Networking / Beginners

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Technical Considerations

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

Important Technical Considerations: IS-IS Versus OSPF

	OSPF	IS-IS
Fast Convergence	Yes	Yes
Routing Updates	Fast, change only	Fast, change only
VLSM Support and CIDR	Yes	Yes
Load Sharing	Yes, equal cost	Yes, equal cost
Metric Range	0-65,535	0-1023
Static Metric Pieces	Sum of bandwidth	Sum of bandwidth
Dynamic Metric Pieces	None	None
Scalability	Very Strong	Strong
Physical Media Support	All types	Most, but some issues
Extensibility	Yes w/Opaque LSAs	Yes

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.