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 ...