Assign a totally stubby area
By default, the device sends summary LSAs (LSA type 3) into stub areas. You can further reduce the number of link state
advertisements (LSA) sent into a stub area by conguring the device to stop sending summary LSAs (type 3 LSAs) into the area. You can
disable the summary LSAs when you are conguring the stub area or later after you have congured the area.
This feature disables origination of summary LSAs, but the device still accepts summary LSAs from OSPF neighbors and oods them to
other neighbors. The device can form adjacencies with other routers regardless of whether summarization is enabled or disabled for
areas on each router.
When you enter a command or apply a Web management option to disable the summary LSAs, the change takes eect immediately. If
you apply the option to a previously congured area, the device ushes all of the summary LSAs it has generated (as an ABR) from the
area.
NOTE
This feature applies only when the device is congured as an Area Border Router (ABR) for the area. To completely prevent
summary LSAs from being sent to the area, disable the summary LSAs on each OSPF router that is an ABR for the area.
To disable summary LSAs for a stub area, enter commands such as the following.
device(config-ospf-router)# area 40 stub 99 no-summary
Syntax: [no] area { num | ip-addr stub cost [ no-summary ] }
The num and ip-addr parameters specify the area number, which can be a number or in IP address format. If you specify a number, the
number can be from 0 - 2,147,483,647.
The stub cost parameter
species an additional cost for using a route to or from this area and can be from 1 - 16777215. There is no
default. Normal areas do not use the cost parameter.
The no-summary parameter applies only to stub areas and disables summary LSAs from being sent into the area.
Assign a Not-So-Stubby Area (NSSA)
The OSPF Not So Stubby Area (NSSA) feature enables you to congure OSPF areas that provide the benets of stub areas, but that also
are capable of importing external route information. OSPF does not ood external routes from other areas into an NSSA, but does
translate and ood route information from the NSSA into other areas such as the backbone.
NSSAs are especially useful when you want to summarize Type-5 External LSAs (external routes) before forwarding them into an OSPF
area. The OSPF specication (RFC 2328) prohibits summarization of Type-5 LSAs and requires OSPF to ood Type-5 LSAs
throughout a routing domain. When you congure an NSSA, you can specify an address range for aggregating the external routes that
the NSSA's ABR exports into other areas.
The implementation of NSSA is based on RFC 1587.
Conguring OSPF
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04 235
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