Extreme Networks ExtremeWare XOS 11.3, Manual

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Overview of OSPF
ExtremeWare XOS 11.3 Concepts Guide
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Backbone Area (Area 0.0.0.0)
Any OSPF network that contains more than one area is required to have an area configured as area
0.0.0.0, also called the
backbone . All areas in an AS must be connected to the backbone. When designing
networks, you should start with area 0.0.0.0 and then expand into other areas.
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 vlan <vlan-name> area <area-identifier>
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 <area-identifier>
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 computational
requirements on OSPF routers. Use the following command to configure an OSPF area as a stub area:
configure ospf area <area-identifier> stub [summary | nosummary] stub-default-cost
<cost>
Not-So-Stubby-Areas
Not-so-stubby-areas (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 command line interface (CLI) command to control the NSSA function is similar to the command
used for configuring a stub area, as follows:
configure ospf area <area-identifier> nssa [summary | nosummary] stub-default-cost
<cost> {translate}
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