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| OSPF allows collections of contiguous
networks and hosts to be grouped together. Such a group, together with the
routers having interfaces to any one of the included networks, is called an
Area. With this new definition, it is no longer true that all routers
in the AS have an identical LS-database. A router actually has
a separate LS-database for each area it is connected to. Two
routers belonging to the same area, have for that area, identical
LS-databases. Routers connected to more than one area are called Area
Border Routers (ABRs). |
| When the source and destination of a packet
reside in the same area, intra-area routing is used. In this case the
packet is routed solely on information contained within the area. |
| The backbone |
| The backbone is the special OSPF Area
0 or Area 0.0.0.0 (using a 32-bit address format). The
backbone always contains all ABRs and is responsible for
distributing routing information between non-backbone areas. The
backbone must be logical contiguous, but not necessarely
physical contiguous. Using what is known as Virtual Links (VLs), the
backbone connectivity can be achieved. VLs can be
configured between any two backbone routers that have an interface to
a common non-backbone area. Virtual Links belong to the
backbone. |
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| Inter-area routing |
| When routing a packet between two
non-backbone areas the backbone is used. The path that the packet
will travel can be broken up into three contiguous pieces: an intra-area
path from the source to an ABR, a backbone path
between the source and destination areas, and another intra-area path
to the destination. The algorithm finds the set of such path that have the
smallest cost. The topology of the backbone dictates the backbone
paths used between areas. |
| The correct ABR to be used as
the packet exits the source area is chosen as follows: each ABR in an area summarize for the area its cost to all networks
external to the area. After the SPF tree is calculated for the area,
routes to all inter-area destinations are calculated by examining the
summaries of the ABRs. Routing tables are created on
each router including these inter-area destination routes and their cost.
Then a route is selected with the lower cost to the network to be reached. |
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