checking for optimal egress point in ibgp routing

Transcription

1 heking for optimal egress point in ibgp routing Mar Olivier Buob (CORE/CPN/NEO), Mikaël Meulle (CORE/CPN/RIV), Steve Uhlig (Delft Universit of Tehnolog) 0/0/07

2 agenda 2 3 introdution internet routing border gatewa protool (BGP): basis drawbaks of ibgp route refletion fm optimalit onepts ontet and definition how to hek fm optimalit? wh ensuring fm optimalit? onlusion titre du doument/date/auteur p2

3 introdution titre du doument/date/auteur p3

4 Internet routing (/2): IGP Internet is made of more than AS an AS is a olletion of IP networks and routers under the ontrol of one entit that presents a ommon routing poli e: AS55 = OpenTransit an IGP (suh as OSPF or IS IS) ensures routing inside a single AS 2 b 3 e? e a 2 d 2 e? 4 e e? d AS55 titre du doument/date/auteur p4

5 Internet routing (2/2): BGP ebgp: eah AS announes routes to its neighbouring AS aording to its routing poli to a given destination prefi (e: /24) ibgp disseminates the BGP routes inside the AS BGP omputes the best egress point (=BGP net hops) to reah p IGP routes are used to forward traffi to this egress point p? b a AS3 b d f p p? f e g AS2 AS titre du doument/date/auteur p5

6 BGP (/3) a router selets its best route among the routes it learns toward a given prefi. the BGP deision proess is a leiographi omparison of eah BGP attributes highest loal pref shortest AS path IGP > BGP lowest MED a router onl forwards its best route aording to its outbound filters the BGP path pattern do not forward ibgp routes to ibgp peers quasi equivalent ebgp > ibgp losest egress point tie break rules titre du doument/date/auteur p6

7 BGP(2/3): ibgp full mesh (fm) small ASes ommonl use a full mesh (fm) onfiguration this topolog ensures that eah router alwas selets its best possible route for eah prefi fm does not sale if there are more than 00 routers (n(n )/2 ibgp sessions) p a b p3 p2 e p4 d titre du doument/date/auteur p7 p5

8 BGP (2/3) : route refletors (RR) a RR forwards routes learned b an ibgp lient to an BGP peer routes learned b an ibgp peer to its ibgp lients 3 kind of ibgp sessions UP, OVER, DOWN UP DOWN rr OVER OVER IBGP path pattern: (UP)*(OVER)?(DOWN)* UP a UP b OVER UP d UP e titre du doument/date/auteur p8

9 drawbaks of route refletion when the best route is seleted up to the IGP ost, a BGP topolog under route refletion ma lead to man routing problems. suboptimalit non determinism forwarding loops a b 2 4 d!?? b d 5 5 a b d 5 5 a titre du doument/date/auteur p9

10 possible solutions make the RR more verbose (send all equall good BGP routes, up to IGP ost) make the RR more intelligent (send ustomized routes for eah lient) [Bonaventure,Uhlig04] [Caeser,Feamster,Reford04] drafts/draft bonaventure bgp route refletors 00.tt nsdi.pdf appl a multiast protool to distribute messages [Poduri & al 03] /ppt/van.pdf BGP onfederations (sub ASes) hek route refletion verifing the orretness of wide area Internet routing [Feamster04] thanks to CBGP [Quoitin,Van den Shriek,Tandel,...] find a good ibgp design reliabilit aware ibgp route refletion topolog design [Xiao03] how to onstrut a orret and salable ibgp onfiguration[balakrishnan06] titre du doument/date/auteur p0

11 how to hek egress point optimalit? n n5 problem: opait indued b RR without installing too man ibgp sessions rr rr5 solution: ensure that eah router is able to selet its losest BGP net hop, like in a full mesh topolog. n2 rr2 rr3 rr4 n4 fm optimalit n5 for all subset of net hops toda no tool heks this riterion 2 do we need to simulate BGP onvergene? for eah net hop subset? the quasi full mesh is a fm optimal topolog titre du doument/date/auteur p

12 2 fm optimalit onept titre du doument/date/auteur p2

13 ontet ontet: quasi equivalent onurrent BGP routes toward a given prefi. notations: N: the BGP net hop set ; R: the BGP router set ; r R a BGP router ; n N its losest BGP net hop among if r learns and selets the route announed b n, the (n,r) ouple is said to be fm optimal. if n is the losest BGP net hop of r toward a given prefi, is r alwas able to learn the route announed b n for an onurrent net hop set? titre du doument/date/auteur p3

14 shadowing route refletors let be (n,r) N R N(n,r) = { n' N, r,n' > r,n } N W(n,r) = { r' R, n' N(n,r), r',n r',n' } R R \ W(n,r) is the shadowing route refletor set: these routers ma hide the route announed b n to r. a 2 b 4 d N = {,} R = {a,b,,d} n = ; r = d N(,d) = {} W(,d) = {b,d} no fm optimal and valid ibgp path from to d! titre du doument/date/auteur p4

15 etended graph onept we use an graph transformation to ompute valid ibgp routing a soure node and a target node is mapped to eah router eah valid path in G is mapped with eatl one path in G et from a soure node to a target node p OVER UP rr DOWN rr OVER UP DOWN p titre du doument/date/auteur p5

16 how to hek fm optimalit? let be (n,r) N R ompute N(n,r) ompute W(n,r) ompute Gw(n,r) (remove from G eah verte of (R W(n,r))) verif if there eists a path from soure_node(n) to target_node(r) in Gw(n,r) with a DFS algorithm a 2 b d UP b UP d b d titre du doument/date/auteur p6

17 wh ensuring fm optimalit? if eah (n,r) pair is fm optimal the topolog is fm optimal fm optimal routing = routing in a full mesh fm optimalit implies : optimal routing deterministi routing: a router will ever selet the losest route announed b its losest net hop no forwarding loop (if no null weighted IGP link) this riterion is eas to hek! polnomial algorithm titre du doument/date/auteur p7

18 onlusion fm optimalit is eas to hek we do not simulate BGP polnomial algorithm an effiient propert optimal routing deterministi routing avoid route defletion and forwarding loop. future work ibgp network design: Gigp, N, R > G ibgp? thank ou! an question? titre du doument/date/auteur p8