KB.15.15

You can configure the stub area ABR to do the following:

• Suppress advertising some or all of the area's summarized internal routes into the backbone

area.

• Suppress LSA traffic from other areas in the AS by replacing type-3 summary LSAs and the

default external route from the backbone area with the default summary route (0.0.0.0/0.)

Virtual links are not allowed for stub areas.

OSPF RFC compliance

The OSFP features covered in this guide comply with the following:

• RFC 2328 OSPF version 2

• RFC 3101 OSPF NSSA option (s/w release K.12.xx and greater)

• RFC 1583 (Enabled in the default OSPF configuration. See the following Note.)

• RFC 4750 MIB variables.

NOTE: If all of the routers in your OSPF domain support RFC 2178, RFC 2328, or later, you

should disable RFC 1583 compatibility on all routers in the domain. See “Changing the RFC 1583

OSPF compliance setting” (page 141).

Reducing AS external LSAs and Type-3 summary LSAs

An OSPF ASBR uses AS external LSAs to originate advertisements of a route to another routing

domain, such as an RIP domain. These advertisements are

• Flooded in the area in which the ASBR operates

• Injected into the backbone area and then propagated to any other OSPF areas (except stub

areas) within the local OSPF AS. If the AS includes an NSSA, there are two additional options:

• If the NSSA includes an ASBR, you can suppress advertising some or all of its summarized

external routes into the backbone area.

• Replace all type-3 summary LSAs and the default external route from the backbone area

with the default summary route (0.0.0.0/0.)

In some cases, multiple ASBRs in an AS can originate equivalent external LSAs. The LSAs are

equivalent when they have the same cost, the same next hop, and the same destination. In such

cases, the HP switch optimizes OSPF by eliminating duplicate AS external LSAs. That is, the ASBR

with the highest router ID floods the AS external LSAs for the external domain into the OSPF AS,

while the other ASBRs flush the equivalent AS external LSAs from their databases. As a result, the

overall volume of route advertisement traffic within the AS is reduced and the switches that flush

the duplicate AS external LSAs have more memory for other OSPF data.

This enhancement implements the portion of RFC 2328 that describes AS external LSA reduction.

This enhancement is enabled by default, requires no configuration, and cannot be disabled.

OSPF RFC compliance 195