Figure 96 The T3 route cannot be discovered by routers 1 and

• The problem is that Router 4 has a vastly superior connection to the data center than does Router 3. As such, Router 4 should be the next hop to the data center from Router 3, even though this router has its own direct connection! This dilemma can be solved in several ways. One obvious approach would be to violate the company's network architecture rules by configuring RIP-2 for the interfaces connecting Routers 3 and 4. This would enable end systems in networks 193.168.124.0 and 193.168.125.0 to discover routes through Router 4. RIP-2 is still a simple routing protocol that uses one, dimensionless routing metric, however: hop count. To end systems in these two networks, the data center would appear further away using the T3 route through Router 4 than it would using the 56 kbps link via Router 3. Therefore, merely configuring Router 4 for the RIP-2 protocol wouldn't bias traffic heading to the data center to use this router.

The architects of RIP-2 understood that this could pose problems. Their answer was to develop a mechanism that would force "next hops" regardless of metrics. This metric would enable a RIP-2 node (Router 3, in this example) to inform end systems on other RIP-2 nodes of the presence of a superior path that it would otherwise not be able to discover.

The IP address of the next hop router would be passed to the neighboring routers via routing table updates. These routers would force datagrams to use a specific route, regardless of whether that route would have been calculated to be optimal using available metrics. RIP-2 nodes would check the Next Hop field of an inbound datagram. If unpopulated, it would be forwarded along the path determined by that router to be optimal. If the Next Hop field was populated with an IP address, however, that datagram would be forwarded according to this Next Hop information.

Note This type of mechanism is frequently called source routing in the context of other protocols. The difference between RIP-2's Next Hop field and source routing is that source routing is performed by end systems. RIP-2's Next Hop field is populated by a RIP-2 node as it forwards datagrams on behalf of end systems.

Figure 9-7 illustrates the impact of this use of the Next Hop field. Figure 9-7: Using the Next Hop field.

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