H3C Technologies H3C SR8800 - Manual (Page 94)

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OSPF sham link
An OSPF sham link is a point-to-point link between two PE routers on the MPLS VPN backbone.
In general, BGP peers exchange routing information on the MPLS VPN backbone by using the BGP
extended community attributes. OSPF running on the remote PE uses the routing information to originate
Type-3 summary LSAs (inter-area routes) transmitted to CEs.
If a CE has an intra-area route (backdoor route) to another CE, VPN traffic will always travel on the
backdoor route rather than the corresponding inter-area route because an intra-area route has a higher
priority than an inter-area route. To avoid this, an unnumbered sham link can be configured between PEs
to connect the CEs via an intra-area route over the backbone.
NOTE:
For more information about sham link, see MPLS Configuration Guide.
BFD
NOTE:
For more information about BFD, see High Availability Configuration Guide.
Bidirectional forwarding detection (BFD) provides a single mechanism to quickly detect and monitor the
connectivity of links between OSPF neighbors, reducing network convergence time.
Protocols and standards
• RFC 1765, OSPF Database Overflow
• RFC 2328, OSPF Version 2
• RFC 3101, OSPF Not-So-Stubby Area (NSSA) Option
• RFC 3137, OSPF Stub Router Advertisement
• RFC 3630, Traffic Engineering Extensions to OSPF Version 2
• RFC 4811, OSPF Out-of-Band LSDB Resynchronization
• RFC 4812, OSPF Restart Signaling
• RFC 4813, OSPF Link-Local Signaling
OSPF configuration task list
Make a proper plan before configuring OSPF.
To run OSPF in a routing domain, you must first enable OSPF on the routers. Then you can either use the
default settings of parameters such as the hello interval, LSA delay timer, and SPF calculation interval.
You can also configure them as needed. OSPF routers should be configured on an area basis. Wrong
configurations may cause communication failures, routing information block, and routing loops.
Complete the following tasks to configure OSPF:
Task Remarks
Enabling OSPF Required
Configuring OSPF areas Configuring a stub area Optional