CS551Multi-homing in BGPBill Chenghttp://merlot.usc.edu/cs551-f121 Computer Communications - CSCI 551 Copyright © William C. ChengISPn provides transit service to Customern2Transit vs. Nontransit Services Computer Communications - CSCI 551 Copyright © William C. ChengISPn provides non-transit service from another ISP fortraffic for its customerISP1Customer1ISP2Customer2ISP3Customer3singly-homedsubscriberMultiple connections provide load sharing but not loadbalancingWith multi-homing, a single network has more than oneconnections to the Internetcan accommodate link failure3Multi-homing Computer Communications - CSCI 551 Copyright © William C. ChengImproves reliability and performance:bandwidth is sum of links to InternetBGP cannot do load balancingwhile conventional wisdom prefers symmetric paths,many are asymmetricSymmetric routing4Issues With Multi-homing Computer Communications - CSCI 551 Copyright © William C. Chengmay trigger TCP’s fast retransmit algorithmPacket re-orderingaddressing, DNS, aggregationOther concerns: Note: using BGP in multi-homing situation is not anoff-the-shelf use of the protocol5Static Routing May Not Work Computer Communications - CSCI 551 Copyright © William C. ChengISP1Static routing may send traffic to ISPs 2-n from customerover one link and traffic to ISP1 over the other linkLacks flexibility (especially when ISP1 grows and shrink)CustomerISPnISP2ISP3Inter-connectR2 R3R1traffic forISP2-ntraffic forISP1no BGP, but use IMUXor Multilink PPPEasy solution:6Multi-homing to a SingleProvider: Case 1 Computer Communications - CSCI 551 Copyright © William C. Chenguse BGPHarder solution:makes assumptionsabout traffic (sameamount of prefixes canbe reached from bothlinks)CustomerR2ISPR1use MED in Customer orLOCAL-PREF in ISPFor ISP-> Customer traffic:7Multi-homing to a SingleProvider: Case 2 Computer Communications - CSCI 551 Copyright © William C. Chengbreak-down prefix andadvertise differentprefixes over differentlinks with default routesFor Customer->ISP traffic:CustomerR2ISPR1R3138.39/16 204.70/16Good if traffic load to/fromprefixes is equalif single prefix in Customer, only 1 link will be used forISP->Customer trafficFor ISP->Customer traffic,same as before:use MED in Customer orLOCAL-PREF in ISP8Multi-homing to a SingleProvider: Case 3 Computer Communications - CSCI 551 Copyright © William C. ChengFor Customer->ISP traffic:R3 alternates links(reordering?)Customer learns full BGProutes and load-sharesCustomerISP138.39/16 204.70/16R1R3R2Good if traffic load to/from prefixes is equalno equipment sharingMost reliable approach9Multi-homing to a SingleProvider: Case 4 Computer Communications - CSCI 551 Copyright © William C. Chengsame as case 2Customer -> ISP:same as case 3ISP -> Customer:CustomerISP138.39/16 204.70/16R1 R2R3 R4CustomerISP3ISP1 ISP2addressingMajor issues:10Multi-homing to MultipleProviders Computer Communications - CSCI 551 Copyright © William C. Chengaggregationdelegated by ISP1Customer address space:(what are the advantages anddisadvantages of each approach?)delegated by ISP2delegated by ISP1 and ISP2obtained independentlyISP3ISP1 ISP2Customer uses address space from ISP111Case 1: Customer Uses AddressSpace From One ISP (1 or 2) Computer Communications - CSCI 551 Copyright © William C. ChengCustomer138.39/16138.39.1/24ISP1 advertises /16 aggregateCustomer advertises /24 route to ISP2ISP2 relays route to ISP1 andISP3ISP2-3 use /24 routeISP1 routes directlyProblems with traffic load?(longest prefix becomes a"traffic magnet")Note: this can actually work wellif the relative sizes of the providershave a good matchISP1 aggregates to a /19 at border routerto reduce internal tables12Pitfalls Computer Communications - CSCI 551 Copyright © William C. ChengISP3ISP1 ISP2Customer138.39/16138.39.1/24138.39.0/19ISP1 still announces /16ISP1 hears /24 from ISP2ISP1 routes packets forcustomer to ISP2!Workaround: ISP1 mustinject /24 into I-BGPISP1 and ISP2 continue toannounce aggregates13Case 2: Customer Uses AddressSpace From Both ISPs Computer Communications - CSCI 551 Copyright © William C. ChengISP3ISP1 ISP2Customer138.39.1/24 204.70.1/24Load sharing depends on trafficto two prefixesLack of reliability: if ISP1 linkgoes down, part of customerbecomes inaccessibleCustomer may announceprefixes to both ISPs, but stillproblems with longest match asin case 1suppose ISP1 large, ISP2-3smallOffers the most control, but at thecost of aggregation14Case 3: Customer Uses Its OwnAddress Space Computer Communications - CSCI 551 Copyright © William C. ChengStill need to control paths:customer advertises longpath to ISP1, but LOCAL-PREFattribute used to overrideISP3 learns shorter path from ISP2ISP3ISP1 ISP2CustomerBottom line: no good and generalsolution for multi-homing to multiple providers1 will use the blue path for packets destined to 4 and the red forpackets destined to 5 15How Can BGP Express the Following Policies: Computer Communications - CSCI 551 Copyright © William C. Cheng1.21.1232.1 2.22.2.13.13.255.15.244.14.212 will not act as transit to 32 will not accept packets sourced in 11.21.1232.1 2.22.2.13.13.255.15.244.14.211 will use the blue path for packets destined to 4 and the red forpackets destined to 5 (cannot control path, just first hop) 16How Can BGP Express the Following Policies: Computer Communications - CSCI 551 Copyright © William C. Cheng2 will not act as transit to 3 (do not tell anyone about 3)2 will not accept packets sourced in 1 (no way)17 Computer Communications - CSCI 551 Copyright © William C. Cheng Route Flap DampeningBGP sessions disappear and reappearProblem: route flap when a flaky link constantly goes up anddown: routes are withdrawn and re-advertisedglobal effects (does the flap of a butterfly’s wing in Brazilset off a tornado in Texas?)BGP was extended to dampen route flapsincrease when route flapsAssociate a penalty with each routeexponentially decay penalty with timemust never forget routesWhen penalty reaches threshold, suppress route18 Computer Communications - CSCI 551 Copyright © William C. Cheng Route Flap Dampening (Cont...)012340 2 4 6 8 10 12 14 16 18 20 22 241 3 5 7 9 11 13 15 17 19 21 23 25Reuse-LimitSuppress-LimitTimePenalty[CISCO - Intro to BGP]19 Computer Communications - CSCI 551 Copyright © William C. Cheng Tricky Issuesinteraction with aggregation"Synchronizing" intra and inter-domain routingGetting packets to the right exit router without introducingtoo much flux into intra-domain
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