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Deterministic Route Redistribution in BGP Networks

This paper discusses the challenges of non-deterministic routing behavior in route redistribution into BGP networks and proposes a solution using administrative distance in BGP route selection.

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Deterministic Route Redistribution in BGP Networks

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  1. Deterministic Route Redistribution into BGPEnke Chen (enke@redback.com)Jenny Yuan (jenny@redback.com)

  2. Introduction • Routing protocols download routes to RIB • RIB uses the “administrative distance” to break ties between routes from different protocols, and then program the forwarding engine • BGP uses the LOCAL_PREF, AS_PATH, MED, IGP metric for route selection • Currently the two route selection algorithms are independent of each other • Non-deterministic routing behavior can occur involving route redistribution to BGP by RIB • Route redistribution into BGP is common

  3. The Problem • A is the primary connection, and B the backup. • X is statically routed on R1, R2, and redistributed into BGP. On R2, X is configured with the least preferred admin distance. R3 IBGP R1 R2 B A CUST X

  4. The Problem (cont.) • There are potentially two BGP paths for X on R2 • One received from R1 via IBGP • One locally redistributed • With the default setting, the ordering of the arrival of the paths impacts the routing behavior! • When the IBGP path arrives before the locally configured static route is redistributed into BGP • Then the IBGP path will be downloaded to RIB • RIB will select the IBGP path as the active path due to the admin distance • The locally configured static route will not be redistributed • R1, R2 and R3 will converge to the primary path

  5. The Problem (cont.) • When the locally configured static route is redistributed into the BGP before the IBGP path arrives • Then the locally redistributed path will be selected as the BGP best path (due to IGP metric) • It will then be advertised to IBGP peers • R2, and possibly R3 will use the backup path! • Additional route-specific BGP configuration can be added during route redistribution to eliminate the non-deterministic routing behavior • Expensive operationally.

  6. The Proposed Solution • Use the “admin distance” in BGP route selection • Add it as the first step in the route selection • Favor the path with a more preferred admin distance when comparing a locally redistributed path with a path received from a peer • For a locally redistributed path, its admin distance is inherited from the RIB • For a path from a peer, its admin distance is the same value as the admin distance assigned to the path for route download

  7. The Benefits • Eliminates the non-deterministic routing behavior • Avoids the extra BGP configuration • Very low computational overhead • Backward compatible

  8. Reference & Revision • draft-chen-bgp-redist-00.txt • Future revisions (as suggested by Curtis) • Expand on “admin distance” • Re-title: Use Admin Distance in BGP Route Selection

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