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IS-IS: Deployment in IP Networks by Alvaro Retana, Russ White

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Level 1 Domains and Next Hop Reachability

A common situation in which hiding topology and reachability information from intermediate systems within a level 1 domain will affect the operation of a network is when one IP destination is reachable via another (the final destination subnet recurses through an intermediary next hop). Figure 3-12 illustrates a network in which this problem shows up; the network has been divided into domains over which iBGP is running. There are some eBGP neighbors outside the network. Note this topology is a simplification of what a typical ISP would look like with a two-level design.

Figure 3-12. Level 1 domains and next hop reachability

As you can see from Figure 3-12, this entire network is within a single Border ...

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