Connected: An Internet Encyclopedia
9.1.2.1 Breaking Ties (Phase 2)
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9.1 Decision Process
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9.1.2 Phase 2: Route Selection
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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:
- 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.
- 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)