Open Shortest Path
First version 3 (OSPFv3) operates in nondefault VPN routing and forwarding
(VRF) instances for both IPv6 and IPv4 address families and, transports the
routes across a Border Gateway Protocol (BGP) or a Multiprotocol Label
Switching (MPLS) backbone. On the provider edge (PE) device, customer routes
are installed together by OSPFv3 and BGP in a common VRF or address family and
each protocol is configured to redistribute the routes of the other. BGP
combines the prefixes redistributed into it with a route-distinguisher value
defined for the VRF and advertises them to other MPLS-BGP speakers in the same
autonomous system using the VPNv4 or VPNv6 address family as appropriate.
The OSPFv3 route
selection algorithm prefers intra-area routes across the back-door link over
inter-area routes through the MPLS backbone. Sham-links are a type of virtual
link across the MPLS backbone that connect OSPFv3 instances on different PEs.
OSPFv3 instances tunnel protocol packets through the backbone and form
adjacencies. Because OSPFv3 considers the sham-link as an intra-area
connection, sham-link serves as a valid alternative to an intra-area back-door
Domain IDs are used to
determine whether the routes are internal or external. They describe the
administrative domain of the OSPFv3 instance from which the route originates.
Every PE has a 48-bit primary domain ID (which may be NULL) and zero or more
secondary domain IDs.