Comparison of Routing Protocol Convergence

As shown in Figure 7-8, different routing protocols need different amounts of time to converge in a given network. Although the convergence depends on the network's topology and structure, pure distance vector protocols are slower to converge than link-state protocols. The use of periodic updates and the hold-down mechanism are the main reasons for slow convergence. As a result, the fast-converging protocols should be used when the network's convergence time is crucial.

Figure 7-8 Routing Protocol Convergence Comparison for the Network Shown in Figure 7-7

Figure 7-8 Routing Protocol Convergence Comparison for the Network Shown in Figure 7-7

Protocol

Convergence Time to Router E

RIP

Hold-down + 1 or 2 Update Intervals

IGRP

Hold-down +1 or 2 Update Intervals

EIGRP

Matter of Seconds

OSPF

Matter of Seconds

Link-state protocols usually converge much more quickly because they instantly propagate routing updates. Whenever a change occurs in a link's state, a link-state update is flooded through the entire network. There is no need to wait for the hold-down timer to expire or for the next periodic update, as with distance vector protocols.

EIGRP is a special case because it incorporates the distance vector principle of metric propagation (it sends only the best routes to the neighbors). However, it does not have periodic updates, nor does it implement the principle of hold-downs. The most distinct feature of EIGRP is that it stores all feasible backup routes in its topology table. When a backup route exists for a lost destination, the switchover to the best backup route is almost immediate and involves no action from other routers. Therefore, very fast convergence can be achieved with proper EIGRP deployment.

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