How can I put a redistributed network on nssa ospf?

[Erico Verissimo]

Hi there!

The scenario is attached. I have a router CORE1, it is ABR router and I config some filters on it and area 282 as nssa. I configured two statics routers on router JBQ and redistrituted it on ospf process, but these prefixes do not arrive to the SW3750 (282 area). Please help me!

Thanks.

[Rolf Fischer]

Hi Enrico,

external routing information is allowed in NSSAs, but the originating router must have an interface in this NSSA. Router JBQ is an internal backbonerouter, in other words: It does not have an OSPF interface in area 282.

What you can do to make external networks generally reachable from inside a NSSA is configure the (NSS)ABR(s) to generate default routes:

*************************************CORE1***********************************

router ospf 100
 area 282 nssa default-information-originate [metric <cost>] [metric-type 1]

The scope of this default route is the NSSA only (Type-7 LSA), it is not propagated to other areas.

A "Totally" NSSA ('no summary' option) generates a default-route by default, for a "normal" NSSA its optional so you have to configure it if needed.

HTH

Rolf

[Erico Verissimo]

Hi Rolf,

Thank you for the tip, the default route was injected into route table on SW3750, but I need to put the prefix 24.24.24.0/24. This network is being advertised on JBQ by static route. Is it possible this prefix reach the route table in the SW3750?

Thank you!

[Rolf Fischer · Accepted solution]

One main characteristic of stub area types is that they block Type-5 external LSAs and replace them by default routes in order to reduce the amount of control traffic and memory requirements of internal routers. This also applies for NSSAs; and although they allow the injection of external routing information, this is only possible within the area.

SW3750 should be able to reach the 24.24.24.0/24 network via the default route. If you, for whatever reasons, want to inject the exact prefix in the NSSA, you'd have to configure static routes and redistribution on the (NSS)ABR(s) as well.

HTH

Rolf

[Erico Verissimo]

wow Rolf, that was so nice tip. I need to study more ospf, this characteristic is cool, so nssa block LSA type 5 and replace them by default. I configured a static router on ABR CORE 1 and redistributed them by ospf and the result in SW3750 was:

O N1    24.24.24.0 [110/21] via 192.168.1.2, 00:00:53, FastEthernet0/0

Thank you so much!!!

[Rolf Fischer]

You're welcome! And thanks for marking this thread as solved.

When you decide to use a stub area type instead of a regular area, it is normally because of exactly this properties. By doing this, you reduce the amount of protocol traffic and -data, but on the other hand you can loose some routing granularity when more than one ABR exist, which can result in suboptimal paths for some external destinations because the respective forwarding decisions now rely on default routes only.

[Erico Verissimo]

My goal is reduce the route table to optimize the memory of the router. In the real life this SW3750 have little space of memory and the ospf is the process that get high holding.

Thanks!