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9.1.2.1 Breaking Ties (Phase 2) Connected: An Internet Encyclopedia
9.1.2.1 Breaking Ties (Phase 2)

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Up: 9.1.2 Phase 2: Route Selection
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Next: 9.1.3 Phase 3: Route Dissemination

9.1.2.1 Breaking Ties (Phase 2)

9.1.2.1 Breaking Ties (Phase 2)

In its Adj-RIBs-In a BGP speaker may have several routes to the same destination that have the same degree of preference. The local speaker can select only one of these routes for inclusion in the associated Loc-RIB. The local speaker considers all equally preferable routes, both those received from BGP speakers located in neighboring autonomous systems, and those received from other BGP speakers located in the local speaker's autonomous system.

The following tie-breaking procedure assumes that for each candidate route all the BGP speakers within an autonomous system can ascertain the cost of a path (interior distance) to the address depicted by the NEXT_HOP attribute of the route. Ties shall be broken according to the following algorithm:

  1. If the local system is configured to take into account MULTI_EXIT_DISC, and the candidate routes differ in their MULTI_EXIT_DISC attribute, select the route that has the lowest value of the MULTI_EXIT_DISC attribute.

  2. Otherwise, select the route that has the lowest cost (interior distance) to the entity depicted by the NEXT_HOP attribute of the route. If there are several routes with the same cost, then the tie-breaking shall be broken as follows:

    • if at least one of the candidate routes was advertised by the BGP speaker in a neighboring autonomous system, select the route that was advertised by the BGP speaker in a neighboring autonomous system whose BGP Identifier has the lowest value among all other BGP speakers in neighboring autonomous systems;

    • otherwise, select the route that was advertised by the BGP speaker whose BGP Identifier has the lowest value.


Next: 9.1.3 Phase 3: Route Dissemination

Connected: An Internet Encyclopedia
9.1.2.1 Breaking Ties (Phase 2)

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