You work as a network designer for a company that is replacing their Frame Relay WAN with an MPLS VPN service, where the PE-to-CE routing protocol is BGP. The company has 3000 routes in their distribu

Prefix lists filter which specific routes are advertised but do not aggregate them, so the total count can still exceed 1000 prefixes and some destinations would become unreachable.

Configuring aggregate routes on distribution routers reduces many specific prefixes into a single summarized advertisement, and pairing each aggregate with a null route prevents routing loops by dropping traffic locally when the specific destination is unreachable. This keeps the advertised prefix count within the provider's 1000-prefix-per-VRF limit while maintaining reachability.

Advertising only a default route into the MPLS network tells the provider how to send traffic toward the distribution routers, but does not give the provider the prefix information needed to route return traffic back to all 3000 specific destinations.

Summarizing routes on the MPLS WAN interfaces of the distribution routers reduces the number of prefixes injected into the BGP PE-CE session, allowing the 3000-route network to be represented by far fewer aggregated prefixes that fit within the 1000-prefix constraint.