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Implementing OSPF on Cisco IOS XR Software
Information About Implementing OSPF on Cisco IOS XR Software
RC-185
Cisco IOS XR Routing Configuration Guide
OL-14356-01
Figure 3 Backdoor Paths Between OSPF Client Sites
For example, Figure 3 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 only used 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 PEs routers 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.
MPLS VPN Backbone
Area 1
Winchester
10.3.1.7
Brighton
242094
PE3
10.3.1.2
PE2
10.3.1.5
backdoor
link
backdoor
link
backdoor link
PE1
10.3.1.6
Area 1
Stockholm
10.3.1.3
Area 1
Vienna
10.3.1.38