Intel 480T, User Manual

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C H A P T E R 1 1 RIP and OSPF
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Area 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.
The backbone allows summary information to be exchanged
between ABRs. Every ABR hears the area summaries from all other
ABRs. The ABR then forms a picture of the distance to all networks
outside of its area by examining the collected advertisements, and
adding in the backbone distance to each advertising router.
When a VLAN is configured to run OSPF, by default it is
automatically joined to the backbone area (0.0.0.0). If you want to
configure the VLAN to be part of a different OSPF area, use the
following command:
config ospf vlan <name> area <areaid>
If this is the first instance of the OSPF area being used, you must
create the area first using the following command:
create ospf area <areaid>
Stub Areas
OSPF allows certain areas to be configured as stub areas . A stub
area is connected to only one other area. The area that connects to a
stub area can be the backbone area. External route information is
not distributed into stub areas. Stub areas are used to reduce
memory and computation requirements on OSPF routers.
Not-So-Stubby-Areas (NSSA)
NSSAs are similar to the existing OSPF stub area configuration
option, but have the following two additional capabilities:
• External routes originating from an ASBR connected to the
NSSA can be advertised within the NSSA.
• External routes originating from the NSSA can be propagated to
other areas, including the backbone area.