Foundry Networks Switch and Router Installation And Configuration Manual page 585

Configuring OSPF
Assign a Totally Stubby Area
By default, the Layer 3 Switch 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 configuring the Layer 3 Switch to stop sending
summary LSAs (type 3 LSAs) into the area. You can disable the summary LSAs when you are configuring the
stub area or later after you have configured the area.
This feature disables origination of summary LSAs, but the Layer 3 Switch still accepts summary LSAs from OSPF
neighbors and floods them to other neighbors. The Layer 3 Switch 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
effect immediately. If you apply the option to a previously configured area, the Layer 3 Switch flushes all of the
summary LSAs it has generated (as an ABR) from the area.
NOTE: This feature applies only when the Layer 3 Switch is configured 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.
This feature does not apply to Not So Stubby Areas (NSSAs).
To disable summary LSAs for a stub area, use the following CLI method.
USING THE CLI
To disable summary LSAs for a stub area, enter commands such as the following:
BigIron(config-ospf-router)# area 40 stub no-summary
Syntax: area <num> | <ip-addr> [nssa <cost> | stub <cost> [no-summary]]
The <num> | <ip-addr> parameter specifies the area number, which can be a number or in IP address format. If
you specify an number, the number can be from 0 – 2,147,483,647.
The nssa parameter specifies that this is an NSSA. For more information about configuring NSSAs, see "Assign
a Not-So-Stubby Area (NSSA)" on page 17-11.
The <cost> specifies an additional cost for using a route to or from this area and can be from
1 – 16777215. If you configure a stub area or NSSA, you must specify the cost. 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.
NOTE: You can assign one area on a router interface. For example, if the system or chassis module has 16
ports, 16 areas are supported on the chassis or module.
USING THE WEB MANAGEMENT INTERFACE
You can configure a stubby area using the Web management interface, but you cannot disable summary LSAs for
the area. You must use the CLI to disable the summary LSAs.
Assign a Not-So-Stubby Area (NSSA)
The OSPF Not So Stubby Area (NSSA) feature enables you to configure OSPF areas that provide the benefits of
stub areas, but that also are capable of importing external route information. OSPF does not flood external routes
from other areas into an NSSA, but does translate and flood 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 specification (RFC 2328) prohibits summarization of Type-5 LSAs
and requires OSPF to flood Type-5 LSAs throughout a routing domain. When you configure an NSSA, you can
specify an address range for aggregating the external routes that the NSSA's ABR exports into other areas.
December 2000 17 - 11