Ospf Area Partition And Route Summarization - 3Com 5500-EI PWR Install Manual

Type-7 LSAs can only be advertised in an NSSA area. When Type-7 LSAs reach an ABR, the ABR
can convert part of the routing information carried in the Type-7 LSAs into Type-5 LSAs and
advertise the Type-5 LSAs. Type-7 LSAs are not directly advertised to other areas (including the
backbone area).
Neighbor and Adjacency
In OSPF, neighbor and adjacency are two different concepts.
Neighbor: Two routers that have interfaces to a common network. Neighbor relationships are
maintained by, and usually dynamically discovered by, OSPF's hello packets. When a router starts, it
sends a hello packet through the OSPF interface, and the router that receives the hello packet checks
parameters carried in the packet. If parameters of the two routers match, they become neighbors.
Adjacency: A relationship formed between selected neighboring routers for the purpose of exchanging
routing information. Not every pair of neighboring routers become adjacent, which depends on network
types. Only by synchronizing the LSDB via exchanging DD packets and LSAs can two routers become
adjacent.

OSPF Area Partition and Route Summarization

Area partition
If all the routers on an ever-growing large network run OSPF, the large number of routers will result in an
enormous LSDB, which will consume an enormous storage space, complicate the running of SPF
algorithm, and increase CPU load.
Furthermore, as a network grows larger, it is more likely to have changes in the network topology.
Hence, the network will often be "flapping", and a great number of OSPF packets will be generated and
transmitted in the network. This will lower the network bandwidth utilization. Even worse, any change of
the topology will cause all the routers on the network to re-perform route calculation.
OSPF solves the above-mentioned problem by dividing an AS into multiple areas. Areas refer to groups
into which routers are logically divided. Each group is identified by an Area ID, as shown in
4-4
Figure 4-1.