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Additional BGP Configuration
The peering router would also use a loopback interface in this scenario. The neighbor address
configured on each router would be the IPv4 address of the loopback interface on the peering router.
Since the loopback address enables the BGP neighbors to use an IPv4 address that is not reachable by
a directly connected interface, separate static routes would need to be specified for each individual link
that can reach the neighbor’s loopback IPv4 address.This ensures that the loopback address on the
BGP peer remains reachable if one of the links goes down.
Establishing Routing Preference
When a route is redistributed into BGP from a static route, another routing protocol, or when a prefix is
specifically advertised using a network statement in BGP Configuration mode, BGP must consider it in its
route calculations.
Assuming the LOCAL_PREF value of the two routes is equal, the BGP decision logic prefers routes
learned by the local router from statically configured routes or routing protocols over routes learned from a
BGP peer.
Assuming both routes have an equal LOCAL_PREF value, it doesn’t matter if the
administrative distance of a locally learned route is configured higher than that of BGP.
The local route will still be used instead of the route learned from the BGP peer. This
scenario occurs only when the same route is learned locally and by BGP.
After the best valid route has been selected by the BGP decision logic, the administrative distance for BGP
is applied to the route. The routing table then chooses the best route to install based on the lowest
administrative distance. If the BGP algorithm chooses a locally learned route with a higher administrative
distance that is already installed in the routing table, it will not replace that existing route since it points to
the same next-hop IPv4 address. As this specific scenario is manifested, it can be observed in the output of
the show ip route command. The routing source will show the locally learned route from its original
source instead of a BGP sourced route (denoted with a B).
Despite administrative distance and assuming equal LOCAL_PREF value, BGP learned
routes are always less preferable than routes learned by the local router from statically
configured routes or routing protocols.
The best way to make a static route or a prefix learned from another routing protocol more or less
preferable than any route learned using BGP is to redistribute the protocol into BGP (using the
redistribute [connected | ospf | rip | static] [metric | route-map ] command) and use a
route map to modify the local preference of the intended routes. If this approach is used, an additional
prefix list might need to be configured and applied outbound to all neighbors to prevent unwanted
redistributed subnets from being advertised.
The default LOCAL_PREF value for BGP routes is 100. To prefer the redistributed route over a route
learned through BGP, the LOCAL_PREF of the redistributed route must be set to a value higher than 100.
To prefer a route learned through BGP over a redistributed route, an inbound route map should be applied
to the appropriate BGP neighbor to set the LOCAL_PREF of the intended route higher than 100.
The following example configuration demonstrates the basic commands used to prefer a static route or a
route learned through another routing protocol (local prefix) over a BGP learned route:
!
route-map REDISTRIBUTE permit 10
match ip address prefix-list
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