OSPF over sham links - cost Q.

[jigsaw2026]

If I were to use the following command on a sham link:

Router1(config-if)# area <area-id> sham-link <source-address> <destination-address> cost <number>

Would the cost act the same as in a normal OSPF network, i.e. increment the existing metric (of an advertised network) by X, or would it overwrite the existing metric and the metric would then appear as X (as apposed to old metric plus X)?

If it just adds to the existing, is there any other commands that force the metric to be of a certain value?

Hope that makes sense!

Many thanks,

J

[pankajkulkarni]

It is the cost of sending a packet on a OSPF sham link. The cost configured would eventually get added to compute the total cost to reach a destination network.

Regards,

Pankaj

[jigsaw2026]

Many thanks for your response Pankaj.

Can I just confirm, if I have:

CE1-----PE1PE2-----CE2

And I've entered the cost at 10 the sham link, the total cost of PE1-PE2 would be 10? And then would get added to cost of PE1-CE1 and PE2-CE2?

Or would 10 get added to the existing cost of the sham link (and then subsequently to the rest of the link)?

[attrgautam]

Sham-link is a end to end OSPF on demand link across the SP backbone. You can really simplify it to be a point-to-point link across a network. So the cost of the sham-link by default is 1 and can be modified by you and this cost will add the PE-CE cost for OSPF LSA in the CE.

Trust this clarifies.

[pankajkulkarni]

Yes, 10 would get added to the existing cost. Cost of the shamlink needs to be carefully choosen after studying the metric received for the destination network over the backdoor link.

A shamlink is treated a virtual link and has the DN bit set to avoid necessary flooding.

NOTE - For shamlink to be operational the endpoints need to be advertised through MPBGP (redistribute). Shamlink will be operational only when two communication between them is established.

Shamlink endpoints shouldn't be advertised through OSPF.

Hope this clarifies. Kindly rate all posts.

Regards,

Pankaj

[jigsaw2026]

Many thanks for your replies.

Regards,

J