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Route Summarization for BGP

431

that the more specific routes have passed through. The command and the
syntaxes are as follows:

aggregate-address

ip-address mask [summary-only]

[as-set]

Let's take a look at a sample configuration using this command:

RouterA(config)#router bgp 65000

RouterA(config-router)#network 172.16.0.0 mask 255.255.0.0

RouterA(config-router)#neighbor 10.1.1.2 remote-as 64500

RouterA(config-router)#neighbor 172.16.1.50 remote-as

65000

RouterA(config-router)#network 172.16.10.0 mask

255.255.255.0

RouterA(config-router)#network 172.16.1.0 mask

255.255.255.0

RouterA(config-router)#no synchronization

RouterA(config-router)#neighbor 172.16.1.50 next-hop-self

RouterA(config-router)#aggregate-address 172.16.0.0

255.255.0.0 summary-only

Advertising Networks into BGP

Redistribution of routing information occurs in a number of ways. The pri-
mary way is the network command, which was discussed in Chapters 8, "Con-
figuring Basic BGP" and 9, "BGP Scalability and Advanced Features." The
network

command allows BGP to advertise a network that is already in the

IP table. When using the network command, you must identify all the net-
works in the AS that you want to advertise.

You can also use the ip route command to create a static route. The

static route is then redistributed into BGP. Redistribution occurs when a
router uses different protocols to advertise routing information received
between the protocols. BGP considers a static route to be a protocol. Static
route information is advertised to BGP.

The third way to create a static route is to redistribute dynamically learned

routes (routes learned through an IGP) into BGP. In Chapter 8 we learned the
commands to enable this; however, Cisco does not recommend this
approach because of convergence issues and the possibility of introducing
routing loops into the network. Convergence is the time it takes for the net-
work to recover from a change in the network's topology.

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