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Versions: 00 01 02
Network Working Group A. D. Zinin
Internet Draft Cisco Systems
Expiration Date: January 2001 July 2000
File name: draft-ietf-ospf-shortcut-abr-02.txt
OSPF Shortcut ABR
Enhanced OSPF ABR Behavior
draft-ietf-ospf-shortcut-abr-02.txt
Status of this Memo
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Abstract
OSPF [Ref1] is a link-state intra-domain routing protocol used for
routing in IP networks. Though the definition of the ABR in the
current OSPF specification does not require a router with multiple
attached areas to have a backbone connection, it is actually
necessary to provide successful routing to the inter-area and
external destinations. If this requirement is not met, all traffic,
destined for the areas not connected to such an ABR or out of the
OSPF domain, is dropped. The rules of originating and processing
Summary-LSAs given in the current OSPF standard [Ref1] can also
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result in suboptimal inter-area routing. Though all these problems
can be fixed using virtual links, this memo describes an alternative
implementation of the OSPF ABR behavior, which allows the
administrator to avoid this or, if virtual links are still used, to
decrease the number of configured virtual links.
This memo also describes possible situations where the proposed
implementation can be used.
Acknowledgements
The author would like to acknowledge Christian Hopps and Peter Psenak
for their help in finding weak points in early versions of this
document, Thomas M. Thomas for reviewing the preceding version of it,
and James Huang for pointing out potential problems described in
section 7.
Special thanks go to John Moy who contributed a lot to this document
and provided a simpler algorithm representation, used herein.
Table of Contents
1 Overview ..................................................... 3
1.1 Introduction ............................................... 3
1.2 Motivation ................................................. 3
2 Description of Shortcut ABR behavior ......................... 5
3 Proposed changes to OSPF ABR behavior ........................ 6
3.1 Changes to Area Data Structure ............................. 6
3.2 Changes to Router-LSA Origination .......................... 8
3.3 Changes to Routing Table Calculation ....................... 8
3.4 Changes to Summary-LSA Origination ......................... 10
4 Implementation Details ....................................... 12
5 Compatibility ................................................ 12
6 Deployment Considerations .................................... 12
6.1 Necessity of Virtual Links ................................. 12
6.2 Change of Traffic Patterns ................................. 13
6.3 Optimized Inter-area Routing ............................... 13
6.4 Gradual Deployment of Shortcut ABRs ........................ 14
7 Routing Loops in Transition Periods .......................... 15
8 Security Considerations ...................................... 17
9 Appendixes ................................................... 17
A.1 Router-LSA ................................................. 17
10 References .................................................. 19
11 Author's Address ............................................ 19
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1 Overview
1.1 Introduction
An OSPF routing domain can be split into several subdomains, called
areas, which limit the scope of LSA flooding. A router having attach-
ments to multiple areas is called an "area border router" (ABR). The
primary function of an ABR is to provide its attached areas with
Type-3 and Type-4 LSAs (which are used for describing routes and
ASBRs in other areas) as well as to perform actual inter-area rout-
ing.
1.2 Motivation
In OSPF, flooding of Type-1 and Type-2 LSAs is limited to the area
borders, so routers in other areas must somehow know how to reach
destinations and ASBRs residing in different areas. OSPF uses
Distance-Vector (DV) approach to achieve this goal, i.e., Area Border
Routers announce networks and ASBRs internal to directly connected
areas in Type-3 and Type-4 Summary-LSAs.
If routers using a DV protocol announce only directly attached net-
works, they must be fully meshed to provide complete routing informa-
tion to each other. This condition cannot always be met, so routers
also announce the networks they heard about from their neighbors.
This is the main reason for loops of routing updates in DV protocols,
which are solved using methods like split-horizon, limitting the max-
imum metric value or hop count, and hold-down timers. Application of
these rules to OSPF inter-area routing would make the code very com-
plex, but since areas in OSPF need not be fully meshed, ABRs are
allowed to reannounce inter-area routes. In order to prevent loops of
summaries in OSPF, ABRs reannounce only those inter-area routes which
are associated with the backbone area. Summaries from non-backbone
areas are just not considered by ABRs. Because inter-area routes are
not reannounced back into the backbone area, the latter functions as
a loop-free inter-area routing information repository. In order to
achieve normal routing to inter-area and AS-external destinations,
all areas in OSPF should be connected to the backbone either physi-
cally (via an interface) or logically (via a virtual link). This is
to ensure that all areas are provided with inter-area routes from the
backbone.
A basic discussion of the disadvantages of the standard inter-area
approach are given in [Ref2] and are applicable to this document as
well. In addition to that, consider another problem caused by stan-
dard OSPF ABR behavior (Figure 1).
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. Area 2 .
. .
. +--+ .
......|R5|.......
. +--+ .
. / \ .
. / \ .
. BB /10Mb \ 2Mb .
. / \ .
. | \ .
. +--+ +--+ .
..|R1|......|R2|..
. +--+ +--+ .
. | 10Mb |\ .
. ------------ | .
. | .
. +--+ .
. |R4| .
. Area 1 +--+ .
..................
Figure 1. Suboptimal inter-area routing
In this example router R2 has a 2Mb link to R5. At the same time R1
has a better link (10 Mbps), but R2 cannot route traffic going to
area 2 through R1. This is because according to [Ref1] R2 is not
allowed to consider summary-LSAs from non-backbone areas and, conse-
quently, does not have routes covering destinations in area 2 via R1.
The situation looks even more interesting if R4's routing table is
considered. Since R2 floods summary-LSAs from R1 to R4, router R4
will have routes to the area 2 via R1 (the best path), expecting
traffic to go via 10Mbps links. In reality, R2 will not direct
traffic to R1, but will forward it via 2Mbps link attached to itself.
The last example shows how the main principle of OSPF---prefer the
shortest path---is broken due to distance vector approach used for
inter-area routing. Again, the problem can be fixed using the virtual
links between R1 and R2 in standard OSPF, but the solution proposed
in this document appears to be more elegant and involving less admin-
istrative and traffic overhead. More sophisticated examples of how
Shortcut ABR approach improves inter-area routing are given in sec-
tion 6.
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2 Description of Shortcut ABR behavior
This section describes an alternative implementation of OSPF ABR
behavior, named "Shortcut ABR". It is an improvement on standard ABR
behavior, based on relaxation of the restrictions applied to the cal-
culation of the inter-area routes.
With the Shortcut ABR approach, ABRs are allowed to consider
summary-LSAs from all (or a subset of) attached areas by performing a
modified version of section 16.3 of [Ref1]. This gives Shortcut ABRs
a chance to install inter-area routes through non-backbone areas (if
non-backbone paths are really better), i.e. to "shortcut" through
them. The routing loop prevention is ensured by restricting the ori-
gination of summary-LSAs---inter-area routes are readvertised only if
they are associated with the backbone area (there is a valid LSA for
the destination learned from the backbone). Origination of summary-
LSAs for intra-area routes is done as in standard OSPF [Ref1].
There is a probability of forming constant routing loops if
Shortcut-capable and standard ABRs are present in an OSPF domain and
can see each other through the backbone. Also, if transit or shortcut
areas form a circle, it is possible (though the probability is really
low) to have temporary routing loops as described in section 7. To
prevent these types of loops, two new variables (ShortcutConfigured
and ShortcutCapability) are introduced to the OSPF area data struc-
ture for non-backbone areas, and a new bit (S-bit) is announced in
the router-LSAs by the ABRs.
If the ABR doesn't have the backbone area connected, it considers
summary-LSAs from all attached areas. This is safe, because no
inter-area routes are associated with the backbone and get readver-
tised. The relaxation of the routing table calculation allows ABRs
without a backbone connection to route traffic between the attached
areas, as well as to route traffic destined for the backbone and
other areas using the routes derived from the summary-LSAs in each
attached area. This approach also enables router R2 in Figure 1 to
route inter-area traffic via R1.
Note that the proposed solution does not obviate the need of virtual
link configuration in case an ABR has no physical backbone connection
at all, but at the same time should reannounce inter-area routes
(intra-area routes are always announced to other areas). However,
this approach requires only a single backbone link per ABR or no
backbone link at all (if the ABR does not have to reannounce inter-
area routes and just needs to find the best routes through attached
areas itself).
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3 Proposed changes to OSPF ABR behavior
This section describes the changes made to the base OSPF described in
[Ref1].
3.1 Changes to Area Data Structure
Two new flags are introduced to OSPF area data structure---
ShortcutConfigured and ShortcutCapability.
The ShortcutConfigured flag can be assigned three values: Default,
Enable, and Disable. The flag is set to Default value when an area
data structure is created. Description of the flag values is given
below.
Default
If area A's ShortcutConfigured flag is set to Default, and
the ABR has an active backbone connection, area A is not
used for shortcutting and the ABR does not set the S-bit in
the router-LSA originated for that area. If the ABR has no
backbone connection, area A is always used for shortcutting
and the ABR sets the S-bit in the router-LSA for that area.
Enable
If area A's ShortcutConfigured flag is set to Enable, and
the ABR has an active backbone connection, it sets the S-
bit in the router-LSA for area A and uses it for shortcut-
ting, provided that all other ABRs seen through this area
also report the S-bit. If the ABR has no backbone connec-
tion, it unconditionally uses area A for shortcutting and
sets the S-bit in the router-LSA originated for that area.
Disable
If an area's ShortcutConfigured flag is set to Disable, the
ABR doesn't use this area for shortcutting and doesn't set
the S-bit in the router-LSA originated for it.
Treamtment of the ShortcutConfigured flag described above ensures
that Shortcut ABRs operate correctly and efficiently without
explicit configuration. For example, when a Shortcut ABR is
attached to non-backbone areas only, the Default value will allow
it to shortcut through these areas. When a Shortcut ABR is con-
nected to the backbone, it doesn't shortcut through non-backbone
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areas until it is explicitly configured to do so by setting the
ShortcutConfigured flag for specific (or all) areas to Enable
value and all other ABRs announce the S-bit (either because they
are not connected to the backbone, or because they were also con-
figured to shortcut through that area). Again, this behavior
ensures that no routing loop is established between a shortcut-
ting and not shortcutting ABR, as well as that shortcut areas do
not form a circle.
In addition to ShortcutConfigured, Shortcut ABRs maintain
ShortcutCapability flag in Area Data Structure for every non-
backbone area. These two flags are used to prevent permanent
routing loops in the networks where Shortcut-incapable ABRs are
used along with Shortcut ABRs.
While ShortcutConfigured flag indicates what the administrator
has configured for a particular area, ShortcutCapability indi-
cates that the area may actually be used for shortcutting either
because all other ABRs in the area agree on this using the S-bit
or because the calculating ABR does not have a backbone connec-
tion and was not explicitly configured not to do so.
Note that backbone-connected Shortcut ABRs are allowed to con-
sider summary-LSAs from a non-backbone area only if that area's
ShortcutCapability flag is set to TRUE. An area's ShortcutCapa-
bility flag, in turn, is set to TRUE when the ABR does not have a
backbone attachment and area's ShortcutConfigured is not set to
Disables, or when the ABR has a backbone connection, area's
ShortcutConfigured is set to Enable, and all other ABRs connected
to the area set their S bits in their router-LSAs. This means
that the calculating ABR and all other ABRs connected to that
area should be allowed to consider that area's summary-LSAs.
If, during the routing table calculation, a Shortcut ABR notices
that there is an ABR which does not announce the S-bit in any
area, the Shortcut ABR will probably need to clear the Shortcut-
Capability flag for that area (depending on whether it has a
backbone connection and the value of ShortcutConfigured flag).
Should the ABR in question find that all ABRs in an area agree on
the S-bit it may need to set the ShortcutCapability flag for that
area.
Note that announcement of S-bit does not depend on the results of
routing table calculation, but only on the setting of Shortcut-
Configured and backbone attachment.
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3.2 Changes to Router-LSA Origination
The algorithm of Type 1 LSA (router-LSA) origination is changed
to have the Shortcut ABR announce its Shortcut capability in the
Router-LSA as described in A.1. A Shortcut ABR should set the S-
bit in the Router-LSA for Area A only if:
o the router does not have a backbone connection and
ShortcutConfigured flag for this area is NOT set to Dis-
able value, or
o the router has a backbone connection and area A's
ShortcutConfigured flag is set to Enable.
As in [Ref1] Shortcut ABRs identify themselves as ABRs by setting
the bit B in their Router-LSAs when they have more than one
attached area.
3.3 Changes to Routing Table Calculation
In order to maintain correct state of the ShortcutCapability
flag, steps 1 and 2 in section 16.1 of [Ref1] are changed as fol-
lows:
Step 1:
"Initialize the algorithm's data structures. Clear the
list of candidate vertices. Initialize the shortest-path
tree to only the root (which is the router doing the calcu-
lation). Set Area A's TransitCapability to FALSE.
ShortcutCapability flag is set as follow.
o If the router is not connected to the backbone and
Area A's ShortcutConfigured flag is NOT set to Dis-
able, or the router is connected to the backbone and
Area A's ShortcutConfigured flag is set to Enable,
set ShortcutCapability flag to TRUE.
o Otherwise, set Area A's ShortcutCapability flag to
FALSE."
Step 2:
"Call the vertex just added to the tree vertex V. Examine
the LSA associated with vertex V. This is a lookup in the
Area A's link state database based on the Vertex ID. If
this is a router-LSA, and bit V of the router-LSA (see Sec-
tion A.4.2) is set, set Area A's TransitCapability to TRUE.
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If this is a router-LSA, and bit B of the router-LSA is set
(the router is an ABR), and bit S of the router-LSA is not
set (the ABR is either not Shortcut-capable or has a back-
bone connection and is not configured to use Area A for
shortcutting), and the ABR has a backbone connection, set
Area A's ShortcutCapability to FALSE. In any case, each
link described by the LSA gives the cost to an adjacent
vertex. For each described link, (say it joins vertex V to
vertex W):"
Note that the above algorithm, ensures that it is enough to check
only ShortcutCapability flag while deciding whether summary-LSAs
of a particular area should be considered or not.
The algorithm of calculating inter-area routes is changed as fol-
lows.
ABRs consider summary-LSAs only from those attached non-backbone
areas that have ShortcutCapability flag set to TRUE. This is
achieved by applying section 16.3 of [Ref1] to such areas. The
following changes to 16.3 are made.
Paragraph 1 of 16.3 is changed to be as follows:
"This step is only performed by area border routers attached
to one or more non-backbone areas that are either capable of
carrying transit traffic (i.e., "transit areas", or those
areas whose TransitCapability parameter has been set to TRUE
in Step 2 of the Dijkstra algorithm (see Section 16.1) or can
be used for shortcutting (those areas whose ShortcutCapability
parameter has NOT been set to FALSE during the Dijkstra algo-
rithm otherwise)."
Paragraph 4 of 16.3 is changed to be as follows:
"The calculation proceeds as follows. All summary-LSAs of the
areas with TransitCapability or ShortcutCapability parameter
set to TRUE are examined in turn. Each such summary-LSA
describes a route through a non-backbone area Area A to a Net-
work N (N's address is obtained by masking the LSA's Link
State ID with the network/subnet mask contained in the body of
the LSA) or in the case of a Type 4 summary-LSA, to an AS
boundary router N. Suppose also that the summary-LSA was ori-
ginated by an area border router BR."
Step (3) of the algorithm in 16.3 is changed to be as follows:
"Look up the routing table entry for N. (If N is an AS
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boundary router, look up the "router" routing table entry
associated with the backbone area). If the route type is other
than backbone intra-area or inter-area (associated with any
area) then examine the next LSA.
In other words, this calculation updates backbone intra-area
routes found in Section 16.1, inter-area routes found in Sec-
tion 16.2 and installs new inter-area routes if the ABR does
not have a backbone connection."
Step (5) of the algorithm in 16.3 is changed to be as follows:
"If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC is
the same as N's current cost, add BR's list of next hops to
N's list of next hops. If the area associated with N's routing
table entry is the backbone, then the area and the type of the
path (either intra-area or inter-area) should remain
unchanged. Otherwise (the routing table entry does not exist
or the associated area is not the backbone), the type of the
route should be set to inter-area and associated area should
be set to the area associated with the summary-LSA being pro-
cessed."
In order to prevent routing loops, section 16.2 of [Ref1] is
changed. Step (3) of section 16.2 is changed to instruct the ABRs
to ignore summary defaults received from stub areas:
"If it is a Type 3 summary-LSA, and the collection of destina-
tions described by the summary-LSA equals one of the router's
configured area address ranges (see Section 3.5), and the par-
ticular area address range is active, then the summary-LSA
should be ignored. "Active" means that there are one or more
reachable (by intra-area paths) networks contained in the area
range. The summary-LSA should also be ignored if it is a sum-
mary default (Destination ID = DefaultDestination, Address
Mask = 0x00000000) and the area it has been received from is
a stub area. This is to prevent possible routing loops."
It is also reemphasized that routers are supposed to install dis-
card routing entries for active area ranges per 11.1 of [Ref1]
3.4 Changes to Summary-LSA Origination
The algorithm of the summary-LSAs origination is changed to
include an explicit restriction not to originate summary-LSAs for
inter-area routes if the route to the destination is not
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associated with the backbone.
Note that if there are multiple alternative paths to a destina-
tion, some of which are via the backbone and the rest are via
non-backbone areas, the area associated with the corresponding
routing table entry will remain the backbone area, but the set of
next hops will actually direct traffic along the best path even
through non-backbone areas.
If the ABR in question has no backbone connection, it will not
originate summary-LSA for any inter-area route in any area,
because the area associated with the routing table entry will
never be the backbone area.
The ABR will also not readvertise an inter-area route from non-
backbone area if its backbone link state database does not con-
tain a summary-LSA, router-LSA, or network-LSA covering a
specific destination.
In order to implement described policy, the paragraph 2 in sec-
tion 12.4.3 of [Ref1] should be read as follows:
"... Note that only intra-area routes are advertised into the
backbone, while both intra-area and inter-area routes are
advertised into the other areas. Also, summary-LSAs for
inter-area routes are originated if and only if these routes
are associated with the backbone area (to prevent loops of
summary-LSAs)."
The 6th step of the algorithm given in sections 12.4.3 of [Ref1]
should be interpreted as shown below:
"Else, if the destination of this route is an AS boundary
router, a summary-LSA should be originated if and only if the
routing table entry describes the preferred path to the AS
boundary router (see Step 3 of Section 16.4) and it is associ-
ated with the backbone area. If so, a Type 4 summary-LSA is
originated for the destination, with Link State ID equal to
the AS boundary router's Router ID and metric equal to the
routing table entry's cost. Note: these LSAs should not be
generated if Area A has been configured as a stub area."
The 7th step of the algorithm given in sections 12.4.3 of [Ref1]
should be interpreted as shown below:
"Else, the Destination type is network. If this is an inter-
area route and it is associated with the backbone area, gen-
erate a Type 3 summary-LSA for the destination, with Link
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State ID equal to the network's address (if necessary, the
Link State ID can also have one or more of the network's host
bits set; see Appendix E for details) and metric equal to the
routing table cost."
Described changes in the ABR behavior allow selection of most optimal
paths to inter-area and external destinations. Note that backbone
intra-area routes can be updated with better non-backbone inter-area
one, thus directing internal backbone traffic along more optimal
paths through other areas.
4 Implementation Details
If the current implementation of OSPF uses the standard described in
[Ref1], then support of the proposed Shortcut ABR behavior strategy
should be implemented as configurable options, allowing to change the
ABR behavior and set the ShortcutConfigured flag for a given area.
Note that the nature of the changes to OSPF presented in this docu-
ment is so that standard ABR behavior is not altered until at least
one area is used for shortcutting.
5 Compatibility
ABRs following the approach described in this document are required
to announce their Shortcut capability for a given area in Router-
LSAs. Since backbone-attached Shortcut ABRs do not consider Summary-
LSAs from an area until all ABRs agree on the S-bit, and ABRs not
attached to the backbone do not readvertise the inter-area routes,
the approach described in this document is compatible with standard
OSPF described in [Ref1].
6 Deployment Considerations
This section discusses the deployment details of Shortcut ABR.
6.1 Necessity of Virtual Links
It should be repeated that Shortcut ABR behavior does not obviate the
need for virtual links in case an ABR has no physical backbone con-
nection. The difference with standard OSPF is that the administrator
does not need to configure virtual links through all areas he or she
wants the inter-area traffic to go through. Shortcut ABR needs a
single backbone connection (physical or virtual) to be able to rean-
nounce optimal inter-area routes to other areas. Note that it is not
necessary for a Shortcut ABR itself to have a backbone connection in
order to find the best inter-area paths, since it considers summary-
LSAs from all attached areas if the backbone is not configured.
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6.2 Change of Traffic Patterns
Use of Shortcut ABR can lead to changes in the paths inter-area
traffic flows take comparing to those experienced with standard OSPF.
This happens because the Shortcut ABR approach allows a router to
find paths better than it is possible with the standard OSPF. While
standard OSPF tries to forward all inter-area traffic through the
backbone area (though it is not guaranteed), the Shortcut ABR finds
best routes in the domain even across non-backbone areas. With
Shortcut ABR the backbone area is used as a dedicated place of
inter-area routing information exchange and inter-area traffic is
allowed to cross non-backbone area borders if such a path is really
the best.
6.3 Optimized Inter-area Routing
Use of Shortcut ABR improves inter-area routing in OSPF domains by
allowing ABRs to consider summary-LSAs from all attached area and
consequently readvertise them into non-backbone areas. Consider an
example show in the Figure 2:
.......................
. Backbone . .........
. . . .
. +--+ +--+ .
. |R1| |R5|--| .
. +--+ +--+ 1| .
. 8/ 8\ 1/ . | .
. / \ -------- . | .
. 8/ 8\ /1 1\ . | Net N .
.+--+ +--+ +--+ 1| .
......|R2|.....|R3|.......|R4|--| .
. +--+ +--+ +--+ .
. .|1 1/ \1 1/ . . Area 3 .
. .-------- -------- . ..........
. . .
. . .
.Area 1 . Area 2 .
....... ...................
Figure 2. Optimized inter-area routing
In case all ABRs use standard OSPF approach, routing to the net N
would be as follows:
o R4 and R5 inject summary-LSAs into the backbone
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o R4 also injects a summary-LSA into area 2
o R3 is limited to consider summary-LSAs from the backbone
only, so it doesn't see the alternative path through area 2
and always routes through the backbone (though parallel
paths are available)
o R3 injects summary-LSA for the inter-area routes derived
from the backbone summary-LSAs and received from R4 and R5
into Area 2
o R2 is not allowed to consider non-backbone summary-LSAs and
routes via serial links to R1, though more optimal paths do
exist
If R2, R3, and R4 use Shortcut ABR approach inter-area routing is
improved as shown below:
o R4 and R5 inject summary-LSAs into the backbone
o R4 also injects a summary-LSA into area 2
o R3 considers summary-LSAs from both attached areas and
installs the route through area 2 (it has three routes in
the routing table---via R5, via R4 through the backbone, and
via R4 through area 2) and performs traffic sharing between
the two ethernet links.
o R3 injects summary-LSA for the inter-area routes to N (it
will be the same as in the previous case, actually)
o R2 considers summary-LSAs from all attached areas and
prefers the route through area 2 rather than the backbone.
6.4 Gradual Deployment of Shortcut ABRs
Shortcut ABR behavior is designed in such a way that the administra-
tor can enable shortcutting through non-backbone OSPF areas gradu-
ally.
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Since Shortcut ABRs are allowed to consider summaries only of those
areas that were configured as Shortcut (ShortcutConfigured flag in
area data structure is set to TRUE) and whose ShortcutCapability flag
is set to TRUE, it is easy to control which areas will accept addi-
tional inter-area traffic. For an area to become Shortcut-capable,
all ABRs that have links in it must agree on their configuration.. If
a single ABR in an area does not announce the S-bit in its Router-LSA
for this area, no other Shortcut ABRs connected to this area will
direct inter-area traffic through it (except for the cases when stan-
dard OSPF behavior leads to it).
The implementers should note that support of the configurable option
described in section 4 is very important for traffic control and suc-
cessful deployment.
7 Routing Loops in Transition Periods
As it was noted before standard OSPF ABR behavior uses DV approach to
distribute routing information among the areas. While the basic tech-
nique used in OSPF for this purpose provides loop-free environment,
existence of circular virtual link topology may lead to temporary
routing loops basically because of the section 16.3 that can update
backbone routes with non-backbone inter-area ones. The routing loops
formed in such situations are similar to those experienced in DV
routing protocols when the originator of a route loses its connec-
tivity to the network, sends messages withdrawing the route to all
neighbors, but not all messages manage to get through and the origi-
nator receives a false update from an upstream neighbor. An example
of such a temporary loop is illustrated in Figure 3.
In this example routers R1, R2, R3, and R4 are ABRs. All of them are
connected to the backbone ring that constitutes the backbone area.
Note that the link from R4 to the backbone ring (marked with aster-
isks) does not belong to area 2, but to the backbone area. The cost
of the backbone intra-area route between any given two ABRs is 10,
since they are all connected via a broadcast segment. The cost of
non-backbone intra-area path from R1 to R2, from R2 to R3, and from
R3 to R1 is 1. In this example, it doesn't matter what the cost
between R3 and R4 is. We are interested in network N residing in area
3. Both ABRs (R3 and R4) can reach this network. Suppose R3 can
reach it via a path with cost 1, while R4 via a path with cost 100.
Both routers announce summary-LSAs for network N into the backbone
area. If all ABRs follow the standard ABR behavior, and there are no
virtual links in the domain, R1 and R2 will install inter-area routes
to network N through the backbone area. If virtual links are esta-
blished between R1 and R2, R2 and R3, and R3 and R1, then routers R1
and R2 will choose more optimal paths through areas 4 and 2
correspondingly, according to the algorithm described in 16.3 of
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INTERNET DRAFT Shortcut ABR July 2000
[Ref1]. Also, R1 and R2 will announce summary-LSAs with cost 2 into
area 1, since inter-area routes to network N in their routing tables
are still backbone-associated. The same happens when R1, R2, and R3
are Shortcut-capable ABRs and agree to use areas 2 and 4 for
shortcutting.
.............. ................
. . . .
. Area 1 +----+ Area 2 .
. ______| |________ .
. / 1| R2 |1 \ .
. / +----+ \ . 10
. / . 10| . ***************
. 1/ . | . * \1 . *--+
+----+..... __|_ *......+----+....|R4|.....
| |10 / \ * 10| | +--+ .
| R1 |------* *--------| R3 | |100 .
+----+ \____/ +----+ 1 | .
. 1| . Backbone . |1.. \ | .
. | . . / . . \ | .
. | ................... / . . ----------- .
. | / . . Network N .
. \_______________________/ . . .
. . . Area 3 .
. Area 4 . ............
...............................
Figure 3. Sample topology with temporary routing loop
Now assume R3 loses its connectivity to area 3. After the routing
table is recalculated, R3 has an inter-area route to network N
through the backbone area via R4 with cost 110. R3 withdraws its
summary-LSA covering network N advertised into the backbone by flood-
ing corresponding MaxAge LSA (premature aging, as described in 14.1
of [Ref1]) and updates areas 2 and 4 with a new version of the
summary-LSA, containing the cost of 110. Now assume that R2 success-
fully receives and installs the new LSA, while R1 does not (due to
packet drops or other potential problems). R2 recalculates the rout-
ing table and installs the inter-area route via R1, because R1 did
not withdraw its summary-LSA with cost 2. After the new route is
installed into R2's routing table, R2 originates a summary-LSA for
network N with cost 3 into area 2. R3 uses this LSA to calculate the
route to N via R2 and announces a new summary-LSA with cost 4 into
area 4. This LSA is used by R1. A loop is formed. Note that R1, R2,
and R3 will not use the backbone path, because it will always be
updated by a non-backbone path with smaller metric. Described looping
stops when the cost of non-backbone inter-area path to network N
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INTERNET DRAFT Shortcut ABR July 2000
becomes greater than the cost of backbone-associated path, that is
greater than 110.
The loop described above is not Shortcut ABR-specific, i.e., it can
be seen even with standard ABR behavior when virtual links create a
circle. However, this document encourages administrators to configure
areas as shortcut and thus potentially increases the probability of
such a temporary loop. The author would like to emphasize that 1)
such routing loops are hardly to be seen in real networks with proper
domain design, and 2) even if such a loop forms, it is temporary, the
network finally converges and proper routes are installed. There
exist several methods to minimize the probability of described rout-
ing loop formation and to provide faster convergence in case a loop
has formed, but specification of these methods is outside the scope
of this document and will be delayed until the problem becomes
apparent.
8 Security Considerations
Shortcut ABR behavior specified in this document does not raise any
security issues that are not already covered in [Ref1].
9 Appendixes
A.1 Router-LSA
An OSPF router originates a router-LSA into each of its attached
areas. The router-LSA describes the state and cost of the router's
interfaces to the area. The contents of the router-LSA are described
in detail in Section A.4.2 of [Ref1]. One more flag has been added
to the router-LSA, called bit S below. This flag indicates whether
the area has been configured as Shortcut on the ABR. See Sections 2
and 3 for more details.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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INTERNET DRAFT Shortcut ABR July 2000
| rtype | 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Link ID | P
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E
| Link Data | R
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | TOS 0 metric | #
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
# | TOS | 0 | metric | I
T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
O | ... | K
S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
| | TOS | 0 | metric | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
The router LSA
+---+---+---+---+---+---+---+---+
| * | * | S | Nt| W | V | E | B |
+---+---+---+---+---+---+---+-+-+
The rtype field
The following defines the flags found in the rtype field. Each flag
classifies the router by function:
o bit B. When set, the router is an area border router (B is for
border). These routers forward unicast data traffic between OSPF
areas.
o bit E. When set, the router is an AS boundary router (E is for
external). These routers forward unicast data traffic between Auto-
nomous Systems.
o bit V. When set, the router is an endpoint of an active virtual
link (V is for virtual) which uses the described area as its Tran-
sit area.
o bit W. Used in MOSPF, when set, the router is a wild-card multicast
receiver. These routers receive all multicast datagrams, regardless
of destination. Inter-area multicast forwarders and inter-AS mul-
ticast forwarders are sometimes wild-card multicast receivers.
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INTERNET DRAFT Shortcut ABR July 2000
o bit Nt. Used in NSSA [Ref3], when set, the router is an NSSA border
router which is unconditionally translating type-7 LSAs into type-5
LSAs (Nt is for NSSA translation).
o bit S. When set, the router is a Shortcut-capable ABR and intends
to use the area for shortcutting provided that all other ABRs in
this area agree on that (also announce the S-bit into this area).
See sections 2 and 3 for more details.
10 References
[Ref1] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
Engineering Task Force, 1998. ftp://ftp.isi.edu/in-
notes/rfc2328.txt.
[Ref2] Zinin, Lindem, Yeung. Alternative OSPF ABR Implementations.
Work in progress, Internet Engineering Task Force.
http://www.ietf.org/internet-drafts/draft-ietf-ospf-abr-alt-
02.txt
[Ref3] Coltun, Fuller, Murphy. The OSPF NSSA Option. Work in
progress, Internet Engineering Task Force, 1999.
http://www.ietf.org/internet-drafts/draft-ietf-ospf-nssa-
update-08.txt
11 Author's Address
Alex Zinin
Cisco Systems
150 West Tasman Dr.
San Jose,CA
95134
E-mail: azinin@cisco.com
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