draft-ietf-grow-bgp-med-considerations-03.txt   draft-ietf-grow-bgp-med-considerations-04.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: September 2005 March 2005 Expires: December 2005 June 2005
BGP MED Considerations BGP MED Considerations
<draft-ietf-grow-bgp-med-considerations-03.txt> <draft-ietf-grow-bgp-med-considerations-04.txt>
Status of this Memo
Status of this Memo Status of this Memo
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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 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
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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 1.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4
1.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 5 1.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 5
2. Implementation and Protocol Considerations . . . . . . . . . . 6 2. Implementation and Protocol Considerations . . . . . . . . . . 7
2.1. MULTI_EXIT_DISC is a Optional Non-Transitive 2.1. MULTI_EXIT_DISC is a Optional Non-Transitive
Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7 2.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7
2.3. Comparing MEDs Between Different Autonomous 2.3. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. MEDs, Route Reflection and AS Confederations 2.4. MEDs, Route Reflection and AS Confederations
for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9 2.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9
2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 9 2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10
2.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 10 2.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 10
3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 10 3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11
3.1. Comparing MEDs Between Different Autonomous 3.1. Comparing MEDs Between Different Autonomous
Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 11 3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12
4. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12
4.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 12
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Normative References. . . . . . . . . . . . . . . . . . . . 14 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Informative References. . . . . . . . . . . . . . . . . . . 15 6.1. Normative References. . . . . . . . . . . . . . . . . . . . 14
6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 6.2. Informative References. . . . . . . . . . . . . . . . . . . 15
7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
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.
This document discusses implementation and deployment considerations While reading this document it's important to keep in mind that the
regarding BGP MEDs and provides information which implementors and goal is to discuss both implementation and deployment considerations
network operators should be familiar with. regarding BGP MEDs and provide and guidance which both implementors
and network operators should be familiar with. In some instances
implementation advice varies from deployment advice.
1.1. About the MULTI_EXIT_DISC (MED) Attribute 1.1. 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.
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get rid of it quickly. Hot potato routing is accomplished by not get rid of it quickly. Hot potato routing is accomplished by not
passing the EGBP learned MED into IBGP. This minimizes transit passing the EGBP learned MED into IBGP. This minimizes transit
traffic for the provider routing the traffic. Far less common is traffic for the provider routing the traffic. Far less common is
"cold potato routing" (or best-exit) where the transit provider uses "cold potato routing" (or best-exit) where the transit provider uses
their own transit capacity to get the traffic to the point that their own transit capacity to get the traffic to the point that
adjacent transit provider advertised as being closest to the adjacent transit provider advertised as being closest to the
destination. Cold potato routing is accomplished by passing the EBGP destination. Cold potato routing is accomplished by passing the EBGP
learned MED into IBGP. learned MED into IBGP.
If one transit provider uses hot potato routing and another uses cold If one transit provider uses hot potato routing and another uses cold
potato, traffic between the two tends to be more symmetric. 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 Depending on the business relationships, if one provider has more
capacity or a significantly less congested backbone network, then capacity or a significantly less congested backbone network, then
that provider may use cold potato routing. An example of widespread that provider may use cold potato routing. An example of widespread
use of cold potato routing was the NSF funded NSFNET backbone and NSF use of cold potato routing was the NSF funded NSFNET backbone and NSF
funded regional networks in the mid 1990s. funded regional networks in the mid 1990s.
In some cases a provider may use hot potato routing for some In some cases a provider may use hot potato routing for some
destinations for a given peer AS and cold potato routing for others. destinations for a given peer AS and cold potato routing for others.
An example of this is the different treatment of commercial and An example of this is the different treatment of commercial and
research traffic in the NSFNET in the mid 1990s. Today many research traffic in the NSFNET in the mid 1990s. Today many
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In addition, some implementations have been shown to internally In addition, some implementations have been shown to internally
employ a maximum possible MED value (2^32-1) as an "infinity" metric 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 (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 all MED variances, many network operators today explicitly reset (i.e., set
values on ingress to conform to their internal routing policies to zero or some other 'fixed' value) all MED values on ingress to
(i.e., to include policy that requires that MED values of 0 and conform to their internal routing policies (i.e., to include policy
2^32-1 NOT be used in configurations, whether the MEDs are directly that requires that MED values of 0 and 2^32-1 NOT be used in
computed or configured), so as to not have to rely on all their configurations, whether the MEDs are directly computed or
routers having the same missing-MED behavior. 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.
2.3. Comparing MEDs Between Different Autonomous Systems 2.3. 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
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2.4. MEDs, Route Reflection and AS Confederations for BGP 2.4. 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 (which Comparing MEDs between differing adjacent autonomous systems
is discussed in other sections), or not utilizing MEDs at all, discussed in section 2.3), or not utilizing MEDs at all,
significantly decreases the probability of introducing potential significantly decreases the probability of introducing potential
route oscillation conditions into the network. 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 2.5. Route Flap Damping and MED Churn
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decrease in update packing efficiency. decrease in update packing efficiency.
2.7. Temporal Route Selection 2.7. 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 3. 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.
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3.2. Effects of Aggregation on MEDs` 3.2. 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. Security Considerations 4. IANA Considerations
This document introduces no new IANA considerations.
5. 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.
4.1. Acknowledgments 5.1. 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.
5. References 6. References
5.1. Normative References 6.1. 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 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.
5.2. Informative References 6.2. 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.
6. Authors' Addresses 7. 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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