4.12 MPLS L3 VPN and OSPF Domain-ID - help

I think for the most part, I understand this workbook lab.

However, I am getting stumped on why the Loopback1 interfaces (redistributed into OSPF via connected) are chosing the backdoor link instead of the MPLS route.

It would "seem" based on the higher cost given to the backdoor link, this would not be the preference.

Any help would be greatly appreciated



  • With respect to a particular OSPF instance associated with a VRF, a VPN-IPv4 route that is installed in the VRF and then selected as the preferred route is treated as an External Route if one of the following conditions holds:

    - The route type field of the OSPF Route Type Extended Community has an OSPF route type of “external”

    - The route is from a different domain from the domain of the OSPF instance

    What this means is that if a route comes into a PE as an External or NSSA-External , it will always be so. It can never change. If a route comes in with a type of 1, 2, or 3; and the domain-id matches – then the local PE will originate a new type-3 LSA. i.e. the route will appear inter-area on the other customer sites.
    If a route comes in with a type of 1, 2, or 3; and the domain-id does not match, then it becomes an external route.
    All my routers are currently running IOS and OSPF process ID 100.


    lets say the domain dont match then routes are seen ext so in example backlink will be prefereed coz it sees them as intra area-remeber OSPF will route according to route types O OIA E2 E1....

    and if they match they will be seen as inter area .......

    for this elab for you to have solid understanding revise your ospf to see which routes are more preferedd

  • So yes, I agree - a solid understanding is indeed the goal...

    I might not have been clear in my description, but I do believe I have the answer so I will post here in case others are interested.

    The route types were not the source of confusion but more specifically what was being done with Loopback1 as these were injected into OSPF as External routes.

    From rfc4577 section 4.1.2:

    If a VRF contains both an OSPF-distributed route and a VPN-IPv4 route
    for the same IPv4 prefix, then the OSPF-distributed route is

    This is the case with Lo1 so this is why it prefers the backdoor link.

    Verification is found when looking at the "show bgp vpnv4 unicast all" on the PEs and seeing that the Loopback1 interfaces are there,
    but are in a "rib-failure" state.

    Once the backdoor link is shut, Loopback1 uses the vpnv4 as its path.

    Thank you again for the response!

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