draft-ietf-grow-bgp-med-considerations-04.txt   draft-ietf-grow-bgp-med-considerations-05.txt 
INTERNET-DRAFT Danny McPherson INTERNET-DRAFT Danny McPherson
Arbor Networks, Inc. Arbor Networks, Inc.
Vijay Gill Vijay Gill
AOL AOL
Category Informational Category Informational
Expires: December 2005 June 2005 Expires: June 2006 December 2005
BGP MED Considerations BGP MULTI_EXIT_DISC (MED) Considerations
<draft-ietf-grow-bgp-med-considerations-04.txt> <draft-ietf-grow-bgp-med-considerations-05.txt>
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved. Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract Abstract
The BGP MED attribute provides a mechanism for BGP speakers to convey The BGP MULTI_EXIT_DISC (MED) attribute provides a mechanism for BGP
to an adjacent AS the optimal entry point into the local AS. While speakers to convey to an adjacent AS the optimal entry point into the
BGP MEDs function correctly in many scenarios, there are a number of local AS. While BGP MEDs function correctly in many scenarios, there
issues which may arise when utilizing MEDs in dynamic or complex are a number of issues which may arise when utilizing MEDs in dynamic
topologies. or complex topologies.
This document discusses implementation and deployment considerations This document discusses implementation and deployment considerations
regarding BGP MEDs and provides information which implementors and regarding BGP MEDs and provides information which implementors and
network operators should be familiar with. network operators should be familiar with.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4 2. Specification of Requirements. . . . . . . . . . . . . . . . . 4
1.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 5 2.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4
2. Implementation and Protocol Considerations . . . . . . . . . . 7 2.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 6
2.1. MULTI_EXIT_DISC is a Optional Non-Transitive 3. Implementation and Protocol Considerations . . . . . . . . . . 7
3.1. MULTI_EXIT_DISC is a Optional Non-Transitive
Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7 3.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7
2.3. Comparing MEDs Between Different Autonomous 3.3. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. MEDs, Route Reflection and AS Confederations 3.4. MEDs, Route Reflection and AS Confederations
for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9 3.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9
2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10 3.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10
2.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 10 3.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 11
3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11 4. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11
3.1. Comparing MEDs Between Different Autonomous 4.1. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12 4.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12
4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12 5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations. . . . . . . . . . . . . . . . . . . . 12
5.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Normative References. . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References. . . . . . . . . . . . . . . . . . . . 15
6.2. Informative References. . . . . . . . . . . . . . . . . . . 15 8.2. Informative References. . . . . . . . . . . . . . . . . . . 16
7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
The BGP MED attribute provides a mechanism for BGP speakers to convey 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 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 BGP MEDs function correctly in many scenarios, there are a number of
issues which may arise when utilizing MEDs in dynamic or complex issues which may arise when utilizing MEDs in dynamic or complex
topologies. topologies.
While reading this document it's important to keep in mind that the While reading this document it's important to keep in mind that the
goal is to discuss both implementation and deployment considerations goal is to discuss both implementation and deployment considerations
regarding BGP MEDs and provide and guidance which both implementors regarding BGP MEDs. In addition, the intention is to provide
and network operators should be familiar with. In some instances guidance that both implementors and network operators should be
implementation advice varies from deployment advice. familiar with. In some instances implementation advice varies from
deployment advice.
1.1. About the MULTI_EXIT_DISC (MED) Attribute 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 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 INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as
follows: follows:
The MULTI_EXIT_DISC is an optional non-transitive attribute which The MULTI_EXIT_DISC is an optional non-transitive attribute which
is intended to be used on external (inter-AS) links to discriminate is intended to be used on external (inter-AS) links to discriminate
among multiple exit or entry points to the same neighboring AS. among multiple exit or entry points to the same neighboring AS.
The value of the MULTI_EXIT_DISC attribute is a four octet unsigned 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 number which is called a metric. All other factors being equal, the
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implementation chooses to remove MULTI_EXIT_DISC, then the optional implementation chooses to remove MULTI_EXIT_DISC, then the optional
comparison on MULTI_EXIT_DISC if performed at all MUST be performed comparison on MULTI_EXIT_DISC if performed at all MUST be performed
only among EBGP learned routes. The best EBGP learned route may only among EBGP learned routes. The best EBGP learned route may
then be compared with IBGP learned routes after the removal of the then be compared with IBGP learned routes after the removal of the
MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a
subset of EBGP learned routes and the selected "best" EBGP learned subset of EBGP learned routes and the selected "best" EBGP learned
route will not have MULTI_EXIT_DISC removed, then the route will not have MULTI_EXIT_DISC removed, then the
MULTI_EXIT_DISC must be used in the comparison with IBGP learned 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 routes. For IBGP learned routes the MULTI_EXIT_DISC MUST be used in
route comparisons which reach this step in the Decision Process. route comparisons which reach this step in the Decision Process.
Including the MULTI_EXIT_DISC of an EBGP learned route in the Including the MULTI_EXIT_DISC of an EBGP learned route in the
comparison with an IBGP learned route, then removing the comparison with an IBGP learned route, then removing the
MULTI_EXIT_DISC attribute and advertising the route has been proven MULTI_EXIT_DISC attribute and advertising the route has been proven
to cause route loops. to cause route loops.
1.2. MEDs and Potatos 2.2. MEDs and Potatos
In a situation where traffic flows between a pair of hosts, each In a situation where traffic flows between a pair of hosts, each
connected to different transit networks, which are themselves connected to different transit networks, which are themselves
interconnected at two or more locations, each transit network has the interconnected at two or more locations, each transit network has the
choice of either sending traffic to the closest peering to the choice of either sending traffic to the closest peering to the
adjacent transit network or passing traffic to the interconnection adjacent transit network or passing traffic to the interconnection
location which advertises the least cost path to the destination location which advertises the least cost path to the destination
host. host.
The former method is called "hot potato routing" (or closest-exit) The former method is called "hot potato routing" (or closest-exit)
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commercial networks exchange MEDs with customers but not bilateral commercial networks exchange MEDs with customers but not bilateral
peers. However, commercial use of MEDs varies widely, from peers. However, commercial use of MEDs varies widely, from
ubiquitous use of MEDs to no use of MEDs at all. ubiquitous use of MEDs to no use of MEDs at all.
In addition, many deployments of MEDs today are likely behaving In addition, many deployments of MEDs today are likely behaving
differently (e.g., resulting is sub-optimal routing) than the network differently (e.g., resulting is sub-optimal routing) than the network
operator intended, thereby resulting not in hot or cold potatos, but operator intended, thereby resulting not in hot or cold potatos, but
mashed potatos! More information on unintended behavior resulting mashed potatos! More information on unintended behavior resulting
from MEDs is provided throughout this document. from MEDs is provided throughout this document.
2. Implementation and Protocol Considerations 3. Implementation and Protocol Considerations
There are a number of implementation and protocol peculiarities There are a number of implementation and protocol peculiarities
relating to MEDs that have been discovered that may affect network relating to MEDs that have been discovered that may affect network
behavior. The following sections provide information on these behavior. The following sections provide information on these
issues. issues.
2.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute 3.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute
MULTI_EXIT_DISC is a non-transitive optional attribute whose MULTI_EXIT_DISC is a non-transitive optional attribute whose
advertisement to both IBGP and EBGP peers is discretionary. As a advertisement to both IBGP and EBGP peers is discretionary. As a
result, some implementations enable sending of MEDs to IBGP peers by result, some implementations enable sending of MEDs to IBGP peers by
default, while others do not. This behavior may result in sub- default, while others do not. This behavior may result in sub-
optimal route selection within an AS. In addition, some optimal route selection within an AS. In addition, some
implementations send MEDs to EBGP peers by default, while others do implementations send MEDs to EBGP peers by default, while others do
not. This behavior may result in sub-optimal inter-domain route not. This behavior may result in sub-optimal inter-domain route
selection. selection.
2.2. MED Values and Preferences 3.2. MED Values and Preferences
Some implementations consider an MED value of zero as less preferable Some implementations consider an MED value of zero as less preferable
than no MED value. This behavior resulted in path selection than no MED value. This behavior resulted in path selection
inconsistencies within an AS. The current draft version of the BGP inconsistencies within an AS. The current draft version of the BGP
specification [BGP4] removes ambiguities that existed in [RFC 1771] specification [BGP4] removes ambiguities that existed in [RFC 1771]
by stating that if route n has no MULTI_EXIT_DISC attribute, the 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 lowest possible MULTI_EXIT_DISC value (i.e. 0) should be assigned to
the attribute. the attribute.
It is apparent that different implementations and different versions It is apparent that different implementations and different versions
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(i.e., the MED value is used to tag routes as unfeasible), and would (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 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 value to 2^32-2. Subsequently, the new MED value would be propagated
and could result in routing inconsistencies or unintended path and could result in routing inconsistencies or unintended path
selections. selections.
As a result of implementation inconsistencies and protocol revision As a result of implementation inconsistencies and protocol revision
variances, many network operators today explicitly reset (i.e., set variances, many network operators today explicitly reset (i.e., set
to zero or some other 'fixed' value) all MED values on ingress to to zero or some other 'fixed' value) all MED values on ingress to
conform to their internal routing policies (i.e., to include policy 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 that requires that MED values of 0 and 2^32-1 not be used in
configurations, whether the MEDs are directly computed or configurations, whether the MEDs are directly computed or
configured), so as to not have to rely on all their routers having configured), so as to not have to rely on all their routers having
the same missing-MED behavior. the same missing-MED behavior.
Because implementations don't normally provide a mechanism to disable Because implementations don't normally provide a mechanism to disable
MED comparisons in the decision algorithm, "not using MEDs" usually MED comparisons in the decision algorithm, "not using MEDs" usually
entails explicitly setting all MEDs to some fixed value upon ingress entails explicitly setting all MEDs to some fixed value upon ingress
to the routing domain. By assigning a fixed MED value consistently to the routing domain. By assigning a fixed MED value consistently
to all routes across the network, MEDs are a effectively a non-issue to all routes across the network, MEDs are a effectively a non-issue
in the decision algorithm. in the decision algorithm.
2.3. Comparing MEDs Between Different Autonomous Systems 3.3. Comparing MEDs Between Different Autonomous Systems
The MED was intended to be used on external (inter-AS) links to The MED was intended to be used on external (inter-AS) links to
discriminate among multiple exit or entry points to the same discriminate among multiple exit or entry points to the same
neighboring AS. However, a large number of MED applications now neighboring AS. However, a large number of MED applications now
employ MEDs for the purpose of determining route preference between employ MEDs for the purpose of determining route preference between
like routes received from different autonomous systems. like routes received from different autonomous systems.
A large number of implementations provide the capability to enable A large number of implementations provide the capability to enable
comparison of MEDs between routes received from different neighboring comparison of MEDs between routes received from different neighboring
autonomous systems. While this capability has demonstrated some autonomous systems. While this capability has demonstrated some
benefit (e.g., that described in [RFC 3345]), operators should be benefit (e.g., that described in [RFC 3345]), operators should be
wary of the potential side effects with enabling such a function. wary of the potential side effects with enabling such a function.
The deployment section below provides some examples as to why this The deployment section below provides some examples as to why this
may result in undesirable behavior. may result in undesirable behavior.
2.4. MEDs, Route Reflection and AS Confederations for BGP 3.4. MEDs, Route Reflection and AS Confederations for BGP
In particular configurations, the BGP scaling mechanisms defined in In particular configurations, the BGP scaling mechanisms defined in
"BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC 2796] "BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC 2796]
and "Autonomous System Confederations for BGP" [RFC 3065] will and "Autonomous System Confederations for BGP" [RFC 3065] will
introduce persistent BGP route oscillation [RFC 3345]. The problem introduce persistent BGP route oscillation [RFC 3345]. The problem
is inherent in the way BGP works: a conflict exists between is inherent in the way BGP works: a conflict exists between
information hiding/hierarchy and the non-hierarchical selection information hiding/hierarchy and the non-hierarchical selection
process imposed by lack of total ordering caused by the MED rules. process imposed by lack of total ordering caused by the MED rules.
Given current practices, we see the problem most frequently manifest Given current practices, we see the problem most frequently manifest
itself in the context of MED + route reflectors or confederations. itself in the context of MED + route reflectors or confederations.
One potential way to avoid this is by configuring inter-Member-AS or One potential way to avoid this is by configuring inter-Member-AS or
inter-cluster IGP metrics higher than intra-Member-AS IGP metrics inter-cluster IGP metrics higher than intra-Member-AS IGP metrics
and/or using other tie breaking policies to avoid BGP route selection and/or using other tie breaking policies to avoid BGP route selection
based on incomparable MEDs. Of course, IGP metric constraints may be based on incomparable MEDs. Of course, IGP metric constraints may be
unreasonably onerous for some applications. unreasonably onerous for some applications.
Comparing MEDs between differing adjacent autonomous systems Not comparing MEDs between multiple paths for a prefix learned from
discussed in section 2.3), or not utilizing MEDs at all, different adjacent autonomous systems, as discussed in section 2.3),
significantly decreases the probability of introducing potential or not utilizing MEDs at all, significantly decreases the probability
route oscillation conditions into the network. of introducing potential route oscillation conditions into the
network.
Although perhaps "legal" as far as current specifications are Although perhaps "legal" as far as current specifications are
concerned, modifying MED attributes received on any type of IBGP concerned, modifying MED attributes received on any type of IBGP
session (e.g., standard IBGP, AS confederations EIBGP, route session (e.g., standard IBGP, AS confederations EIBGP, route
reflection, etc..) is NOT recommended. reflection, etc..) is not recommended.
2.5. Route Flap Damping and MED Churn 3.5. Route Flap Damping and MED Churn
MEDs are often derived dynamically from IGP metrics or additive costs MEDs are often derived dynamically from IGP metrics or additive costs
associated with an IGP metric to a given BGP NEXT_HOP. This associated with an IGP metric to a given BGP NEXT_HOP. This
typically provides an efficient model for ensuring that the BGP MED typically provides an efficient model for ensuring that the BGP MED
advertised to peers used to represent the best path to a given advertised to peers used to represent the best path to a given
destination within the network is aligned with that of the IGP within destination within the network is aligned with that of the IGP within
a given AS. a given AS.
The consequence with dynamically derived IGP-based MEDs is that The consequence with dynamically derived IGP-based MEDs is that
instability within an AS, or even on a single given link within the instability within an AS, or even on a single given link within the
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dampening behavior because it many cause routes to be re- advertised dampening behavior because it many cause routes to be re- advertised
solely to reflect an internal topology change. solely to reflect an internal topology change.
Many implementations don't have a practical problem with IGP Many implementations don't have a practical problem with IGP
flapping, they either latch their IGP metric upon first advertisement flapping, they either latch their IGP metric upon first advertisement
or they employ some internal suppression mechanism. Some or they employ some internal suppression mechanism. Some
implementations regard BGP attribute changes as less significant than implementations regard BGP attribute changes as less significant than
route withdrawals and announcements to attempt to mitigate the impact route withdrawals and announcements to attempt to mitigate the impact
of this type of event. of this type of event.
2.6. Effects of MEDs on Update Packing Efficiency 3.6. Effects of MEDs on Update Packing Efficiency
Multiple unfeasible routes can be advertised in a single BGP Update Multiple unfeasible routes can be advertised in a single BGP Update
message. The BGP4 protocol also permits advertisement of multiple message. The BGP4 protocol also permits advertisement of multiple
prefixes with a common set of path attributes to be advertised in a prefixes with a common set of path attributes to be advertised in a
single update message, this is commonly referred to as "update single update message, this is commonly referred to as "update
packing". When possible, update packing is recommended as it packing". When possible, update packing is recommended as it
provides a mechanism for more efficient behavior in a number of provides a mechanism for more efficient behavior in a number of
areas, to include: areas, to include:
o Reduction in system overhead due to generation or receipt of o Reduction in system overhead due to generation or receipt of
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matching in the database as you don't have different matching in the database as you don't have different
representations representations
of the same data. of the same data.
Update packing requires that all feasible routes within a single Update packing requires that all feasible routes within a single
update message share a common attribute set, to include a common update message share a common attribute set, to include a common
MULTI_EXIT_DISC value. As such, potential wide-scale variance in MED MULTI_EXIT_DISC value. As such, potential wide-scale variance in MED
values introduces another variable and may resulted in a marked values introduces another variable and may resulted in a marked
decrease in update packing efficiency. decrease in update packing efficiency.
2.7. Temporal Route Selection 3.7. Temporal Route Selection
Some implementations have had bugs which lead to temporal behavior in Some implementations have had bugs which lead to temporal behavior in
MED-based best path selection. These usually involved methods used MED-based best path selection. These usually involved methods used
to store the oldest route along with ordering routes for MED in to store the oldest route along with ordering routes for MED in
earlier implementations that cause non-deterministic behavior on earlier implementations that cause non-deterministic behavior on
whether the oldest route would truly be selected or not. whether the oldest route would truly be selected or not.
The reasoning for this is that older paths are presumably more The reasoning for this is that older paths are presumably more
stable, and thus more preferable. However, temporal behavior in stable, and thus more preferable. However, temporal behavior in
route selection results in non-deterministic behavior, and as such, route selection results in non-deterministic behavior, and as such,
is often undesirable. is often undesirable.
3. Deployment Considerations 4. Deployment Considerations
It has been discussed that accepting MEDs from other autonomous It has been discussed that accepting MEDs from other autonomous
systems have the potential to cause traffic flow churns in the systems have the potential to cause traffic flow churns in the
network. Some implementations only ratchet down the MED and never network. Some implementations only ratchet down the MED and never
move it back up to prevent excessive churn. move it back up to prevent excessive churn.
However, if a session is reset, the MEDs being advertised have the However, if a session is reset, the MEDs being advertised have the
potential of changing. If an network is relying on received MEDs to potential of changing. If an network is relying on received MEDs to
route traffic properly, the traffic patterns have the potential for route traffic properly, the traffic patterns have the potential for
changing dramatically, potentially resulting in congestion on the changing dramatically, potentially resulting in congestion on the
network. Essentially, accepting and routing traffic based on MEDs network. Essentially, accepting and routing traffic based on MEDs
allows other people to traffic engineer your network. This may or may allows other people to traffic engineer your network. This may or may
not be acceptable to you. not be acceptable to you.
As previously discussed, many network operators choose to reset MED As previously discussed, many network operators choose to reset MED
values on ingress. In addition, many operators explicitly do not values on ingress. In addition, many operators explicitly do not
employ MED values of 0 or 2^32-1 in order to avoid inconsistencies employ MED values of 0 or 2^32-1 in order to avoid inconsistencies
with implementations and various revisions of the BGP specification. with implementations and various revisions of the BGP specification.
3.1. Comparing MEDs Between Different Autonomous Systems 4.1. Comparing MEDs Between Different Autonomous Systems
Although the MED was meant to only be used when comparing paths Although the MED was meant to only be used when comparing paths
received from different external peers in the same AS, many received from different external peers in the same AS, many
implementations provide the capability to compare MEDs between implementations provide the capability to compare MEDs between
different autonomous systems as well. AS operators often use different autonomous systems as well. AS operators often use
LOCAL_PREF to select the external preferences (primary, secondary LOCAL_PREF to select the external preferences (primary, secondary
upstreams, peers, customers, etc.), using MED instead of LOCAL_PREF upstreams, peers, customers, etc.), using MED instead of LOCAL_PREF
would possibility lead to an inconsistent distribution of best routes would possibility lead to an inconsistent distribution of best routes
as MED is compared only after the AS_PATH length. as MED is compared only after the AS_PATH length.
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This allows MEDs to reasonably reflect IGP topologies when This allows MEDs to reasonably reflect IGP topologies when
advertising routes to peers. While this is fine when comparing MEDs advertising routes to peers. While this is fine when comparing MEDs
between multiple paths learned from a single AS, it can result in between multiple paths learned from a single AS, it can result in
potentially "weighted" decisions when comparing MEDs between potentially "weighted" decisions when comparing MEDs between
different autonomous systems. This is most typically the case when different autonomous systems. This is most typically the case when
the autonomous systems use different mechanisms to derive IGP the autonomous systems use different mechanisms to derive IGP
metrics, BGP MEDs, or perhaps even use different IGP protocols with metrics, BGP MEDs, or perhaps even use different IGP protocols with
vastly contrasting metric spaces (e.g., OSPF v. traditional metric vastly contrasting metric spaces (e.g., OSPF v. traditional metric
space in IS-IS). space in IS-IS).
3.2. Effects of Aggregation on MEDs` 4.2. Effects of Aggregation on MEDs`
Another MED deployment consideration involves the impact that Another MED deployment consideration involves the impact that
aggregation of BGP routing information has on MEDs. Aggregates are aggregation of BGP routing information has on MEDs. Aggregates are
often generated from multiple locations in an AS in order to often generated from multiple locations in an AS in order to
accommodate stability, redundancy and other network design goals. accommodate stability, redundancy and other network design goals.
When MEDs are derived from IGP metrics associated with said When MEDs are derived from IGP metrics associated with said
aggregates the MED value advertised to peers can result in very aggregates the MED value advertised to peers can result in very
suboptimal routing. suboptimal routing.
4. IANA Considerations 5. IANA Considerations
This document introduces no new IANA considerations. This document introduces no new IANA considerations.
5. Security Considerations 6. Security Considerations
The MED was purposely designed to be a "weak" metric that would only 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 be used late in the best-path decision process. The BGP working
group was concerned that any metric specified by a remote operator group was concerned that any metric specified by a remote operator
would only affect routing in a local AS IF no other preference was 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 specified. A paramount goal of the design of the MED was to ensure
that peers could not "shed" or "absorb" traffic for networks that that peers could not "shed" or "absorb" traffic for networks that
they advertise. As such, accepting MEDs from peers may in some sense they advertise. As such, accepting MEDs from peers may in some sense
increase a network's susceptibility to exploitation by peers. increase a network's susceptibility to exploitation by peers.
5.1. Acknowledgments 7. Acknowledgments
Thanks to John Scudder for applying his usual keen eye and Thanks to John Scudder for applying his usual keen eye and
constructive insight. Also, thanks to Curtis Villamizar, JR Mitchell constructive insight. Also, thanks to Curtis Villamizar, JR Mitchell
and Pekka Savola for their valuable feedback. and Pekka Savola for their valuable feedback.
6. References 8. References
6.1. Normative References 8.1. Normative References
[RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless [RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993. Aggregation Strategy", RFC 1519, September 1993.
[RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995. (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 [RFC 2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection
- An Alternative to Full Mesh IBGP", RFC 2796, April - An Alternative to Full Mesh IBGP", RFC 2796, April
2000. 2000.
[RFC 3065] Traina, P., McPherson, D., Scudder, J.. "Autonomous System [RFC 3065] Traina, P., McPherson, D., Scudder, J.. "Autonomous System
Confederations for BGP", RFC 3065, February 2001. Confederations for BGP", RFC 3065, February 2001.
[BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border [BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border
Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress. Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress.
6.2. Informative References 8.2. Informative References
[RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping", [RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping",
RFC 2439, November 1998. RFC 2439, November 1998.
[RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP [RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP
Persistent Route Oscillation Condition", RFC 3345, Persistent Route Oscillation Condition", RFC 3345,
August 2002. August 2002.
7. Authors' Addresses 9. Authors' Addresses
Danny McPherson Danny McPherson
Arbor Networks Arbor Networks
Email: danny@arbor.net Email: danny@arbor.net
Vijay Gill Vijay Gill
AOL AOL
Email: VijayGill9@aol.com Email: VijayGill9@aol.com
Intellectual Property Statement Intellectual Property Statement
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