OSPF Cost

[amit-singh]

Hi All,

We have a point to point(E1) link between Site A and Site B with a cost of 48 in Area 0. Today I added one more E1 Point to point connection between Site A and B.

Traffic was not going over the new link as the metric was 64 for the new link, which is true as the OSPF doesnot do unequal cost path load balancing . I change the cost to 48 manually on one end I was just seeing the out going traffic, no incoming traffic, Then I had set the cost to 48 on the other link as well and got the both incomin and outgoing traffic.

My questions are :

1) How come the cost can be diffrent for 2 point to point E1 links using the same 30 timeslots.

2) Do I have to manually change the Cost value on both the sides. Does OSPF negotiates the cost or should only be manually set.

regards,

-amit singh

[Richard Burts]

The OSPF cost is locally determined and locally significant. The cost is not negotiated and the cost is not advertised. An OSPF router looks at its local interface and determines the cost and this is entirely separate from what its neighbor may do. So it is quite possible for the cost on one end of a /30 to be different from the cost of the other end.

So it is a good practice that if you set the cost on one interface, that you should manually set the cost of every other interface that shares that subnet.

HTH

Rick

[Kevin Dorrell]

Was the declared bandwidth of the old link 2048, while the new one had been left at the default 1544? If so, then even without explicitly configuring the OSPF cost, it would give you the results you describe. If you don't configure a cost for the link, then it takes 100/BW, where BW is the declared bandwidth in Mbps.

Kevin Dorrell

Luxembourg

[amit-singh]

Thanks Kevin and Rick for the info.

regards,

-amit singh