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Implementing OSPF on Cisco IOS XR Software
Information About Implementing OSPF on Cisco IOS XR Software
RC-184
Cisco IOS XR Routing Configuration Guide
OL-14356-01
In the case in which an area cannot be connected to Area 0, you must configure a virtual link between
that area and Area 0. The two endpoints of a virtual link are ABRs, and the virtual link must be
configured in both routers. The common nonbackbone area to which the two routers belong is called a
transit area. A virtual link specifies the transit area and the router ID of the other virtual endpoint (the
other ABR).
A virtual link cannot be configured through a stub area or NSSA.
Figure 2 illustrates a virtual link from Area 3 to Area 0.
Figure 2 Virtual Link to Area 0
OSPFv2 Shamlink Support for MPLS VPN
In an MPLS VPN environment, several VPN client sites can be connected in the same OSPF area. If these
sites are connected over a backdoor link (intra-area link) and connected over the VPN backbone, all
traffic passes over the backdoor link instead of over the VPN backbone, because provider edge routers
advertise 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 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.
88722
Area 0
Backbone
Area 1
Transit Area
Area 3
ABR 1ABR 2
OSPF Domain (BGP autonomous system 109)
ABR 3
Router ID 5.5.5.5 Router ID 4.4.4.4
ASBR 1
ASBR 2