How to configure BGP to accept only default routes using prefix lists

[TCC_2]

Core Issue

Routers in the network run the risk of being overwhelmed by huge amounts of routing information when configured for the Border Gateway Protocol (BGP). Therefore, you want to filter the routing information exchanged with the BGP peer. Permit default routes to be learned and deny all other BGP updates. The size of the local routing table can be controlled without losing IP connectivity to remote networks. This is useful when, based on the existence of other routes, the default route learned from the BGP neighbor is conditionally advertised by it.

Resolution

To resolve this issue, create a prefix list to permit only default routes and use this list to filter routes learned from BGP neighbors.

To create the prefix list to permit only default routes, issue the ip prefix-list command in global configuration mode. Use the permit keyword and specify the network to be permitted as 0.0.0.0/0.

To filter BGP updates learned from a BGP neighbor as per the configured prefix list, issue the neighbor prefix-list command in router configuration mode. To specify that filtering is applied to incoming advertisements, use the in keyword.

For information on configuring a router to receive only default routes, refer to the Configuration to Receive Default Routes Only section of Sample Configuration for BGP with Two Different Service Providers (Multihoming).

[Philip D'Ath]

If I could give a more specific response; if you have a core router with a "full" table and you want a downstream router only to only get a default route, and assuming a default route is already in the BGP routing table, then you need something like this:

ip prefix-list default-route seq 10 permit 0.0.0.0/0

router bgp xxx
 ...
 neighbor a.b.c.d prefix-list default-route out

[Sann Htet Hlaing]

Please explain me, how to configure default route using prefix-list