I understand the concept of virtual link, which it will connect to backbone area to form a star topology.

For example, there is ABR A connects Area and Area, and ABR B connects Area and Area Then there should be a virtual link between ABR A and ABR B.

But I wonder how it actually works in router. Does ABR B generates summary-LSA of Area and send to ABR A directly? So ABR A will distribute the LSA into Area What if there is a router on virtual link? Will it receive the LSA which ABR B generates or just treat it as a datagram and send to ABR A? And, does ABR B have LSAs from Area

1 Answer 1


You can think of the virtual link as an OSPF tunnel. ABR B acts like it is an Area 0 border router, and the LSAs it sends to ABR A are the same as any Area 0 border sends into Area 0. ABR B will also receive from ABR A any LSAs that any Area 0 router receives.

Any routers in Area 1 will only pass along IP packets from ABR B to ABR A, they will not directly use the LSAs since those routers are not the destination of the IP packets carrying the LSAs. LSA are carried in IP packets which are like any other IP packets and have source and destination IP addresses. The routers routing IP packets do not look in the IP packets unless the packets' destination IP address is an IP address on the router.

There is a full explanation of OSPF, including virtual links, in RFC 2328, OSPF Version 2.

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