In this paper we give examples to show that if an Internal Border Gateway Protocol (IBGP) configuration using route reflections violates even one of the four conditions mentioned in the theorem given in a previous work, then there may be persistent oscillations or forwarding loops

Rawat, Anuj

Shayman, Mark

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ISR develops, applies and teaches advanced methodologies of design and analysis to solve complex, hierarchical,heterogeneous and dynamic problems of engineering technology and systems for industry and government.ISR is a permanent institute of the University of Maryland, within the Glenn L. Martin Institute of Technol-ogy/A. James Clark School of Engineering. It is a National Science Foundation Engineering Research Center.Web site   http://www.isr.umd.eduI RINSTITUTE FOR SYSTEMS RESEARCHTECHNICAL RESEARCH REPORTInteresting Examples of IBGP Configurationby Anuj Rawat, Mark ShaymanTR 2006-61Interesting Examples of IBGP ConfigurationAnuj Rawat and Mark A. ShaymanDepartment of Electrical and Computer EngineeringUniversity of Maryland, College Park, MD 20742Email: {anuj, shayman}@glue.umd.eduAbstract— In this paper we give examples to show that if anInternal Border Gateway Protocol (IBGP) configuration usingroute reflections violates even one of the four conditions men-tioned in the theorem given in [1], then there may be persistentoscillations or forwarding loops.I. INTRODUCTIONIt has been observed that scaling IBGP using route re-flection may lead to problems such as routing oscillationsand forwarding loops. There have been attempts to suppressthese anomalies by changing the IBGP protocol and by usinggraph theoretic analysis. Anuj et al. [1] model the autonomoussystem (AS) as an IGP connectivity graph GI and an IBGPpeering graph GL. They then state and prove conditionson these graphs so that the IBGP configuration using routereflection is free from persistent oscillations and forwardingloops due to MED attribute and IBGP path asymmetry. In thispaper we give example to show that if an IBGP configurationusing route reflection violates even one of the conditionsmentioned in [1] then there may be persistent oscillations orloops in the system.Section II presents the theorem given in [1]. In section IIIwe present the examples to show that violating any conditionmentioned in the theorem can lead to problem. Finally sectionIV concludes the paper.II. THEOREMThe theorem mentioned in [1] is given below.Theorem 2.1: If an AS configuration with route reflectionsatisfies each one of the following conditions then it is free ofpersistent route oscillations as well as forwarding loops.(i) If nodes u, v learn about paths P,Q respectively, hav-ing nextAS(P ) = nextAS(Q) through EBGP sessions,then u, v are IBGP peers.(ii) Clients of same cluster are not IBGP peers.(iii) cost(sp(u, v)) < cost(sp(u, w)) for all nodes u, v, wsuch that u, v ∈ cluster Vi, w ∈ cluster Vj and i = j.(iv) If ui ∈ Vi and uj ∈ Vj are client nodes and i = j,then there is a reflector uk ∈ sp(ui, uj).III. EXAMPLESIn this section we present examples which show that if anIBGP configuration violates even one condition of Theorem2.1 then we may have persistent oscillations or loops.dAS1 AS2V1AS0rc1 c24112(0)(1)P Q RFig. 1. Condition (i) violated: Persistent oscillationsA. Example for Condition 1In Fig. 1, AS0 has one cluster V1 with reflector r andclients c1, c2. The only IBGP sessions are between nodepairs r, c1 and r, c2. The IGP costs for the shortest pathbetween the node pairs are indicated besides the lines joiningthem. There are three available paths to external destinationd. Client c1 learns about paths P,Q through EBGP peersin AS1, AS2 respectively, and client c2 learns about path Rthrough an EBGP peer in AS2. The weights on EBGP links ofc1 represent its preference in BGP tie-breaking (lower value ismore preferred). Also the MED values for paths Q, R (throughsame neighboring AS) are given in parentheses. Now node c2will always select path R and therefore R is always visible atnode r. Suppose path R is invisible at node c1, so out of pathsP,Q it selects Q and advertises it to node r. Now, out of pathsQ,R, node r selects path R (lower MED value) and advertisesit to c1. Now all the three paths P,Q,R are visible at c1 andit selects path P and advertises it to r. Now r sees paths P,Rand selects P (based on lower IGP cost). Now path R is againinvisible at c1 and it changes its path selection from P to Q. Sowe see that the system is in a persistent oscillation and nodesr, c1 keep on changing their path selections. In this example,the IBGP configuration violates condition (i) of Theorem 2.1,2AS0V1c1 c2c3rdP Q113Fig. 2. Condition (ii) violated: Forwarding loopsince nodes c1, c2 learn about paths Q,R through EBGP peersin the same neighboring AS, but are not IBGP peers.B. Example for Condition 2In Fig. 2, AS0 has one cluster V1 with reflector r and clientsc1, c2 and c3. The dotted lines represent the IBGP sessionsand the solid lines represent the IGP links between the twonodes. The IGP costs are indicated besides the lines. Thereare two paths to external node d. Reflector r learns aboutpath P through EBGP session and client c3 learns about pathQ through EBGP session. Now in this example we have anextra IBGP session between clients c1 and c3. Now we cansee that c1 selects Q as its best path (based on lower IGPto NEXT HOP node) whereas c2 selects path P (only pathknown). Now it is easy to see that there is a forwardingloop between nodes c1 and c2. In this example, the IBGPconfiguration violates condition (ii) of Theorem 2.1, sincethere is an IBGP session between the client nodes c1, c3.C. Example for Condition 3In Fig. 3, AS0 has clusters V1 and V2, both having onereflector and one client (r1, c1 and r2, c2 respectively). Thereare two stand alone reflectors r3 and r4 (these may very wellbe considered to be in clusters V3 and V4 respectively, withboth the clusters having only one node). We assume that thereis a full mesh of IBGP sessions between all the reflectors andthe only other IBGP sessions are between r1, c1 and r2, c2.The lines represent the IGP links between two nodes. TheIGP costs are indicated besides the lines. There are two pathsto external node d. Reflector r3 learns about path P throughan EBGP session and reflector r4 learns about path Q throughan EBGP session. Now r1 selects P as its best path (basedon lower IGP to NEXT HOP node) therefore c1 also selectspath P (only path known). Similarly r2 and c2 select path Q.Now it is easy to observe that the shortest path from c1 tor3 goes through c2 and the shortest path from c2 to r4 goesr1 r2r3 r4c1 c2AS0V1 V2d7651576P QFig. 3. Condition (iii) violated: Forwarding loopthrough c1 so there is a forwarding loop between nodes c1and c2. The problem with this configuration is that IGP costof shortest path between c1 and c2 is less than the IGP cost ofthe shortest path between c1 and r1, which violates condition(iii) of Theorem 2.1.D. Example for Condition 4In Fig. 4 the AS has clusters V1, V2 and V3, all having onereflector and one client (r1, c1 ; r2, c2 ; r3, c3). There are threestand alone reflectors r4, r5 and r6 (these may very well beconsidered to be in clusters V4, V5 and V6 respectively, withall the clusters having only one node). We assume that thereis a full mesh of IBGP sessions between the reflectors and theonly other IBGP sessions are between r1, c1 ; r2, c2 ; r3, c3.The lines represent the IGP links between two nodes. The IGPcosts are indicated besides the lines. There are three paths toexternal node d. Reflector r4 learns about path P1 through anEBGP session, reflector r5 learns about path P2 through anEBGP session and reflector r6 learns about path P3 throughEBGP session. We can see that according to the IGP costto the NEXT HOP node r1 selects path P2 and therefore c1also selects P2 (only path known). Similarly c2 selects P3and c3 selects P1. Now it is easy to verify that there is aforwarding loop between node c1, c2 and c3. The problem withthis configuration is that there is no reflector in the shortestpath between two clients c1,c2 (similarly for c2, c3 and c3, c1).This is in violation to condition (iv) of Theorem 2.1.IV. CONCLUSIONIn this paper we presented examples which show that inIBGP configurations with route reflection, even if one of theconditions mentioned in Theorem 2.1 stated in [1] is violated,there may be persistent oscillations or forwarding loops in thesystem.3r1r2r3r4r6c1 c2c31010 10888202222202022151515r5P3P2P1Fig. 4. Condition (iv) violated: Forwarding loopREFERENCES[1] A. Rawat and M. Shayman, “Preventing persistent oscillations and loopsin IBGP configuration with route reflection,” March 2006.

Interesting Examples of IBGP Configuration

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Interesting Examples of IBGP Configuration

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