Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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Overview of OSPF
ExtremeWare 7.2e Installation and User Guide 237
NOTE
Area 0.0.0.0 exists by default and cannot be deleted or changed.
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, you must configure the area for the VLAN. If you want to
configure the VLAN to be part of a different OSPF area, use the following command:
configure ospf add vlan area
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
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 consumption and computation
requirements on OSPF routers. Use the following command to configure an OSPF area as a stub area:
configure ospf area stub stub-default-cost
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.
The CLI command to control the NSSA function is similar to the command used for configuring a stub
area, as follows:
configure ospf area nssa stub-default-cost
The
translate
option determines whether type 7 LSAs are translated into type 5 LSAs. When
configuring an OSPF area as an NSSA, the
translate
should only be used on NSSA border routers,
where translation is to be enforced. If
translate
is not used on any NSSA border router in a NSSA, one
of the ABRs for that NSSA is elected to perform translation (as indicated in the NSSA specification). The
option should not be used on NSSA internal routers. Doing so inhibits correct operation of the election
algorithm.
Normal Area
A normal area is an area that is not:
• Area 0.