Technical Considerations

Table below provides a list of technical issues to consider when selecting a routing protocol.

Fast Convergence

All routing protocols have three important characteristics when dealing with the issue of convergence:

1. Detecting that a change has occurred
2. Adapting to that change
3. Updating the network topology to reflect the change

IS-IS and OSPF detect certain types of network changes instantly. In general, any change that can be detected by a physical change (such as loss of carrier) is detected immediately by any routing protocol.

In addition, both routing protocols use hello packets as keepalives and to detect other failures (such as the loss of an adjacent router or the degradation of an interface to the point where it should no longer be used). Both protocols cause adjacent routers to exchange information periodically.

After the routing protocol has detected the topology change, it needs to adjust the routing tables to accommodate the new topology. OSPF and integrated IS-IS both have mechanisms for updating routing tables. If the topology change were within the area, all the existing routes affected by the change would be discarded and a new routing table would be generated. In general, OSPF and integrated IS-IS converge in less than 2 seconds. The amount of CPU time required to do the recompilation is strongly affected by the number of routes and the amount of redundancy in the network.