User guide

12-6 E
XTREME
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ARE
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OFTWARE
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NTERIOR
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ATEWAY
R
OUTING
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ROTOCOLS
A
REAS
OSPF allows parts of a network to be grouped together into areas. The topology within
an area is hidden from the rest of the autonomous system. Hiding this information
enables a significant reduction in LSA traffic, and reduces the computations needed to
maintain the LSDB. Routing within the area is determined only by the topology of the
area.
The three types of routers defined by OSPF are as follows:
• Internal Router (IR)
An internal router has all of its interfaces within the same area.
• Area Border Router (ABR)
An ABR has interfaces in multiple areas. It is responsible for exchanging summary
advertisements with other ABRs. You can create a maximum of 7 non-zero areas.
• Autonomous System Border Router (ASBR)
An ASBR acts as a gateway between OSPF and other routing protocols, or other
autonomous systems.
A
REA 0
Any OSPF network that contains more than one area is required to have an area
configured as area 0, also called the backbone. All areas in an autonomous system must
be connected to the backbone. When designing networks, you should start with area 0,
and then expand into other areas.
Table 12-1: LSA Type Numbers
Type Number Description
1 Router LSA
2 Network LSA
3 Summary LSA
4 AS summary LSA
5 AS external LSA
7 NSSA external LSA