Reg:sham-links in mpls(ospf)

Hi All,

I have a confusion about sham-links.If we have mpls backbone and we are running ospf as igp in our mpls core.customer edge router is running ospf with provider edge router in all of its location e.g newyork,california.In this case provider edge router will act as intra-area router and on our provider edge router we will use sham-link for this customer vrf to reach the provider-edge router of another site and transmit customer route as LSA-1 OR whatever type of LSA generated by customer router.

This is the only case when we use sham-links whnen customer sunig ospf and we as ISP running ospf as igp in our mpls core want to transmit the customer generated las type to other site unchanged or something else.

please help.



  • Hello Vinod,  ospf sham links has nothing to do with the igp you are using inside the mpls backbone.

    Updates between PEs routers is made via iBGP and ifyou use ospf as the CE-PE routing protocol this has

    to be redistributed in BGP ir order to get the updates travel from one PE to the other PE of the Vpn.

    Imagine 2 CEs conected to 2 PEs and running ospf as the protocol to exchgange routing updates 

    and if you set up a backdoor link , that is a link from one CE to the other CE, not passing via de 

    mpls core ospf updates via this link will be LSA Type 1 intra-area so this updates will be prefered 

    instead of crossing the mpls backbone wich send updates a inter-area LSAs. 

    To make this inter-area LSAs appear as Intra area over the mpls backbone a sham-link

    must be setup between the ingress and egess PEs. and if the backdoor link still wins

    there is a cost option in the area x sham-link command to make the mpls backbone preferable.

    Remember this solution is only useful when a backdoor link is been used outside the scope of

    the mpls backbone.

    If you are using ospf as the IGP inside the mpls backbone this will probabiy be runnig as:

    router ospf 1

    while the ospf process used for a specific VRF would be configured as follows:

    router ospf 2 vrf test

    note that process ids must be different and the vrf must be associated w/ the respective process.

    In this case process 1 is been used for the backbone while 2 is used to connect CEs in the VRF test.


    Hope to help



    Santiago E


    Santiago E.

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