Alcatel Carrier Internetworking Solutions 060187-10 REV D Not-So-Stubby-Areas, Totally Stubby Areas

OSPF Overview Configuring OSPF

page 1-12 OmniSwitch 6600 Family Advanced Routing Configuration Guide March 2005

Not-So-Stubby-Areas

NSSA, or not-so-stubby area, is an extension to the base OSPF specification and is defined in RFC 1587.

An NSSA is similar to a stub area in many ways: AS-external LSAs are not flooded into an NSSA and

virtual links are not allowed in an NSSA. The primary difference is that selected external routing informa-

tion can be imported into an NSSA and then redistributed into the rest of the OSPF routing domain. These

routes are imported into the NSSA using a new LSA type: Type-7 LSA. Type-7 LSAs are flooded within

the NSSA and are translated at the NSSA boundary into AS-external LSAs so as to convey the external

routing information to other areas.

NSSAs enable routers with limited resources to participate in OSPF routing while also allowing the import

of a selected number of external routes into the area. For example, an area which connects to a small exter-

nal routing domain running RIP may be configured as an NSSA. This will allow the import of RIP routes

into this area and the rest of the OSPF routing domain and at the same time, prevent the flooding of other

external routing information (learned, for example, through IP) into this area.

All routers in an NSSA must have their OSPF area defined as an NSSA. To configure otherwise will

ensure that the router will be unsuccessful in establishing an adjacent in the OSPF domain.

Totally Stubby Areas

In Totally Stubby Areas the ABR advertises a default route to the routers in the totally stubby area but

does not advertise any inter-area or external LSAs. As a result, routers in a totally stubby area know only

the routes for destination networks in the stub area and have a default route for any other destination

outside the stub.

Note. Virtual links cannot be configured through totally stubby areas.

The router memory is saved when using stub area networks by filtering Type 4 and 5 LSAs. This concept

has been extended with Totally Stubby Areas by filtering Type 3 LSAs (Network Summary LSA) in addi-

tion to Type 4 and 5 with the exception of one single Type 3 LSA used to advertise a default route within

the area.

The following is an example of a simple totally stubby configuration with Router B being an ABR

between the backbone area 0 and the stub area 1. Router A is in area 1.1.1.1, totally stubby area:

Totally Stubby Area Example

Note. See “Configuring a Totally Stubby Area” on page 1-20 for information on configuring Totally

Stubby Areas.

Router A Router B

OSPF Area 0

192.168.12.1

OSPF Area 1

Totally Stubby 192.168.12.2

192.168.50.0/24

OmniSwitch 6648