OSPF Sham Links Lab Tips

  • Without Sham Links – If two CE sites that are in the same OSPF area, in addition to the MPLS, are also connected by another link i.e. R3 is connected to Area 1 in Site 1 and Area 1 in Site 2, then this is known as the BACKDOOR link.
  • Traffic will be preferred over the backdoor link as routes will appear as INTRA-area routes and therefore will not prefer the MPLS link as those routes are seen as INTER-area or External routes (see below).
  • If both CE sites are in the same area, routes via MPLS will still be seen as Inter-Area routes due to the redistribution between MP-BGP and IGP into the VRF.
  • domain-id needs to match on both ends for routes, as per above, to be seen as Inter-Area routes otherwise it will be seen as External routes when redistribution occurs between MP-BGP and IGP.
  • With Sham Links will create a ‘virtual link’ between PE routers so that it is an extension of that area, i.e. say area 1. This now means the MPLS routes are now seen as INTRA area routers and or we need to do to prefer the MPLS link is to potentially manipulate the OSPF Cost (depending on link speed).
  • Building the Sham Link needs two /32 loopback addresses that has to be advertised by BGP inside the vrf.  This loopback must always be part of the vrf (ip vrf forwarding VRFNAME).
Click here for an excellent short video that explains sham-links and the above very well

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