Network Working Group D. McPherson Request for Comments: 4451 Arbor Networks, Inc. Category: Informational V. Gill AOL March 2006 BGP MULTI_EXIT_DISC (MED) Considerations Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThe 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, a number of issues may arise when utilizing MEDs in dynamic or complex topologies. This document discusses implementation and deployment considerations regarding BGP MEDs and provides information with which implementers and network operators should be familiar.
1. Introduction ....................................................3 2. Specification of Requirements ...................................3 2.1. About the MULTI_EXIT_DISC (MED) Attribute ..................3 2.2. MEDs and Potatoes ..........................................5 3. Implementation and Protocol Considerations ......................6 3.1. MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute ....6 3.2. MED Values and Preferences .................................6 3.3. Comparing MEDs between Different Autonomous Systems ........7 3.4. MEDs, Route Reflection, and AS Confederations for BGP ......7 3.5. Route Flap Damping and MED Churn ...........................8 3.6. Effects of MEDs on Update Packing Efficiency ...............9 3.7. Temporal Route Selection ...................................9 4. Deployment Considerations ......................................10 4.1. Comparing MEDs between Different Autonomous Systems .......10 4.2. Effects of Aggregation on MEDs ............................11 5. Security Considerations ........................................11 6. Acknowledgements ...............................................11 7. References .....................................................12 7.1. Normative References ......................................12 7.2. Informative References ....................................12
RFC2119]. section 5.1.4 of [BGP4], as follows: The MULTI_EXIT_DISC is an optional non-transitive attribute that 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, called a metric. All other factors being equal, the exit point with the lower metric SHOULD be preferred. If received over External BGP (EBGP), the MULTI_EXIT_DISC attribute MAY be propagated over Internal BGP (IBGP) to other BGP speakers within the same AS (see also 220.127.116.11). The MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be propagated to other neighboring ASes. A BGP speaker MUST implement a mechanism (based on local configuration) that allows the MULTI_EXIT_DISC attribute to be 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 prior to 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 prior to performing route selection (Decision Process phases 1 and 2). See Section 18.104.22.168 for necessary restrictions on this. Section 22.214.171.124 (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 neighboring AS is determined from the AS_PATH attribute). Routes that 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 that 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 possible MULTI_EXIT_DISC value (i.e., 0). Similarly, neighborAS(n) is a function that returns the neighbor AS from which the route was received. If the route is learned via IBGP, and the other IBGP speaker didn't originate the route, it is the neighbor AS from which the other IBGP speaker learned the route. If the route is learned via IBGP, and the other IBGP speaker either (a) originated the route, or (b) created the route by aggregation and the AS_PATH attribute of the aggregate route is either empty or begins with an AS_SET, it is the local AS. 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, 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 that reach this step in the Decision Process. 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.
commercial networks exchange MEDs with customers but not with bilateral peers. However, commercial use of MEDs varies widely, from ubiquitous use to none at all. In addition, many deployments of MEDs today are likely behaving differently (e.g., resulting in sub-optimal routing) than the network operator intended, which results not in hot or cold potatoes, but mashed potatoes! More information on unintended behavior resulting from MEDs is provided throughout this document. BGP4] removes ambiguities that existed in [RFC1771] 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 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). 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); upon receiving an update with an MED value of 2^32-1, they would 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 not to 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. RFC3345]), operators should be wary of the potential side effects of enabling such a function. The deployment section below provides some examples as to why this may result in undesirable behavior. RFC2796] and "Autonomous System Confederations for BGP" [RFC3065] will introduce persistent BGP route oscillation [RFC3345]. 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 manifest itself most frequently 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, EBGP sessions between Member-ASes of a BGP confederation, route reflection, etc.) is not recommended. RFC2439]. Employment of MEDs may compound the adverse effects of BGP flap- dampening behavior because it may 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 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.
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP- 4)", RFC 1771, March 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2796] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000. [RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 3065, February 2001. [BGP4] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route Flap Damping", RFC 2439, November 1998. [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, August 2002.
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