ospf cost, bandwidth

Could someone kindly explain. auto-cost reference-bandwidth 1000 and ip ospf cost commands in ospf I am assuming the first command specifies bandwidth and the second one specifies cost directly. cost =10^8 / Band Width.

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AwesomeGarrett

Cisco IOS IP Routing: OSPF Command Reference - Index [Support] - Cisco

EdTheLad

auto-cost reference-bw is specified in megabits, the default is 100 i.e. 10^8 , above you have 1000 i.e. 10^9
Which means a FE 100Mb/s interface has a cost 10^9/10^8 = 10 rather than 1 if you left the default.

satishtech

when and why do we change the reference bandwidth ?

OfWolfAndMan

Reference bandwidth/interface speed

^^^This is the equation to calculate metric. By default, reference is 100 which means anything 100 Mbps or more (1 Gbps, 10 Gbps, etc), is seen as equal, which is not what we want. If you use at most 10 gig in a network, you should at least have the reference bandwidth set to 10000, or 100000 for extensibility.

satishtech

Is this ccent or ccnp level , the actual tweaking of BW ?

james43026

Not likely to see this on the CCNA. You are more likely to see this on the CCNP. CCNA is basic configs for routing protocols.