Question to OSPF

[as00001111]

Hey cisco experts ;-)

which is the default formula for calculating the OSPF routing metric?

and what are the metrics for Fa, E, T1 and T3.

Thanks so far!

[Krishnendu AR]

OSPF calculates the cost of a link using the associated interface bandwidth command. The cost is calculated as follows:

Cost = Reference bandwidth/interface bandwidth.

The reference bandwidth is 100Mbps by default.  So a Fasterthernet interface will have a cost of 1 which is equal to 100/100.

Ethernet interface will have cost of 10 which is equal to 100/10.

Similarly a T1 line will have cost of 64 which is 100/1.544=64.76683937823834(Round off to 64)

However, you cast change the reference bandwidth to a different value using 'auto-cost reference bandwidth' command.

R1(config)#router ospf 1

R1(config-router)#auto-cost reference-bandwidth 1000

After this all cost calculation will be based on 1000Mbps.

Krishna

[Joseph W. Doherty]

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Posting

Per RFC OSPF standards, cost is not auto calculated nor is it directly related to bandwidth.

However, on most (all?) Cisco OSPF (v2) implementations, they auto calculate using 100 Mbps as a reference bandwidth, i.e. gig = 1, 100 Mbps = 1, 10 Mbps = 10, etc.

Other vendors may, or may not, auto calculate OSPF costs based on bandwidth.  If they do, they may also, and often do, use a different reference bandwidth than 100 Mbps.

Cisco implementations often allow you to change the auto cost reference value.