When Should We Use Stub/Totally Stub/Nssa/ Totally NSSA in OSPF?

[kmr_mc011]

Hello Friends

I need To Know Tha  When Should We Use Stub/Totally Stub/Nssa/ Totally NSSA in OSPF?

[dukenuk96]

Hello!

Well.. I prefer to answer such questions in a manner 'if you do not know what is it, so you do not need this'. Joke :) If you have a small network made of a couple routers which handle about tens of routes - special OSPF areas will be a little more than useless. But if your network growth, you will find them very usefeul to reduce memory and CPU usage on routers, thus improving convergence time. I won't write here about each OSPF area type in detail - you can find this info in the Internet, I'll just explain them shortly:

• stub - filter some prefixes/LSAs entering the area
• totally stub - filter even more prefixes/LSAs entering the area
• NSSA - filter some prefixes/LSAs entering the area, but allow external routes being generated from this specific area
• totally NSSA - filter even more prefixes/LSAs entering the area, but allow external routes being generated from this specific area

Plus - for each type of special area - default route will be generated by the ABR.

If you want to get some advice according usage of these areas in your own network - we can try to help you - share your topology and device's configurations.

[kmr_mc011]

Thanks

dukenuk96 

[rosaho]

This discussion has been reposted from Additional Communities to the WAN, Routing and Switching community.

[Joseph W. Doherty]

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Posting

They are all variations on a theme.  They limit the amount of external to the area routes you need to "slosh" about within the area.  When you want to go outside your local area, you're going to go to your ABR(s) or ASBR(s), so stubs tend to provide a method to do that using as little information as possible, often just a default route to an ABR.  Where they tend to be "problem" is when particular ABRs are a better egress to certain destinations than other ABRs.

BTW, a possible alternative to using stub areas is summarizing routes sent out via the ABR(s).  Or, you could also redistribute using just ASBRs.

[allen mert · Accepted solution]

Hi Naveen,

OSPF Areas are used mostly for filtering Routes between ABRs - ASBRs. In an Area, filtering is disabled(Type 1-2 LSAs are allowed only).

Stubby:               Dont Filter Type-3 LSAs & Filter Type-5 LSAs

Totally Stubby:    Filter Type-3 LSAs & Filter Type-5 LSAs

NSSA:                 Don't Filter Type-3 LSAs & Filter Type-5 LSAs

Totally NSSA:      Filter Type-3 LSAs & Filter Type-5 LSAs

Difference between NSSA/Totally NSSA and Stubby/Totally Areas is; NSSAs redistribute external routes(Type-7 LSA) into the Area., Stubbys don't. NSSA was created because Stubby areas block all Type-5 external LSAs. So Stubby Area can not learn any external LSAs. NSSA lets stub area to learn external Areas(Type-7 LSAs).

Type 7 LSA = Type-5 LSA , but Type-7 is for only external routes in Stubby Areas(like EIGRP,RIP Routes redistributed into OSPF)

Please rate if it helps..

Thanks,

Allen

[kmr_mc011]

Thanks allen mert ,& Joseph W. Doherty  It will help.