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Question on OSPF backdoor link.

  • 1.  Question on OSPF backdoor link.

     
    Posted 27 days ago

    Hi.

     

    Here is Junos Documentation on an OSPF Sham link.

     

    https://www.juniper.net/documentation/en_US/junose15.1/topics/concept/ospf-sham-links-overview.html

     

    I understand what a Sham link does.

     

    But the documentation references an OSPF backdoor link between L3VPN customer sites. What exactly is a backdoor link? Is it a physical connection between sites? That would seem unlikely since the sites are geographically distant (hence the use of an L3VPN to interconnect them).

     

    Can anyone describe how a backdoor link is implemented?

     

    Thanks,

    Deepak


    Juniper Business Use Only



  • 2.  RE: Question on OSPF backdoor link.

    Posted 25 days ago
    You are correct about a link connecting 2 sites together as a "backdoor".  In the link you posted, the diagram shows customer site 4 & 5 having this backdoor link.  This link could be a p2p link, LTE, or broadband, etc., using IPSec to provide redundancy for the site.(it could also be a legacy connection of some kind)  It's not often I've seen this in production, but I have actually seen it.  Typically you use domain-id's when dealing with backdoor links.


  • 3.  RE: Question on OSPF backdoor link.

     
    Posted 25 days ago

    Thanks for the reply Steve.

     

    Do you know what makes this backdoor link less preferred than connectivity via the L3VPN ?

     

    Thanks,

    Deepak


    Juniper Business Use Only






  • 4.  RE: Question on OSPF backdoor link.

    Posted 25 days ago
    you could play around w/ the link metrics, but one thing to keep in mind is intra-area lsa's are preferred over inter-area lsa's.  So that could play a factor in how the preferred path is handled.