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
Configuration
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 50.0.0.1 255.255.255.255
router bgp 100
address-family ipv4 vrf c1b1
network 50.0.0.1 mask 255.255.255.255
router ospf 10 vrf c1b1
area 0 sham-link 50.0.0.1 50.0.0.2 cost 1
-> 50.0.0.2 is the loopback IP address on the other PE router
R4(config)# int s0/0
ip ospf cost 10000
