Conditional Advertising in Multihomed Customer Networks

This topic describes how to configure conditional advertising of a customer address space when you are using BGP.

As a rule of thumb, the customer should announce addresses as large as possible (the larger the address space that can be aggregated, the better). The BGP advertisement is configured on the customer edge routers using the network command. Route advertisement is conditioned by the appearance of a corresponding network or subnet in the routing table of the edge router. If the network or subnet is manually entered into the routing table by configuring a static route to null 0, the condition is always true because the static route is always there, and the BGP advertisement is always performed.

If the customer edge router loses connectivity to the rest of the customer network but is still connected to the ISP network, BGP advertisement must cease. In this case, BGP advertisement can be stopped if BGP advertisements are bound to the reachability status of a specific subnet in the core of the customer network, according to the customer IGP.

The problem with using a static route to null 0 is that it conditions the network statement in the BGP configuration so that BGP always advertises the route. If the customer edge router loses connectivity with the rest of the customer network, the router continues to advertise the entire customer address space. The ISP network receives a valid route from the customer edge router. Traffic is sent to this router, but because the router has lost connectivity with the rest of the network, the traffic is dropped (routed to the null 0 interface using the static route).

5-52 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

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