Cisco ASR 9000 Series Routing Configuration Manual page 451

Implementing OSPF
OSPFv2 Sham Link Support for MPLS VPN
OSPF routes learned over the VPN backbone as inter-area or external routes that are less preferred than
intra-area routes advertised over backdoor links.
To correct this default OSPF behavior in an MPLS VPN, configure a sham link between two provider edge
(PE) routers to connect the sites through the MPLS VPN backbone. A sham link represents an intra-area
(unnumbered point-to-point) connection between PE routers. All other routers in the area see the sham link
and use it to calculate intra-area shortest path first (SPF) routes to the remote site. A cost must be configured
with each sham link to determine whether traffic is sent over the backdoor link or sham link.
Configured source and destination addresses serve as the endpoints of the sham link. The source and destination
IP addresses must belong to the VRF and must be advertised by Border Gateway Protocol (BGP) as host
routes to remote PE routers. The sham-link endpoint addresses should not be advertised by OSPF.
Figure 19: Backdoor Paths Between OSPF Client Sites
For example, Figure 19: Backdoor Paths Between OSPF Client Sites , on page 421 shows three client sites,
each with backdoor links. Because each site runs OSPF within Area 1 configuration, all routing between the
sites follows the intra-area path across the backdoor links instead of over the MPLS VPN backbone.
If the backdoor links between the sites are used only for backup purposes, default route selection over the
backbone link is not acceptable as it creates undesirable traffic flow. To establish the desired path selection
over the MPLS backbone, an additional OSPF intra-area (sham link) link between the ingress and egress
PErouters must be created.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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