H3C Technologies H3C S10500 Series Switches - Manual (Page 89)

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VPN support
In BGP MPLS VPNs, multiple sites in the same VPN can use OSPF as the internal routing protocol, but
they are treated as different ASs. An OSPF route learned by a site is forwarded to another site as an
external route. This mechanism leads to heavy OSPF routing traffic and management issues.
To solve this problem, domain IDs are used to differentiate VPNs. Sites in the same VPN are considered
as directly connected. PE routers exchange OSPF routing information like on a dedicated line. Thus
network management and OSPF operation efficiency are improved.
NOTE:
For configuration of this feature, see MPLS Configuration Guide.
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