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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT Danny McPherson 2 Arbor Networks, Inc. 3 Vijay Gill 4 AOL 5 Category Informational 6 Expires: June 2004 December 2003 8 BGP MED Considerations 9 11 Status of this Document 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 33 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 34 document are to be interpreted as described in RFC 2119 [RFC 2119]. 36 This document is a product of an individual. Comments are solicited 37 and should be addressed to the author(s). 39 Copyright Notice 41 Copyright (C) The Internet Society (2003). All Rights Reserved. 43 Abstract 45 The BGP MED attribute provides a mechanism for BGP speakers to convey 46 to an adjacent AS the optimal entry point into the local AS. While 47 BGP MEDs function correctly in many scenarios, there are a number of 48 issues which may arise when utilizing MEDs in dynamic or complex 49 topologies. 51 This document discusses implementation and deployment considerations 52 regarding BGP MEDs and provides information which implementors and 53 network operators should be familiar with. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 1.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4 59 1.2. MEDs and Potatoes . . . . . . . . . . . . . . . . . . . . . 5 60 2. Implementation and Protocol Considerations . . . . . . . . . . 7 61 2.1. MULTI_EXIT_DISC is a Optional Non-Transitive 62 Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 2.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7 64 2.3. Comparing MEDs Between Different Autonomous 65 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 66 2.4. MEDs, Route Reflection and AS Confederations 67 for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 68 2.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9 69 2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10 70 2.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 10 71 3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11 72 3.1. Comparing MEDs Between Different Autonomous 73 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 74 3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12 75 4. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 76 4.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 77 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 78 6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 79 7. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13 81 1. Introduction 83 The BGP MED attribute provides a mechanism for BGP speakers to convey 84 to an adjacent AS the optimal entry point into the local AS. While 85 BGP MEDs function correctly in many scenarios, there are a number of 86 issues which may arise when utilizing MEDs in dynamic or complex 87 topologies. 89 This document discusses implementation and deployment considerations 90 regarding BGP MEDs and provides information which implementors and 91 network operators should be familiar with. 93 1.1. About the MULTI_EXIT_DISC (MED) Attribute 95 The BGP MUTLI_EXIT_DISC (MED) attribute, formerly known as the 96 INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as 97 follows: 99 MULTI_EXIT_DISC is an optional non-transitive attribute which is 100 intended to be used on external (inter-AS) links to discriminate 101 among multiple exit or entry points to the same neighboring AS. 102 The MULTI_EXIT_DISC attribute is a four octet unsigned number which 103 is called a metric. All other factors being equal, the exit point 104 with lower metric SHOULD be preferred. If received over EBGP, the 105 MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP 106 speakers within the same AS. An MED attribute received from a 107 neighboring AS MUST NOT be propagated to other neighboring 108 autonomous systems. 110 A BGP speaker MUST IMPLEMENT a mechanism based on local 111 configuration which allows the MULTI_EXIT_DISC attribute to be 112 removed from a route. This MAY be done prior to determining the 113 degree of preference of the route and performing route selection 114 (decision process phases 1 and 2). 116 An implementation MAY also (based on local configuration) alter the 117 value of the MULTI_EXIT_DISC attribute received over EBGP. This 118 MAY be done prior to determining the degree of preference of the 119 route and performing route selection (decision process phases 1 and 120 2). 122 Section 9.1.2.2 (c) of [BGP4] defines the following route selection 123 criteria regarding MEDs: 125 Remove from consideration routes with less-preferred 126 MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable 127 between routes learned from the same neighboring AS (the 128 neighboring AS is determined from the AS_PATH attribute). Routes 129 which do not have the MULTI_EXIT_DISC attribute are considered to 130 have the lowest possible MULTI_EXIT_DISC value. 132 This is also described in the following procedure: 134 for m = all routes still under consideration 135 for n = all routes still under consideration 136 if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m)) 137 remove route m from consideration 139 In the pseudo-code above, MED(n) is a function which returns the 140 value of route n's MULTI_EXIT_DISC attribute. If route n has no 141 MULTI_EXIT_DISC attribute, the function returns the lowest possible 142 MULTI_EXIT_DISC value, i.e. 0. 144 If a MULTI_EXIT_DISC attribute is removed before re- advertising a 145 route into IBGP, then comparison based on the received EBGP 146 MULTI_EXIT_DISC attribute MAY still be performed. If an 147 implementation chooses to remove MULTI_EXIT_DISC, then the optional 148 comparison on MULTI_EXIT_DISC if performed at all MUST be performed 149 only among EBGP learned routes. The best EBGP learned route may 150 then be compared with IBGP learned routes after the removal of the 151 MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a 152 subset of EBGP learned routes and the selected "best" EBGP learned 153 route will not have MULTI_EXIT_DISC removed, then the 154 MULTI_EXIT_DISC must be used in the comparison with IBGP learned 155 routes. For IBGP learned routes the MULTI_EXIT_DISC MUST be used in 156 route comparisons which reach this step in the decision process. 157 Including the MULTI_EXIT_DISC of an EBGP learned route in the 158 comparison with an IBGP learned route, then removing the 159 MULTI_EXIT_DISC attribute and advertising the route has been proven 160 to cause route loops. 162 Routes that have different MULTI_EXIT_DISC attribute SHALL NOT be 163 aggregated. 165 1.2. MEDs and Potatoes 167 In a situation where traffic flows between a pair of hosts, each 168 connected to different transit networks, which are themselves 169 interconnected at two or more locations, each transit network has the 170 choice of either sending traffic to the closest peering to the 171 adjacent transit network or passing traffic to the interconnection 172 location which advertises the least cost path to the destination 173 host. 175 DIAGRAM*** 177 The former method is called "hot potato routing" (or closest-exit) 178 because like a hot potato held in bare hands, whoever has it tries to 179 get rid of it quickly. Hot potato routing is accomplished by not 180 passing the EGBP learned MED into IBGP. This minimizes transit 181 traffic for the provider routing the traffic. Far less common is 182 "cold potato routing" (or best-exit) where the transit provider uses 183 their own transit capacity to get the traffic to the point that 184 adjacent transit provider advertised as being closest to the 185 destination. Cold potato routing is accomplished by passing the EBGP 186 learned MED into IBGP. 188 DIAGRAMS*** 190 If one transit provider uses hot potato routing and another uses cold 191 potato, traffic between the two tends to be more symmetric. 192 Depending on the business relationships, if one provider has more 193 capacity or a significantly less congested backbone network, then 194 that provider may use cold potato routing. An example of widespread 195 use of cold potato routing was the NSF funded NSFNET backbone and NSF 196 funded regional networks in the mid 1990s. 198 In some cases a provider may use hot potato routing for some 199 destinations for a given peer AS and cold potato routing for others. 200 An example of this is the different treatment of commercial and 201 research traffic in the NSFNET in the mid 1990s. Today many 202 commercial networks exchange MEDs with customers but not bilateral 203 peers. However, commercial use of MEDs varies widely, from 204 ubiquitous use of MEDs to no use of MEDs at all. 206 In addition, many deployments of MEDs today are likely behaving 207 differently (e.g., resulting is sub-optimal routing) than the network 208 operator intended, thereby resulting not in hot or cold potatoes, but 209 mashed potatoes! More information on unintended behavior resulting 210 from MEDs is provided throughout this document. 212 2. Implementation and Protocol Considerations 214 There are a number of implementation and protocol peculiarities 215 relating to MEDs that have been discovered that may affect network 216 behavior. The following sections provide information on these 217 issues. 219 2.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute 221 MULTI_EXIT_DISC is a non-transitive optional attribute whose 222 advertisement to both IBGP and EBGP peers is discretionary. As a 223 result, some implementations enable sending of MEDs to IBGP peers by 224 default, while others do not. This behavior may result in sub- 225 optimal route selection within an AS. In addition, some 226 implementations send MEDs to EBGP peers by default, while others do 227 not. This behavior may result in sub-optimal inter-domain route 228 selection. 230 DIAGRAM and MORE TEXT*** 232 2.2. MED Values and Preferences 234 Some implementations consider an MED value of zero as less preferable 235 than no MED value. This behavior resulted in path selection 236 inconsistencies within an AS. The current draft version of the BGP 237 specification [BGP4] removes ambiguities that existed in [RFC 1771] 238 by stating that if route n has no MULTI_EXIT_DISC attribute, the 239 lowest possible MULTI_EXIT_DISC value (i.e. 0) should be assigned to 240 the attribute. 242 It is apparent that different implementations and different versions 243 of the BGP draft specification have been all over the map with 244 interpretation of missing-MED. For example, earlier versions of the 245 specification called for a missing MED to be assigned the highest 246 possible MED value (i.e., 2^32-1). 248 In addition, some implementations have been shown to internally 249 employ a maximum possible MED value (2^32-1) as an "infinity" metric 250 (i.e., the MED value is used to tag routes as unfeasible), and would 251 upon on receiving an update with an MED value of 2^32-1 rewrite the 252 value to 2^32-2. Subsequently, the new MED value would be propagated 253 and could result in routing inconsistencies or unintended path 254 selections. 256 As a result of implementation inconsistencies and protocol revision 257 variances, many network operators today explicitly reset all MED 258 values on ingress to conform to their internal routing policies 259 (i.e., to include policy that requires that MED values of 0 and 260 2^32-1 NOT be used in configurations, whether the MEDs are directly 261 computed or configured), so as to not have to rely on all their 262 routers having the same missing-MED behavior. 264 2.3. Comparing MEDs Between Different Autonomous Systems 266 The MED was intended to be used on external (inter-AS) links to 267 discriminate among multiple exit or entry points to the same 268 neighboring AS. However, a large number of MED applications now 269 employ MEDs for the purpose of determining route preference between 270 like routes received from different autonomous systems. 272 A large number of implementations provide the capability to enable 273 comparison of MEDs between routes received from different neighboring 274 autonomous systems. While this capability has demonstrated some 275 benefit (e.g., that described in [RFC 3345]), operators should be 276 wary of the potential side effects with enabling such a function. 277 The deployment section below provides some examples as to why this 278 may result in undesirable behavior. 280 2.4. MEDs, Route Reflection and AS Confederations for BGP 282 In particular configurations, the BGP scaling mechanisms defined in 283 "BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC 2796] 284 and "Autonomous System Confederations for BGP" [RFC 3065] will 285 introduce persistent BGP route oscillation [RFC 3345]. The problem 286 is inherent in the way BGP works: a conflict exists between 287 information hiding/hierarchy and the non-hierarchical selection 288 process imposed by lack of total ordering caused by the MED rules. 289 Given current practices, we see the problem most frequently manifest 290 itself in the context of MED + route reflectors or confederations. 292 One potential way to avoid this is by configuring inter-Member-AS or 293 inter-cluster IGP metrics higher than intra-Member-AS IGP metrics 294 and/or using other tie breaking policies to avoid BGP route selection 295 based on incomparable MEDs. Of course, IGP metric constraints may be 296 unreasonably onerous for some applications. 298 Comparing MEDs between differing adjacent autonomous systems (which 299 will be discussed in later sections), or not utilizing MEDs at all, 300 significantly decreases the probability of introducing potential 301 route oscillation conditions into the network. 303 Although perhaps "legal" as far as current specifications are 304 concerned, modifying MED attributes received on any type of IBGP 305 session (e.g., standard IBGP, AS confederations EIBGP, route 306 reflection, etc..) is NOT recommended. 308 2.5. Route Flap Damping and MED Churn 310 MEDs are often derived dynamically from IGP metrics or additive costs 311 associated with an IGP metric to a given BGP NEXT_HOP. This 312 typically provides an efficient model for ensuring that the BGP MED 313 advertised to peers used to represent the best path to a given 314 destination within the network is aligned with that of the IGP within 315 a given AS. 317 The consequence with dynamically derived IGP-based MEDs is that 318 instability within an AS, or even on a single given link within the 319 AS, can result in wide-spread BGP instability or BGP route 320 advertisement churn that propagates across multiple domains. In 321 short, if your MED "flaps" every time your IGP metric flaps, you're 322 routes are likely going to be suppressed as a result of BGP Route 323 Flap Damping [RFC 2439]. 325 Employment of MEDs may compound the adverse effects of BGP flap 326 dampening behavior because it many cause routes to be re- advertised 327 solely to reflect an internal topology change. 329 Many implementations don't have a practical problem with IGP 330 flapping, they either latch their IGP metric upon first advertisement 331 or they employ some internal suppression mechanism. Some 332 implementations regard BGP attribute changes as less significant than 333 route withdrawals and announcements to attempt to mitigate the impact 334 of this type of event. 336 2.6. Effects of MEDs on Update Packing Efficiency 338 Multiple unfeasible routes can be advertised in a single BGP Update 339 message. In addition, one or more feasible routes can be advertised 340 in a single Update message so long as all prefixes share a common 341 attribute set. 343 The BGP4 protocol permits advertisement of multiple prefixes with a 344 common set of path attributes to be advertised in a single update 345 message, this is commonly referred to as "update packing". When 346 possible, update packing is recommended as it provides a mechanism 347 for more efficient behavior in a number of areas, to include: 349 o Reduction in system overhead due to generation or receipt of 350 fewer Update messages. 352 o Reduction in network overhead as a result of fewer packets and 353 lower bandwidth consumption. 355 o Allows processing of path attributes and searches for matching 356 sets in your AS_PATH database (if you have one) less frequently. 357 Consistent ordering of the path attributes allows for ease of 358 matching in the database as you don't have different 359 representations 360 of the same data. 362 Update packing requires that all feasible routes within a single 363 update message share a common attribute set, to include a common 364 MULTI_EXIT_DISC value. As such, potential wide-scale variance in MED 365 values introduces another variable and may resulted in a marked 366 decrease in update packing efficiency. 368 2.7. Temporal Route Selection 370 Some implementations have had bugs which lead to temporal behavior in 371 MED-based best path selection. These usually involved methods used 372 to store the oldest route along with ordering routes for MED in 373 earlier implementations that cause non-deterministic behavior on 374 whether the oldest route would truly be selected or not. 376 The reasoning for this is that "older" paths are presumably more 377 stable, and thus more preferable. However, temporal behavior in 378 route selection results in non-deterministic behavior, and as such, 379 is often undesirable. 381 3. Deployment Considerations 383 Empirical data [MFN/Ixia Monitoring Project] has shown that accepting 384 MEDs from other autonomous systems have the potential to cause 385 traffic flow churns in the network. Some implementations only 386 ratchet down the MED and never move it back up to prevent excessive 387 churn. 389 However, if that session is reset, the MEDs being advertised have the 390 potential of changing. If an network is relying on received MEDs to 391 route traffic properly, the traffic patterns have the potential for 392 changing dramatically, potentially resulting in congestion on the 393 network. Essentially, accepting and routing traffic based on MEDs 394 allows other people to traffic engineer your network. This may or may 395 not be acceptable to you. 397 As previously discussed, many network operators choose to reset MED 398 values on ingress. In addition, many operators explicitly do not 399 employ MED values of 0 or 2^32-1 in order to avoid inconsistencies 400 with implementations and various revisions of the BGP specification. 402 3.1. Comparing MEDs Between Different Autonomous Systems 404 Although the MED was meant to only be used when comparing paths 405 received from different external peers in the same AS, many 406 implementations provide the capability to compare MEDs between 407 different autonomous systems as well. 409 Though this may seem a fine idea for some configurations, care must 410 be taken when comparing MEDs between different autonomous systems. 411 BGP speakers often derive MED values by obtaining the IGP metric 412 associated with reaching a given BGP NEXT_HOP within the local AS. 413 This allows MEDs to reasonably reflect IGP topologies when 414 advertising routes to peers. While this is fine when comparing MEDs 415 between multiple paths learned from a single AS, it can result in 416 potentially "weighted" decisions when comparing MEDs between 417 different autonomous systems. This is most typically the case when 418 the autonomous systems use different mechanisms to derive IGP 419 metrics, BGP MEDs, or perhaps even use different IGP protocols with 420 vastly contrasting metric spaces (e.g., OSPF v. traditional metric 421 space in IS-IS). 423 3.2. Effects of Aggregation on MEDs` 425 Another MED deployment consideration involves the impact that 426 aggregation of BGP routing information has on MEDs. Aggregates are 427 often generated from multiple locations in an AS in order to 428 accommodate stability, redundancy and other network design goals. 429 When MEDs are derived from IGP metrics associated with said 430 aggregates the MED value advertised to peers can result in very 431 suboptimal routing. 433 DIAGRAM AND EXPAND*** 435 4. Security Considerations 437 The MED was purposely designed to be a "weak" metric that would only 438 be used late in the best-path decision process. The BGP working 439 group was concerned that any metric specified by a remote operator 440 would only affect routing in a local AS IF no other preference was 441 specified. A paramount goal of the design of the MED was to ensure 442 that peers could not "shed" or "absorb" traffic for networks that 443 they advertise. As such, accepting MEDs from peers may in some sense 444 increase a network's susceptibility to exploitation by peers. 446 4.1. Acknowledgments 448 Thanks to John Scudder for applying his usual keen eye and 449 constructive insight. Also, thanks to Curtis Villamizar and JR 450 Mitchell. 452 Others to be supplied. 454 5. References 456 [RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless 457 Inter-Domain Routing (CIDR): an Address Assignment and 458 Aggregation Strategy", RFC 1519, September 1993. 460 [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 461 (BGP-4)", RFC 1771, March 1995. 463 [RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping", 464 RFC 2439, November 1998. 466 [RFC 2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection 467 - An Alternative to Full Mesh IBGP", RFC 2796, April 468 2000. 470 [RFC 3065] Traina, P., McPherson, D., Scudder, J.. "Autonomous System 471 Confederations for BGP", RFC 3065, February 2001. 473 [RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP 474 Persistent Route Oscillation Condition", RFC 3345, 475 August 2002. 477 [BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border 478 Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress. 480 [MFN/Ixia Monitoring Project] Vijay to Provide. 482 6. Authors' Addresses 484 Danny McPherson 485 Arbor Networks 486 Email: danny@arbor.net 488 Vijay Gill 489 AOL 490 Email: VijayGill9@aol.com 492 7. Full Copyright Statement 494 Copyright (C) The Internet Society (2003). 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