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108/17 = 0SPF Use OSPF = Auto Cost Cost Command

TIP Cisco IOS Software Release 11.2 addressed this cost calculation issue with the introduction of the ospf auto-cost reference bandwidth command. This command alleviates the problem of how OSPF would calculate cost on a Gigabit Ethernet interface.

OSPF Convergence

When all routers know the current state of a network, OSPF convergence is extremely fast when compared to that of other protocols; this was one of the main features included in OSPF's initial design. To keep this desirable feature fully functional in your network, consider the following components, which determine how long it takes for OSPF to converge:

• The length of time it takes OSPF to detect a link or interface failure

• The length of time it takes the routers to exchange routing information via link-state advertisements (LSAs), rerun the shortest path first (SPF) algorithm, and build a new routing table

• A built-in SPF delay time of 5 seconds (the default value)

Therefore, the average time for OSPF to propagate LSAs and rerun the SPF algorithm is approximately 1 second. Then the SPF delay timer of 5 seconds must elapse. OSPF convergence can therefore be from 6 to 46 seconds, depending on the type of failure, SPF timer settings, size of the network, and size of the LSA database. The worst-case scenario is when a link fails but the destination is still reachable via an alternate route because the 40-second default dead timer needs to expire before the SPF is rerun.

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