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OSPF auto-cost reference

Unanswered Question
Nov 16th, 2010
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Hi all,


As I know, By default, if I use vlan interface to form ospf neighbor, the ospf cost should be 1 whatever what speed it is.

But now if I add auto-cost reference 10000 (default is 100). then it show all ospf interface to cost 10.Why it show this ospf cost  value?

How to count it ?

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mtsb Tue, 11/16/2010 - 01:05
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Hi,


By default as you said if interface speed in greater that 100MBPS, OSPF metric would be a fractional value and it is not allowed so it is 1 for links higher than 100MBPS BW.


For Vlan interface, BW is 1000MBPS.

nPE1#show int vlan 1 | i BW

MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

So if you see the reference BW to calculate metric to 10000, the cost is 10,000/1000 which is 10 and this is correct IMHO.

sudjacob Tue, 11/16/2010 - 07:08
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the value for auto cost refrence bandwidth = cost* bandwidth/10 to the power of 6



for example I want cost 10 for 1Gbps


then my vlaue for auto cost refrence bandwidth = 10* 10 to the power 9/10to the power 6 = 10000


This is the method  used for setting cost for higher bandwidth in ospf. By default auto cost refrence bandwidth  vlaue = 100


for cost = 1


   = 1*10 to the power 9/10to the power 6  = 1000

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