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Unable to understand redistribution in NSSA

shoaib sheikh
Level 1
Level 1

Hello everyone, while searching about question for CCNP route, i came across this site digitaltut.com.

Under ospf it has two question which are to me confusing.

Refer to the exhibit. Will redistributed RIP routes from OSPF Area 2 be allowed in Area 1?

Then answer is

A. Because Area 1 is an NSSA, redistributed RIP routes will not be allowed.

My question is RIP routes in area 2 will be as type 5, in area 0 they will be as type 5 and then in area 1 (as it is NSSA )it will be as type 7. But still we are getting RIP routes in area 1 as type 7.

The next question:

Refer to the exhibit. OSPF has been configured on all routers in the  network and Area 1 has been configured as a NSSA. Which statement is  true about the NSSA Area 1?

OSPF_Redistribute_IGRP_OSPF.jpg

A. Redistributed RIP and IGRP routes will appear in Area 1. They will be advertised via type 5 LSAs.

B. Only redistributed RIP routes will appear in Area 1. They will be advertised via type 7 LSAs.

C. Only redistributed IGRP routes will appear in Area 1. They will be advertised via type 7 LSAs.

D. No redistributed routes can appear in Area 1, only summary routes.

The answer is option C which is again confusing.

If answer is option C then why can't it be option B.

Please, correct me if i am missing something.

7 Replies 7

Harold Ritter
Cisco Employee
Cisco Employee

Hi Shoaib,

External routes from other area are not alowed in an NSSA area (or regular stubby area for that matter). NSSA does allow ASBR, in the area configured as NSSA, to redistribute and propagate routes from other routing protocols (including static routes). These redistributed routes are propagated as LSA type 7 and are translated into LSA type 5 at the ABR.

Regards

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
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Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Thanks for your reply Harold.

Just one question, According to your explanation the routers in area 1 will never be able to learn about subnets from RIP,

because external routes are not allowed from one area to NSSA area then how the routers in area 1 will be able to learn about RIP subnets.

And is there any specific reason for this behaviour of OSPF routing protocol ?

Your reply will be very helpful...

Hello Shoaib

The NSSA ABR can be configured to inject a default route into its NSSA, so routers in this area  will have access to the other areas redistributed routes

NSSA- ABR-

router ospf xx

area 1 nssa default-information-originate

res

Paul

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Thanks.


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Paul

Shoaib

The routers in area 1 will not be able to learn the RIP routes. These routers may be able to get to the networks that are in RIP but those networks will not appear in the routing table of area 1 routers. They can get to those networks because the routers in area 1 can have a default route which gets them to the Border router between area 1 and area 0. That router will have the RIP networks in its routing table and can forward to them. So area 1 routers can get to those resources but will not learn those specific routes.

This is a feature designed into OSPF. The purpose is to allow some OSPF routers to have Link State Database and IP routing table that is smaller and less complex. This reduces the processing tasks for the OSPF routers by excluding all external routes learned in different areas. The logic of this is that if the routers in area 1 can get to the RIP networks by using a default route to the Border router then why do they need to learn all of the details about the RIP routes?

HTH

Rick

HTH

Rick

Now i got the point.

Thanks Richard and pdriver for your valuable information on this topic.

antonio.guirado
Level 3
Level 3

Hello,

I send you a little diagram that I did:

Hello

plase rate any posts that may have been helpful

res

paul

Sent from Cisco Technical Support Android App


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Paul
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