Understand Open Shortest Path First (ospf) Design Guide

the Backbone
OSPF allows for discontinuous parts of the backbone to link through a virtual link. In some cases,
different area 0s need to be linked together.
This can occur if, for example, a company tries to merge two separate OSPF networks into one
network with a common area 0. In other instances, virtual-links are added for redundancy in case
some router failure causes the backbone to be split into two.
A virtual link can be configured between separate ABRs that touch area 0 from each side and
share a common area (illustrated here).
In this diagram, two area 0s are linked together via a virtual link. In case a common area does not
exist, an additional area, such as area 3, could be created to become the transit area.
In case any area which is different than the backbone becomes partitioned, the backbone takes
care of the partition effort without the use of any virtual links.
One part of the partioned area is known to the other part via inter-area routes rather than intra-
area routes.
Neighbors

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Routers that share a common segment become neighbors on that segment. Neighbors are elected
via the Hello protocol. Hello packets are sent periodically out of each interface through IP
multicast (Appendix B).
Routers become neighbors as soon as they see themselves listed in the neighbor Hello packet.
This way, a two way communication is guaranteed. Neighbor negotiation applies to the primary
address only.
Secondary addresses can be configured on an interface with a restriction that they have to belong
to the same area as the primary address.
Two routers do not become neighbors unless they agree with this criteria.
Area-id:
Two routers which have a common segment; their interfaces have to belong to the
same area on that segment. The interfaces must belong to the same subnet and have a
similar mask.
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Authentication:
OSPF allows for the configuration of a password for a specific area. Routers that
want to become neighbors have to exchange the same password on a particular segment.
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