Digisol DG-GS4826S, Management Manual

Page: 560 / 1344

C
HAPTER
21 | Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
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n
You can further optimize the exchange of OSPF traffic by specifying
an area range that covers a large number of subnetwork addresses.
This is an important technique for limiting the amount of traffic
exchanged between Area Border Routers (ABRs).
n
And finally, you must specify a virtual link to any OSPF area that is
not physically attached to the OSPF backbone. Virtual links can also
be used to provide a redundant link between contiguous areas to
prevent areas from being partitioned, or to merge backbone areas.
(Note that virtual links are not supported for stubs or NSSAs.)
D
EFINING
N
ETWORK
A
REAS
B
ASED ON
A
DDRESSES
OSPF protocol broadcast messages (i.e., Link State Advertisements or
LSAs) are restricted by area to limit their impact on network performance.
A large network should be split up into separate OSPF areas to increase
network stability, and to reduce protocol traffic by summarizing routing
information into more compact messages. Each router in an area shares
the same view of the network topology, including area links, route
summaries for directly connected areas, and external links to other areas.
Use the Routing Protocol > OSPF > Network Area (Add) page to define an
OSPF area and the interfaces that operate within this area. An autonomous
system must be configured with a backbone area, designated by the area
identifier 0.0.0.0. By default, all other areas are created as normal transit
areas.
Routers in a normal area may import or export routing information about
individual nodes. To reduce the amount of routing traffic flooded onto the
network, an area can be configured to export a single summarized route
that covers a broad range of network addresses within the area
( page 575 ). To further reduce the amount of routes passed between areas,
an area can be configured as a stub ( page 568 , page 572 ) or a not-so-
stubby area ( page 568 , page 569 ).
Normal Area – A large OSPF domain should be broken up into several areas
to increase network stability and reduce the amount of routing traffic
required through the use of route summaries that aggregate a range of
addresses into a single route. The backbone or any normal area can pass
traffic between other areas, and are therefore known as transit areas. Each
router in an area has identical routing tables. These tables may include
area links, summarized links, or external links that depict the topology of
the autonomous system.
Figure 353: OSPF Areas
backbone
area
ABR
area
ABR