Figure 19 provides an illustration of when you might configure an OSPF sham link. Router CE1 and Router CE2 are located in the same OSPF area. These CE routers are linked together by a Layer 3 VPN over Router PE1 and Router PE2. In addition, Router CE1 and Router CE2 are connected by an intra-area link used as a backup.
OSPF treats the link through the Layer 3 VPN as an interarea link. By default, OSPF prefers intra-area links to interarea links, so OSPF selects the backup intra-area link as the active path. This is not acceptable in configurations where the intra-area link is not the expected primary path for traffic between the CE routers.
An OSPF sham link is also an intra-area link, except that it is configured between the PE routers as shown in Figure 19. You can configure the metric for the sham link to ensure that the path over the Layer 3 VPN is preferred to a backup path over an intra-area link connecting the CE routers.
Figure 19: OSPF Sham Link
You should configure an OSPF sham link under the following circumstances:
If there is no intra-area link between the CE routers, you do not need to configure an OSPF sham link.
For more information on OSPF sham links, see the Internet draft draft-ietf-l3vpn-ospf-2547-01.txt, OSPF as the PE/CE Protocol in BGP/MPLS VPNs.
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