OSPF auto-cost reference

acbennyma · ‎11-16-2010

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 ?

mtsb · ‎11-16-2010

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 · ‎11-16-2010

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

Sudhin Jacob CCIE R&S,SP#28680