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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-68
The purpose of establishing sham-link between VRF OSPF instances on different PEs
include:
z The approach of using MP-IBGP to carry private-network route will only propagate the route,
and the restoration work after reaching the peer PE is only to introduce the original OSPF
routing information as far as possible, during which the OSPF topology information cannot
be truly communicated. By establishing an OSPF link through sham-link, all OSPF
instances inside each site can be truly connected and work out all-round topology
information.
z Different sites in the same VPN exchanges information via MPLS backbone network, but a
link is established between VPN sites so that VPN sites can still communicate via this link
when MPLS backbone network fails. This link is called the "backdoor link". If two sites of
VPN user fall into the same OSPF area and there is a "backdoor link" connecting these two
sites, then routes will be exchanged via both MPLS backbone network and the "backdoor
link". Since the routes exchanged via MPLS backbone network are inter-area routes and the
routes exchanged via the "backdoor link" are intra-area routes, and the intra-area routes are
apparently superior to inter-area routes, the route forwarding between two sites will hence
use the backdoor link. This goes against the purpose of establishing "backdoor link".
Therefore, the sham link shall be used in such applications.