3.6.g Implement troubleshoot and optimize OSPF convergence and scalability

3.6.g [i] Metrics

OSPF convergence is extremely fast when compared to other protocols. To keep this desirable behavior fully functional in your network, you need to consider the three components that 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 LSAs, rerun the Shortest Path First algorithm, and build a new routing table

● A built-in SPF delay time of five seconds (default value)

Thus, the average time for OSPF to propagate LSAs and rerun the SPF algorithm is approximately 1 second. Then the SPF delay timer of five seconds must elapse. Therefor OSPF convergence can be a anything from 6 to 46 seconds, depending upon 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 will need to expire before the SPF is rerun.

If OSPF interface costs are auto-calculated based on interface bandwidth then the OSPF reference bandwidth on a router should be at least twice the highest interface bandwidth configured on any of the router’s interfaces. OSPF link cost is an integer value calculated by dividing the reference bandwidth by the interface’s bandwidth value. If interface bandwidth values are large and the reference bandwidth is too small, this calculation will result in interfaces with different bandwidths being assigned a metric of 1. To avoid such issues, you can use auto-cost reference-bandwidth command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3607-3616).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/9237-9.html