In the example shown in the figure below, CE 2 and CE 3 are both in OSPF Area 1 and connect to the same service provider network through different PEs. An additional backdoor connection is configured between them over another network. OSPF recognizes the backdoor connection as an Intra-area connection and the connection through the service provider network as an Inter-network connection. Because OSPF favors Intra-area routes over Inter-network routes, most traffic between CE 2 and CE 3 travels across the backdoor link. When this is the preferred link in the network, the configuration is as it should be. However, when the user prefers traffic between the two networks to be routed across the service provider network, this configuration can causes problems.
Problems can be avoided by creating a virtual intra-area OSPF link between two PEs. This virtual link is called a sham link. A sham link directs OSPF to treat the route through the service provider network as an intra-area link. A cost is assigned to the sham link to help the OSPF network determine when to route over the sham link route and when to use the backdoor link. Because this virtual link (sham-link) is an intra-area link, the OSPF areas in which each of the PEs reside must be the same.
Note
For sham links to work, OSPF cannot be configured on the loopback interface in the applicable area.
This configuration example describes the additional configuration required to create a sham link between PE 1 and PE 2 in the example shown in the figure above. In this example, the VRF VPN1 is added to the loopback interface configuration, and a sham link with a cost of 10 is created between the loopback interfaces on PE 1 and PE 2.
After this configuration is implemented, routes between CE 2 and CE 3 over the service provider network is preferred to the backdoor link that exists between these CEs.
device(config)# interface loopback 1
device(config-lbif-1)# vrf forwarding VPN1
device(config-lbif-1)# ip address 10.2.2.1/24
device(config)# vrf VPN1
device(config)# router ospf vrf VLAN1
device(config-ospf-router)# area 1 sham-link 10.2.2.1 10.2.2.2 cost 10
device(config-ospf-router)# redistribution bgp
device(config)# interface loopback 1
device(config-lbif-1)# vrf forwarding VPN1
device(config-lbif-1)# ip address 10.2.2.2/24
device(config)# vrf VPN1
device(config)# router ospf vrf VLAN1
device(config-ospf-router)# area 1 sham-link 10.2.2.2 10.2.2.1 cost 10
device(config-ospf-router)# redistribution bgp