07    |       Offset  |     AddPath (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Offset - Number of 16-bit words in the sub-field. Currently
transmitted as 4

AddPath - Length of this field will vary in length based on whether it
contains IPv4 or IPv6 data.

6.9.3.8.1 AddPath with IPv4 Next-hop

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x07    |       Offset  | Next-hop Address(Upper 2 byes)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (Lower 2 byes)   |       RID (Upper 2 byes)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        RID (Upper 2 byes)     | Admin Tag (Upper 2 byes)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Admin Tag (Upper 2 byes)      |Extern Protocol| Flags Field   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Address - IPv4 address is represented by 4 8-bit values
(total 4 octets). If the value is zero(0), the IPv6 address from the
received IPv4 header is used as the next-hop for the route. Otherwise,
the specified IPv4 address will be used.

Router Identifier (RID) - A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.

Admin Tag - A 32-bit administrative tag assigned by the network. This
allows a network administrator to filter routes based on this value.

If the route is of type external, then 2 addition bytes will be added
as follows:
External Protocol - Defines the external protocol that this route was
learned. See Section 6.2

Flag Field - See Section 6.8.1


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6.9.3.8.2 AddPath with IPv6 Next-hop
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       0x07     |       Offset |         Next-hop Address      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
|                            (16 octets)                        |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                               |       RID (Upper 2 byes)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        RID (Upper 2 byes)     | Admin Tag (Upper 2 byes)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Admin Tag (Upper 2 byes)      | Extern Protocol | Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Hop Address - IPv6 address is represented by 8 groups of 16-bit
values (total 16 octets). If the value is zero(0), the IPv6 address
from the received IPv6 header is used as the next-hop for the route.
Otherwise, the specified IPv6 address will be used.

Router Identifier (RID) - A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.

Admin Tag - A 32-bit administrative tag assigned by the network. This
allows a network administrator to filter routes based on this value.
If the route is of type external, then 2 addition bytes will be added
as follows:

External Protocol - Defines the external protocol that this route was
learned. See Section 6.2

Flag Field - See Section 6.8.1




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6.9.4 Exterior Encoding
Additional routing information so provided for destinations outside of
the EIGRP autonomous system as follows:
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Router Identification (RID)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Autonomous System Number (AS)               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     External Protocol Metric                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Reserved             |Extern Protocol| Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router Identifier (RID) - A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.

Autonomous System (AS) - 32-bit number indicating the external
autonomous system the sending router is a member of. If the source
protocol is EIGRP, this field will be the [VRID|AS] pair.

External Protocol Metric   32-bit value of the metric used by the
routing table as learned by the foreign protocol. If the External
Protocol is IGRP or EIGRP, the value can (optionally) be 0, and the
metric information is stored in the metric section.

External Protocol - Defines the external protocol that this route was
learned. See Section 6.2

Flag Field - See Section 6.8.1


6.9.5 Destination Encoding
Destination information is encoded in Multi-Protocol packets in the
same manner as used by Classic TLVs. This is accomplished by using a
counter to indicate how many significant bits are present in the
variable length address field
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subnet Mask   |    Destination Address (variable length       |
| Bit Count     |         ((Bit Count - 1) / 8) + 1             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Subnet Mask Bit Count - 8-bit value used to indicate the number of
bits in the subnet mask. A value of 0 indicates the default network
and no address is present.



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Destination Address - A variable length field used o carry the
destination address. The length is determined by the number of
consecutive bits in the destination address, rounded up to the nearest
octet boundary, determines the length of the address.


6.9.6 Route Information
6.9.6.1 INTERNAL TYPE
This TLV conveys destination information based on the IANA AFI defined
in the TLV Header (See Section 6.9.1), and associated metric
information. Routes advertised in this TLV are network interfaces that
EIGRP is configured on as well as networks that are learned via other
routers running EIGRP.


6.9.6.2 EXTERNAL TYPE
This TLV conveys destination information based on the IANA AFI defined
in the TLV Header (See Section 6.9.1), and metric information for
routes learned by other routing protocols that EIGRP injects into the
AS. Available with this information is the identity of the routing
protocol that created the route, the external metric, the AS number,
an indicator if it should be marked as part of the EIGRP AS, and a
network administrator tag used for route filtering at EIGRP AS
boundaries.




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7 Security Considerations
By the nature of being promiscuous, EIGRP will neighbor with any
router that sends a valid HELLO packet. Due to security
considerations, this "completely" open aspect requires policy
capabilities to limit peering to valid routers.

EIGRP does not rely on a PKI or a heavy weight authentication system.
These systems challenge the scalability of EIGRP, which was a primary
design goal.

Instead, Denial of Service (DoS) attack prevention will depend on
implementations rate-limiting packets to the control plane as well as
authentication of the neighbor through the use of MD5 or SHA2-256 [6].

8 IANA Considerations

This document serves as the sole reference for two multicast
addresses; IGRP Routers [13], EIGRP Routers [14] and assignment for
protocol number 88 (EIGRP) [15].




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9 References

9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, April 1997.
[2] J.J. Garcia-Luna-Aceves, "A Unified Approach to Loop-Free Routing
using Distance Vectors or Link States", 1989 ACM 089791-332-
9/89/0009/0212, pages 212-223.
[3] J.J. Garcia-Luna-Aceves, "Loop-Free Routing using Diffusing
Computations", Network Information Systems Center, SRI International
to appear in IEEE/ACM Transactions on Networking, Vol. 1, No. 1, 1993.
[4] Rosen, E., "IANA Registries for BGP Extended Communities", RFC
7153, March 2014.

[5] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", RFC 3692, January 2004
[6] Kelly, S., Frankel, S., "Using HMAC-SHA-256, HMAC-SHA-384, and
HMAC-SHA-512 with IPsec", RFC 4868, May 2007.
[7] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
August 1989
[8]   "DARPA Internet Protocol Specification", RFC 791, Sept 1981
[9] [7] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998


9.2 Informative References
[10] Moy, J., "OSPF Version 2" RFC 2328, 1998
[11] Clark, A., Claise, B., "Guidelines for Considering New
Performance Metric Development", RFC 6390, October 2011
[12] Address Family Numbers, http://www.iana.org/assignments/address-
family-numbers/address-family-numbers.xhtml
[13] IPv4 Multicast Address Space Registry,
http://www.iana.org/assignments/multicast-addresses
[14] IPv6 Multicast Address Space Registry,
http://www.iana.org/assignments/ipv6-multicast-addresses
[15] Protocol Numbers, http://www.iana.org/assignments/protocol-
numbers



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10 Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.

An initial thank you goes to Dino Farinacci, Bob Albrightson, and Dave
Katz. Their significant accomplishments towards the design and
development of the EIGRP protocol provided the bases for this
document.

A special and appreciative thank you goes to the core group of Cisco
engineers whose dedication, long hours, and hard work lead the
evolution of EIGRP over the following decade. They are Donnie Savage,
Mickel Ravizza, Heidi Ou, Dawn Li, Thuan Tran, Catherine Tran, Don
Slice, Claude Cartee, Donald Sharp, Steven Moore, Richard Wellum, Ray
Romney, Jim Mollmann, Dennis Wind, Chris Van Heuveln, Gerald Redwine,
Glen Matthews, Michael Wiebe, and others.

The authors would like to gratefully acknowledge many people who have
contributed to the discussions that lead to the making of this
proposal. They include Chris Le, Saul Adler, Scott Van de Houten,
Lalit Kumar, Yi Yang, Kumar Reddy, David Lapier, Scott Kirby, David
Prall, Jason Frazier, Eric Voit, Dana Blair, Jim Guichard, and Alvaro
Retana.

In addition to the tireless work provided by the Cisco engineers over
the years, I would like to personally recognize the teams what created
Open Source versions of EIGRP;
    o Linux implementation developed by the Quagga team: Jan Janovic,
      Matej Perina, Peter Orsag, and Peter Paluch
    o BSD implementation developed, and release by, Renato Westphal




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Author's Address

Donnie V Savage
Cisco Systems, Inc
7025 Kit Creed Rd, RTP, NC

Phone: 919-392-2379
Email: dsavage@cisco.com

Donald Slice
Cumulus Networks
Apex, NC

Phone:
Email: dslice@cumulusnetworks.com

James Ng
Cisco Systems, Inc
7025 Kit Creed Rd, RTP, NC

Phone: 919-392-2582
Email: jamng@cisco.com

Peter Paluch
University of Zilina
Univerzitna 8215/1, Zilina 01026, Slovakia

Phone: 421-905-164432
Email: Peter.Paluch@fri.uniza.sk

Steven Moore
Cisco Systems, Inc
7025 Kit Creed Rd, RTP, NC

Phone: 919-392-2674
Email: smoore@cisco.com

Russ White
LinkedIn
Apex, NC

Phone: 1-877-308-0993
Email: russw@riw.us




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