Avaya Configuring IP, ARP, RARP, RIP, and OSPF Services Manuel d'utilisateur télécharger pdf (Page 40)

Configuring IP, ARP, RARP, RIP, and OSPF Services

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308627-14.20 Rev 00

Each area has a unique ID number. (ID 0.0.0.0 is always reserved for the OSPF

backbone.) The AS in Figure 1-6

has the following four areas:

• Backbone area: 0.0.0.0

• Normal areas: 0.0.0.1 and 0.0.0.4

• Stub area: 0.0.0.2

• Not-so-stubby area (NSSA): 0.0.0.3

For information about associating an OSPF interface with an area ID, see

“Changing the Area ID” on page 6-21.

Stub Area

A stub area does not import AS external routes and may or may not import

internal route summaries. In place of routes to destinations outside the stub area, a

border router connected to a stub (R8 in Figure 1-6

) injects a default route

advertisement. When an internal router encounters a datagram addressed to a

destination outside the stub area, the router forwards it to the border router

specified in the default route advertisement.

Not-So-Stubby Area (NSSA)

A not-so-stubby area (NSSA) is similar to a stub area except that, to a limited

degree, an NSSA can import AS external routes. Like a stub area, an NSSA

consumes less memory and CPU resources by preventing the flooding of type 5

AS external link state advertisements (LSAs) into the area and by relying on

default routing to external destinations. However, unlike a stub area, an NSSA can

import external routes into an OSPF routing domain. You can also configure

type 7 address ranges to allow NSSA border routers to control the summarization

of external routes for the NSSA.

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Avaya Configuring IP, ARP, RARP, RIP, and OSPF Services Manuel d'utilisateur Page 40