Totally NSSA Issue

[Manu Shankar]

Based on the attached topology I configured area 2 as Totally NSSA. So area 2 will receive default route from R3 and R6 (ABRs).

R9 (EIGRP) is advertising a default route to R7. So why that default route is not showing as LSA type 7 in R7?

Thanks,

Manu 

[Rolf Fischer]

Hi,

as far as I can see, there is no 'area 2 nssa default-information-originate' under R7's OSPF process.

Remember that Cisco IOS doesn't allow you to redistribute a default-route into OSPF using the redistribute command.

HTH
Rolf

[Manu Shankar]

Hi Rolf, 

Thank you for the comment.

I checked it again and found where the issue is. Here R7 is getting a default route from EIGRP and redistributing in to OSPF, so the AD is 170. Since we are configured Area 2 as totally NSSA, R7 is receiving default route from R6.  AD is 110. So that route is adding in to the routing table.

If you configure the Area 2 as just a NSSA and add the command area 2 nssa default-information-originate in R7, it will redistribute the default route and send to rest of the ospf domain. 

Thanks, 

Manu

[Manu Shankar]

The question still remains, if we configure area 2 as totally NSSA how we will inject default route in to Area 0, is there any possibility? 

Thanks,

Manu 

[Rolf Fischer]

Manu,

If you configure the Area 2 as just a NSSA and add the command area 2 nssa default-information-originate in R7, it will redistribute the default route and send to rest of the ospf domain.

The question still remains, if we configure area 2 as totally NSSA how we will inject default route in to Area 0, is there any possibility?

From RFC 3101 2.5:

Else if the destination is a Type-7 default route (destination ID = DefaultDestination) and one of the following is true, then do nothing with this LSA and consider the next in the list:
(...)

•   The calculating router is a border router and is suppressing the import of summary routes as Type-3 summary-LSAs.

To understand the reason for this behavior, have a look at this simple diagram:

[external domain]----[ASBR]----[IR]----[NSSABR]--- (...)

                       <------NSSA--------> <-----Backbone----->   

The routing-table of the ASBR has a non-OSPF default-route pointing to the external routing domain. It originates a Type-7 LSA for the default-route into the NSSA with the P-bit set (7-to-5 translation allowed). The internal router (IR) and the NSSABR will install this default-route pointing to the ASBR and the NSSABR will also originate a Type-5 LSA for this default-route into the Backbone Area.

Now if you change the area type to a Totally NSSA (no-summary keyword on the NSSABR), the NSSABR originates a Type-3 Summary default-route, removes the Type-7 default route from the local routing table (as per RFC) and therefore stops originating a default-route into the Backbone Area.

What if the NSSABR wouldn't remove the Type-7? The internal router (IR) would install the Type-3 pointing to the NSSABR, whereas the NSSABR would still have the Type-7 pointing to the IR; in other words: A routing loop.

HTH
Rolf