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INTERNET-DRAFT Danny McPherson
Arbor Networks, Inc.
Vijay Gill
AOL
Category Informational
Expires: June 2006 December 2005
BGP MULTI_EXIT_DISC (MED) Considerations
<draft-ietf-grow-bgp-med-considerations-05.txt>
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McPherson, Gill [Page 1]
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Abstract
The BGP MULTI_EXIT_DISC (MED) attribute provides a mechanism for BGP
speakers to convey to an adjacent AS the optimal entry point into the
local AS. While BGP MEDs function correctly in many scenarios, there
are a number of issues which may arise when utilizing MEDs in dynamic
or complex topologies.
This document discusses implementation and deployment considerations
regarding BGP MEDs and provides information which implementors and
network operators should be familiar with.
McPherson, Gill [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Specification of Requirements. . . . . . . . . . . . . . . . . 4
2.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4
2.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 6
3. Implementation and Protocol Considerations . . . . . . . . . . 7
3.1. MULTI_EXIT_DISC is a Optional Non-Transitive
Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7
3.3. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. MEDs, Route Reflection and AS Confederations
for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9
3.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10
3.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 11
4. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11
4.1. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12
5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References. . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References. . . . . . . . . . . . . . . . . . . 16
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
The BGP MED attribute provides a mechanism for BGP speakers to convey
to an adjacent AS the optimal entry point into the local AS. While
BGP MEDs function correctly in many scenarios, there are a number of
issues which may arise when utilizing MEDs in dynamic or complex
topologies.
While reading this document it's important to keep in mind that the
goal is to discuss both implementation and deployment considerations
regarding BGP MEDs. In addition, the intention is to provide
guidance that both implementors and network operators should be
familiar with. In some instances implementation advice varies from
deployment advice.
2. Specification of Requirements
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].
2.1. About the MULTI_EXIT_DISC (MED) Attribute
The BGP MULTI_EXIT_DISC (MED) attribute, formerly known as the
INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as
follows:
The MULTI_EXIT_DISC is an optional non-transitive attribute which
is intended to be used on external (inter-AS) links to discriminate
among multiple exit or entry points to the same neighboring AS.
The value of the MULTI_EXIT_DISC attribute is a four octet unsigned
number which is called a metric. All other factors being equal, the
exit point with lower metric SHOULD be preferred. If received over
EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to
other BGP speakers within the same AS (see also 9.1.2.2). The
MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT
be propagated to other neighboring ASs.
A BGP speaker MUST implement a mechanism based on local
configuration which allows the MULTI_EXIT_DISC attribute to be
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removed from a route. If a BGP speaker is configured to remove the
MULTI_EXIT_DISC attribute from a route, then this removal MUST be
done prior to determining the degree of preference of the route and
performing route selection (Decision Process phases 1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over EBGP. If a
BGP speaker is configured to alter the value of the MULTI_EXIT_DISC
attribute received over EBGP, then altering the value MUST be done
prior to determining the degree of preference of the route and
performing route selection (Decision Process phases 1 and 2). See
Section 9.1.2.2 of BGP4] for necessary restrictions on this.
Section 9.1.2.2 (c) of [BGP4] defines the following route selection
criteria regarding MEDs:
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS (the neighbor-
ing AS is determined from the AS_PATH attribute). Routes which do
not have the MULTI_EXIT_DISC attribute are considered to have the
lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the lowest possi-
ble MULTI_EXIT_DISC value, i.e. 0.
If a MULTI_EXIT_DISC attribute is removed before re-advertising a
route into IBGP, then comparison based on the received EBGP
MULTI_EXIT_DISC attribute MAY still be performed. If an
implementation chooses to remove MULTI_EXIT_DISC, then the optional
comparison on MULTI_EXIT_DISC if performed at all MUST be performed
only among EBGP learned routes. The best EBGP learned route may
then be compared with IBGP learned routes after the removal of the
MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a
subset of EBGP learned routes and the selected "best" EBGP learned
route will not have MULTI_EXIT_DISC removed, then the
MULTI_EXIT_DISC must be used in the comparison with IBGP learned
routes. For IBGP learned routes the MULTI_EXIT_DISC MUST be used in
route comparisons which reach this step in the Decision Process.
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Including the MULTI_EXIT_DISC of an EBGP learned route in the
comparison with an IBGP learned route, then removing the
MULTI_EXIT_DISC attribute and advertising the route has been proven
to cause route loops.
2.2. MEDs and Potatos
In a situation where traffic flows between a pair of hosts, each
connected to different transit networks, which are themselves
interconnected at two or more locations, each transit network has the
choice of either sending traffic to the closest peering to the
adjacent transit network or passing traffic to the interconnection
location which advertises the least cost path to the destination
host.
The former method is called "hot potato routing" (or closest-exit)
because like a hot potato held in bare hands, whoever has it tries to
get rid of it quickly. Hot potato routing is accomplished by not
passing the EGBP learned MED into IBGP. This minimizes transit
traffic for the provider routing the traffic. Far less common is
"cold potato routing" (or best-exit) where the transit provider uses
their own transit capacity to get the traffic to the point that
adjacent transit provider advertised as being closest to the
destination. Cold potato routing is accomplished by passing the EBGP
learned MED into IBGP.
If one transit provider uses hot potato routing and another uses cold
potato, traffic between the two tends to be more symmetric. However,
if both providers employ cold potato routing, or both providers
employ hot potato routing between their networks, it's likely that a
larger amount of asymmetry would exist.
Depending on the business relationships, if one provider has more
capacity or a significantly less congested backbone network, then
that provider may use cold potato routing. An example of widespread
use of cold potato routing was the NSF funded NSFNET backbone and NSF
funded regional networks in the mid 1990s.
In some cases a provider may use hot potato routing for some
destinations for a given peer AS and cold potato routing for others.
An example of this is the different treatment of commercial and
research traffic in the NSFNET in the mid 1990s. Today many
commercial networks exchange MEDs with customers but not bilateral
peers. However, commercial use of MEDs varies widely, from
ubiquitous use of MEDs to no use of MEDs at all.
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In addition, many deployments of MEDs today are likely behaving
differently (e.g., resulting is sub-optimal routing) than the network
operator intended, thereby resulting not in hot or cold potatos, but
mashed potatos! More information on unintended behavior resulting
from MEDs is provided throughout this document.
3. Implementation and Protocol Considerations
There are a number of implementation and protocol peculiarities
relating to MEDs that have been discovered that may affect network
behavior. The following sections provide information on these
issues.
3.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute
MULTI_EXIT_DISC is a non-transitive optional attribute whose
advertisement to both IBGP and EBGP peers is discretionary. As a
result, some implementations enable sending of MEDs to IBGP peers by
default, while others do not. This behavior may result in sub-
optimal route selection within an AS. In addition, some
implementations send MEDs to EBGP peers by default, while others do
not. This behavior may result in sub-optimal inter-domain route
selection.
3.2. MED Values and Preferences
Some implementations consider an MED value of zero as less preferable
than no MED value. This behavior resulted in path selection
inconsistencies within an AS. The current draft version of the BGP
specification [BGP4] removes ambiguities that existed in [RFC 1771]
by stating that if route n has no MULTI_EXIT_DISC attribute, the
lowest possible MULTI_EXIT_DISC value (i.e. 0) should be assigned to
the attribute.
It is apparent that different implementations and different versions
of the BGP draft specification have been all over the map with
interpretation of missing-MED. For example, earlier versions of the
specification called for a missing MED to be assigned the highest
possible MED value (i.e., 2^32-1).
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In addition, some implementations have been shown to internally
employ a maximum possible MED value (2^32-1) as an "infinity" metric
(i.e., the MED value is used to tag routes as unfeasible), and would
upon on receiving an update with an MED value of 2^32-1 rewrite the
value to 2^32-2. Subsequently, the new MED value would be propagated
and could result in routing inconsistencies or unintended path
selections.
As a result of implementation inconsistencies and protocol revision
variances, many network operators today explicitly reset (i.e., set
to zero or some other 'fixed' value) all MED values on ingress to
conform to their internal routing policies (i.e., to include policy
that requires that MED values of 0 and 2^32-1 not be used in
configurations, whether the MEDs are directly computed or
configured), so as to not have to rely on all their routers having
the same missing-MED behavior.
Because implementations don't normally provide a mechanism to disable
MED comparisons in the decision algorithm, "not using MEDs" usually
entails explicitly setting all MEDs to some fixed value upon ingress
to the routing domain. By assigning a fixed MED value consistently
to all routes across the network, MEDs are a effectively a non-issue
in the decision algorithm.
3.3. Comparing MEDs Between Different Autonomous Systems
The MED was intended to be used on external (inter-AS) links to
discriminate among multiple exit or entry points to the same
neighboring AS. However, a large number of MED applications now
employ MEDs for the purpose of determining route preference between
like routes received from different autonomous systems.
A large number of implementations provide the capability to enable
comparison of MEDs between routes received from different neighboring
autonomous systems. While this capability has demonstrated some
benefit (e.g., that described in [RFC 3345]), operators should be
wary of the potential side effects with enabling such a function.
The deployment section below provides some examples as to why this
may result in undesirable behavior.
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3.4. MEDs, Route Reflection and AS Confederations for BGP
In particular configurations, the BGP scaling mechanisms defined in
"BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC 2796]
and "Autonomous System Confederations for BGP" [RFC 3065] will
introduce persistent BGP route oscillation [RFC 3345]. The problem
is inherent in the way BGP works: a conflict exists between
information hiding/hierarchy and the non-hierarchical selection
process imposed by lack of total ordering caused by the MED rules.
Given current practices, we see the problem most frequently manifest
itself in the context of MED + route reflectors or confederations.
One potential way to avoid this is by configuring inter-Member-AS or
inter-cluster IGP metrics higher than intra-Member-AS IGP metrics
and/or using other tie breaking policies to avoid BGP route selection
based on incomparable MEDs. Of course, IGP metric constraints may be
unreasonably onerous for some applications.
Not comparing MEDs between multiple paths for a prefix learned from
different adjacent autonomous systems, as discussed in section 2.3),
or not utilizing MEDs at all, significantly decreases the probability
of introducing potential route oscillation conditions into the
network.
Although perhaps "legal" as far as current specifications are
concerned, modifying MED attributes received on any type of IBGP
session (e.g., standard IBGP, AS confederations EIBGP, route
reflection, etc..) is not recommended.
3.5. Route Flap Damping and MED Churn
MEDs are often derived dynamically from IGP metrics or additive costs
associated with an IGP metric to a given BGP NEXT_HOP. This
typically provides an efficient model for ensuring that the BGP MED
advertised to peers used to represent the best path to a given
destination within the network is aligned with that of the IGP within
a given AS.
The consequence with dynamically derived IGP-based MEDs is that
instability within an AS, or even on a single given link within the
AS, can result in wide-spread BGP instability or BGP route
advertisement churn that propagates across multiple domains. In
short, if your MED "flaps" every time your IGP metric flaps, you're
routes are likely going to be suppressed as a result of BGP Route
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Flap Damping [RFC 2439].
Employment of MEDs may compound the adverse effects of BGP flap
dampening behavior because it many cause routes to be re- advertised
solely to reflect an internal topology change.
Many implementations don't have a practical problem with IGP
flapping, they either latch their IGP metric upon first advertisement
or they employ some internal suppression mechanism. Some
implementations regard BGP attribute changes as less significant than
route withdrawals and announcements to attempt to mitigate the impact
of this type of event.
3.6. Effects of MEDs on Update Packing Efficiency
Multiple unfeasible routes can be advertised in a single BGP Update
message. The BGP4 protocol also permits advertisement of multiple
prefixes with a common set of path attributes to be advertised in a
single update message, this is commonly referred to as "update
packing". When possible, update packing is recommended as it
provides a mechanism for more efficient behavior in a number of
areas, to include:
o Reduction in system overhead due to generation or receipt of
fewer Update messages.
o Reduction in network overhead as a result of fewer packets and
lower bandwidth consumption.
o Allows processing of path attributes and searches for matching
sets in your AS_PATH database (if you have one) less frequently.
Consistent ordering of the path attributes allows for ease of
matching in the database as you don't have different
representations
of the same data.
Update packing requires that all feasible routes within a single
update message share a common attribute set, to include a common
MULTI_EXIT_DISC value. As such, potential wide-scale variance in MED
values introduces another variable and may resulted in a marked
decrease in update packing efficiency.
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3.7. Temporal Route Selection
Some implementations have had bugs which lead to temporal behavior in
MED-based best path selection. These usually involved methods used
to store the oldest route along with ordering routes for MED in
earlier implementations that cause non-deterministic behavior on
whether the oldest route would truly be selected or not.
The reasoning for this is that older paths are presumably more
stable, and thus more preferable. However, temporal behavior in
route selection results in non-deterministic behavior, and as such,
is often undesirable.
4. Deployment Considerations
It has been discussed that accepting MEDs from other autonomous
systems have the potential to cause traffic flow churns in the
network. Some implementations only ratchet down the MED and never
move it back up to prevent excessive churn.
However, if a session is reset, the MEDs being advertised have the
potential of changing. If an network is relying on received MEDs to
route traffic properly, the traffic patterns have the potential for
changing dramatically, potentially resulting in congestion on the
network. Essentially, accepting and routing traffic based on MEDs
allows other people to traffic engineer your network. This may or may
not be acceptable to you.
As previously discussed, many network operators choose to reset MED
values on ingress. In addition, many operators explicitly do not
employ MED values of 0 or 2^32-1 in order to avoid inconsistencies
with implementations and various revisions of the BGP specification.
4.1. Comparing MEDs Between Different Autonomous Systems
Although the MED was meant to only be used when comparing paths
received from different external peers in the same AS, many
implementations provide the capability to compare MEDs between
different autonomous systems as well. AS operators often use
LOCAL_PREF to select the external preferences (primary, secondary
upstreams, peers, customers, etc.), using MED instead of LOCAL_PREF
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would possibility lead to an inconsistent distribution of best routes
as MED is compared only after the AS_PATH length.
Though this may seem a fine idea for some configurations, care must
be taken when comparing MEDs between different autonomous systems.
BGP speakers often derive MED values by obtaining the IGP metric
associated with reaching a given BGP NEXT_HOP within the local AS.
This allows MEDs to reasonably reflect IGP topologies when
advertising routes to peers. While this is fine when comparing MEDs
between multiple paths learned from a single AS, it can result in
potentially "weighted" decisions when comparing MEDs between
different autonomous systems. This is most typically the case when
the autonomous systems use different mechanisms to derive IGP
metrics, BGP MEDs, or perhaps even use different IGP protocols with
vastly contrasting metric spaces (e.g., OSPF v. traditional metric
space in IS-IS).
4.2. Effects of Aggregation on MEDs`
Another MED deployment consideration involves the impact that
aggregation of BGP routing information has on MEDs. Aggregates are
often generated from multiple locations in an AS in order to
accommodate stability, redundancy and other network design goals.
When MEDs are derived from IGP metrics associated with said
aggregates the MED value advertised to peers can result in very
suboptimal routing.
5. IANA Considerations
This document introduces no new IANA considerations.
6. Security Considerations
The MED was purposely designed to be a "weak" metric that would only
be used late in the best-path decision process. The BGP working
group was concerned that any metric specified by a remote operator
would only affect routing in a local AS IF no other preference was
specified. A paramount goal of the design of the MED was to ensure
that peers could not "shed" or "absorb" traffic for networks that
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they advertise. As such, accepting MEDs from peers may in some sense
increase a network's susceptibility to exploitation by peers.
7. Acknowledgments
Thanks to John Scudder for applying his usual keen eye and
constructive insight. Also, thanks to Curtis Villamizar, JR Mitchell
and Pekka Savola for their valuable feedback.
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8. References
McPherson, Gill Section 8. [Page 14]
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8.1. Normative References
[RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.
[RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection
- An Alternative to Full Mesh IBGP", RFC 2796, April
2000.
[RFC 3065] Traina, P., McPherson, D., Scudder, J.. "Autonomous System
Confederations for BGP", RFC 3065, February 2001.
[BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border
Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress.
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8.2. Informative References
[RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping",
RFC 2439, November 1998.
[RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP
Persistent Route Oscillation Condition", RFC 3345,
August 2002.
9. Authors' Addresses
Danny McPherson
Arbor Networks
Email: danny@arbor.net
Vijay Gill
AOL
Email: VijayGill9@aol.com
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