First, consider the advertising routes into BGP-4. There are three ways of populating the BGP-4 table with IGP routes:
• Using the network command—This is used to advertise routes that are in the IP routing table.
• Redistributing static routes—These are routes that have been summarized to a supernet, such as a Class C address with a prefix mask of 16 bits. This requires statically routing to null 0. This fools the system by creating a route that has no exit point from the router because the route does not exist. The command places the route into the routing table without fear of it being used and creating a black hole. This is because lookups are based on the longest match, and the outgoing interface is null 0, which is nowhere.
The problem is that if the route in the IGP routing table disappears, BGP-4 still advertises the route, causing traffic to journey into the autonomous system only to die. Therefore, Cisco suggests that you use the aggregate-address command for BGP-4 instead.
• Redistributing dynamically learned routes from the IGP—This configuration is not advised because there is a great reliance on the IGP table. It is imperative that external routes carried in IBGP-4 are filtered out; otherwise, routing loops are generated when BGP-4 routes are fed into IGP, only to be advertised back into BGP-4 further down the network.
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