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OSPF Sham Links Notes

OSPF Sham Links

If the customer is using a back door link that is in area 0, then this link will always be preferred over the MPLS L3VPN connection

 – The back door link updates type 1 LSAs

     -> Intra-Area

 – The MPLS L3VPN updates are

     -> type 3 LSAs (Inter-Area) if the OSPF process-ids match

     -> type 5 LSAs (External 2) if the OSPF process-ids do not match

The solution is to change the MPLS L3VPN updates to type 1 LSAs

 – Achieved by using a sham-link


 1. Create new loopback interfaces on the PE routers

 2. Associate the loopbacks with the VRF

 3. Advertise the loopbacks in the BGP address-family VRF

     -> Do not advertise the loopbacks into the PE – CE OSPF process

 4. Create the sham-link in the VRF OSPF on the PE routers

 5. Increase the cost on the interface of the backdoor link

Sham-link loopback interfaces must be /32 bit

R1(config)# int lo10

 ip vrf forwarding c1b1

 ip add

router bgp 100

 address-family ipv4 vrf c1b1

  network mask

router ospf 10 vrf c1b1

 area 0 sham-link cost 1

     -> is the loopback IP address on the other PE router

R4(config)# int s0/0

 ip ospf cost 10000

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