• From the NSSA into the backbone area
• From the backbone into the NSSA
If the ABR detects type-5 external LSAs on the backbone, it injects a corresponding type-7 LSA default route
(0.0.0.0/0) into the NSSA
You can also configure the NSSA ABR to do the following:
• Suppress advertising some or all of the area's summarized internal or external routes into the backbone area.
Configuring ranges on an ABR to reduce advertising to the backbone
on page 207.
• Replace all type-3 summary routes and the type-7 default route with the type-3 default summary route
(0.0.0.0/0.)
Virtual links are not allowed for NSSAs.
Stub area
This area connects to the AS backbone through one or more ABRs. It does not allow an internal ASBR, and does
not allow external (type 5) LSAs. A stub area supports these actions:
• Advertise the area's summary routes to the backbone area.
• Advertise summary routes from other areas.
• Use the default summary (type-3) route to advertise both of the following:
◦ Summary routes to other areas in the AS
◦ External routes to other ASs
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
• 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
on page 198.
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
Chapter 11 Open Shortest Path First Protocol (OSPF)
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