ā07-28-2016 08:19 AM - edited ā03-07-2019 12:17 AM
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
ā07-28-2016 10:40 AM
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
ā07-28-2016 10:58 PM
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
ā07-28-2016 11:25 PM
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
ā07-30-2016 05:18 AM
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
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