OSPF routing links and notes

• This is not a explanation of ospf, just interesting notes.

• sham-links -> ospf + mpls-mp-bgp

  • The problem
    • When using a providers mpls network to connect site, ospf is redistributed into bgp and back out to ospf at the other sites.
    • The problem is that the routes are redistributed back to ospf as inter-area(From another area)
    • this causes ospf to ignore the route, if there is any other intra-area(Same area) links available.
  • Solution

    • Cisco

    • sham-links changes the behavior on PE routers
  • Config of sham links
    • Each PE needs loopback in same vrf as sham endpoints, e.g. 99.99.99.1, should be able to ping loopbacks in vrf from PE to PE
    • Under PE ospf add sham link

      • SPEDGE1(config)#router ospf 2 vrf CUSTOMER
        SPEDGE1(config-router)#area 0 sham-link 99.99.99.1 99.99.99.2
        !

    • Redistribution OSPF -> MP-BGP with sham link ?

      • We dont need any redistribution from BGP into OSPF
      • but from OSPF into BGP, because MP-BGP needs to have the prefixes in its routing table to use LDP for the VPN-label-assignment.

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