Rt updating bgp 0 0 0 00
A sham-link between PE-1 and PE-3 is not necessary in this configuration because the Vienna and Winchester sites do not share a backdoor link.
Figure 4 Sham-Link Example The next example shows forwarding information in which the next hop for the route, 10.3.1.2, is the PE-3 router rather than the PE-2 router (which is the best path according to OSPF).
Figure 3 Using a Sham-Link Between PE Routers to Connect OSPF Client Sites Because the sham-link is seen as an intra-area link between PE routers, an OSPF adjacency is created and database exchange (for the particular OSPF process) occurs across the link.
The PE router can then flood LSAs between sites from across the MPLS VPN backbone.
To reestablish the desired path selection over the MPLS VPN backbone, you must create an additional OSPF intra-area (logical) link between ingress and egress VRFs on the relevant PE routers. 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 the sites, no sham-link is required. A cost is configured with each sham-link and is used to decide whether traffic will be sent over the backdoor path or the sham-link path.
When a sham-link is configured between PE routers, the PEs can populate the VRF routing table with the OSPF routes learned over the sham-link.
When OSPF is used as a protocol between PE and CE routers, the OSPF metric is preserved when routes are advertised over the VPN backbone.
The metric is used on the remote PE routers to select the correct route.