Contents
How does NSSA create default route?
Default Route in NSSA. There are two ways to have a default route in an NSSA. When you configure an area as NSSA, by default the NSSA ABR does not generate a default summary route. In the case of a stub area or an NSSA totally stub area, the NSSA ABR does generate a default summary route.
How OSPF injects a default route into a stub or totally stub area?
The ABR injects a default route into the area and all the routers belonging to this area use the default route to send any traffic outside the area. To define a totally stub area, use the OSPF router configuration command, area stub no-summary, on the ABR.
What is the difference between stub area and NSSA?
An OSPF stub area has no external routes in it, so you cannot redistribute from another protocol into a stub area. A not-so-stubby area (NSSA) allows external routes to be flooded within the area. These routes are then leaked into other areas. However, external routes from other areas still do not enter the NSSA.
Which type of LSA will be restricted by OSPF stub area?
If you configure an area as stub it will block all type 5 external LSAs. All the prefixes that you redistributed into OSPF from another routing protocol are not welcome in the stub area. Since you are not allowed to have type 5 external LSAs in the stub area it’s also impossible to have an ASBR in the stub area.
What is a stub area?
A stub area is an area in which you do not allow advertisements of external routes, which thus reduces the size of the database even more. Like stub areas, they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs and instead rely on default routing to external destinations.
What type of LSAs are blocked in the totally stubby area?
A stub area that only allows routes internal to the area and restricts Type 3 LSAs from entering the stub area is often called a totally stubby area. You can convert area 0.0. 0.3 to a totally stubby area by configuring the ABR to only advertise and allow the default route to enter into the area.
When to inject a default route into NSSA?
A default route is injected into the NSSA totally stub area as a type 3 summary LSA. There are situations where there is no need to inject external routes into the NSSA as type 7. This situation usually occurs when an ASBR is also an NSSA ABR. When redistribution takes place in this scenario, the router generates type 5 as well as type 7 LSAs.
How does the not so stubby area ( NSSA ) feature work?
Introduction. The OSPF not-so-stubby area (NSSA) feature is described by RFC 1587 and is first introduced in Cisco IOS ® Software release 11.2. It is a non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area. This document explains how the NSSA feature works.
Can a Type 3 LSA be injected into an NSSA?
No type 3 or 4 summary LSAs are allowed in Area 1. This means no inter-area routes are allowed in Area 1. A default route is injected into the NSSA totally stub area as a type 3 summary LSA. There are situations where there is no need to inject external routes into the NSSA as type 7.
How is NSSA boundary router propagated into OSPF?
An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain. The network diagram demonstrates this principle. Refer to this network diagram as you use this document: In the network diagram, Area 1 is defined as a stub area.