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Decision Factors at the LOC-Rib Level
Once candidate routes are received from neighbors, the Loc-RIB software applies
additional tie-breakers to routes to the same destination.
If at least one route was learned from an external neighbor (i.e., the route was
learned from eBGP), drop all routes learned from iBGP.
Prefer the route with the lowest interior cost to the NEXT_HOP, according to the
main Routing Table. If two neighbors advertised the same route, but one neighbor
is reachable via a low-bandwidth link and the other by a high-bandwidth link,
and the interior routing protocol calculates lowest cost based on highest
bandwidth, the route through the high-bandwidth link would be preferred and
other routes dropped.
If there is more than one route still tied at this point, several BGP
implementations offer a configurable option to load-share among the routes,
accepting all (or all up to some number).
Prefer the route learned from the BGP speaker the numerically lowest BGP
identifier
Prefer the route learned from the BGP speaker with the lowest peer IP address
Communities
BGP communities are sets of routes with some common attribute (RFC 1997). RFC
1998 shows one technique, based on communities, for multihoming with several
connections to the same AS.
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