Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 757)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 733
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BGP or MPLS VPN sample configurations
6
OSPF sham links
In the example shown in Figure 82 , CE 2 and CE 3 are both in OSPF Area 1 and connect to the
same service provider network through different PEs. An additional backdoor connection is
configured between them over another network. OSPF recognizes the backdoor connection as an
Intra-area connection and the connection through the service provider network as an Inter-network
connection. Because OSPF favors Intra-area routes over Inter-network routes, most traffic between
CE 2 and CE 3 travels across the backdoor link. When this is the preferred link in the network, the
configuration is as it should be. However, when the user prefers traffic between the two networks to
be routed across the service provider network, this configuration can causes problems.
FIGURE 82 BGP or MPLS VPN with OSPF backdoor link
Problems can be avoided by creating a virtual intra-area OSPF link between two PEs. This virtual
link is called a sham link. A sham link directs OSPF to treat the route through the service provider
network as an intra-area link. A cost is assigned to the sham link to help the OSPF network
determine when to route over the sham link route and when to use the backdoor link. Because this
virtual link (sham-link) is an intra-area link, the OSPF areas in which each of the PEs reside must be
the same.
NOTE
For sham links to work, OSPF cannot be configured on the loopback interface in the applicable area.
FIGURE 83 BGP or MPLS VPN with OSPF including Sham link and backdoor link