Ospfv3 Sham Link Support For Mpls Vpn - Cisco ASR 9000 Series Configuration Manual

Implementing OSPF

OSPFv3 Sham Link Support for MPLS VPN

over the MPLS backbone, an additional OSPF intra-area (sham link) link between the ingress and egress
PErouters must be created.
A sham link is required between any two VPN sites that belong to the same OSPF area and share an OSPF
backdoor link. If no backdoor link exists between sites, no sham link is required.
Figure 19: Sham Link Between PE Routers to Connected OSPF Client Sites
Figure 19: Sham Link Between PE Routers to Connected OSPF Client Sites , on page 345 shows an MPLS
VPN topology where a sham link configuration is necessary. A VPN client has three sites, each with a backdoor
link. Two sham links are configured, one between PE-1 and PE-2 and another between PE-2 and PE-3. A
sham link is not required between PE-1 and PE-3, because there is no backdoor link between these sites.
When a sham link is configured between the PE routers, the PE routers can populate the virtual routing and
forwarding (VRF) table with the OSPF routes learned over the sham link. These OSPF routes have a larger
administrative distance than BGP routes. If BGP routes are available, they are preferred over these OSPF
routes with the high administrative distance.
OSPFv3 Sham Link Support for MPLS VPN
OSPFv3 sham link represents the VPN backbone as a single point-to-point connection between the two PEs.
OSPFv3 treats the sham link as a point-to-point unnumbered interface, similar to virtual-link. When OSPFv3
sham link is configured, ensure that the route to the remote endpoint of the sham-link exists in the VRF RIB.
If the route to the remote endpoint exists, sham link interface is brought up. If the route to the remote endpoint
of the sham-link is removed from the VRF RIB, OSPFv3 receives redistribution callback and brings the sham
link down.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
OL-30423-03 345