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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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5.9.6 BGP aggregation<br />

► Redistribute the routes into the IGP used within the transit area. In this<br />

solution, R1 redistributes the 10.0.0.0/8 network into the IGP within AS 1. R3<br />

learns of the network through two routing protocols: BGP <strong>and</strong> the IGP. After<br />

R3 learns of the network through the IGP, it is certain that other routers within<br />

the AS have also learned of the routes. At that time, R3 announces the routes<br />

to its peer in AS 4.<br />

► Encapsulate the transit traffic across the AS. In this solution, transit traffic is<br />

encapsulated within <strong>IP</strong> datagrams addressed to the exit gateway. Because<br />

this does not require the IGP to carry exterior routing information, no<br />

synchronization is required between BGP <strong>and</strong> the IGP. R3 can immediately<br />

announce the routes to its peer in AS 4.<br />

The major improvement introduced in BGP Version 4 was support for CIDR <strong>and</strong><br />

route aggregation. These features allow BGP peers to consolidate multiple<br />

contiguous routing entries into a single advertisement. It significantly enhances<br />

the scalability of BGP into large internetworking environments. Figure 5-27 on<br />

page 229 illustrates these functions.<br />

228 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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