FB DISCUSSIONS: FINDING NSSA IN OSPF


Sumit Sharma

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OSPF Area Type : How to find NSSA ?
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1. What should be the approach to find if there is any NSSA area existing in the network or not ?

2. How to find the routers which are part of NSSA area type ?
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    Amitabh Dey Use the sh ip ospf database command and If we find any LSA type 7 in the LSDB it means nssa area is present and the n/w under lsa 7 will give which routers have that network … but If at the ASBR of the nssa area some filtering or route map has been configured then LSA 7 may not be created or if the ASBR = ABR for that area For these cases this method wont work Also bcoz the ABR of the NSSA converts the LSA 7 to LSA 5 and floods these LSA 5 to entire ospf domain so in the LSDB of other areas these LSA 5 if present signifies that they have come from some nssa Area
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    Sumit Sharma Amitabh Dey .. Bro .. this is a particular scenario, but if we have only OSPF in the network, so there will be no redistribution or LSA-7, even then we can have NSSA area type in the network. How to verify then ?
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    Sumit Sharma Amitabh Dey .. In case we have redistribution going on, then rite, LSA-7 to LSA-5 conversion, in other routers, we will get only LSA-5.
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    Sumit Sharma I got one idea, that might be useful.
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    Sumit Sharma In live network, in NSSA, most probably there will be ASBR’s present. Firstly, we can check how many ASBR’s are present in the network, using command: sh ip ospf border routers. From output, we can find the RID of ASBR’s.
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    Sumit Sharma After finding ASBR’s RID. we can use command, sh ip ospf database router < router id >. Most important thing is, using this command, it is not required to login to all the devices. We can login to any device & get details of any of the OSPF enabled device, we just need the RID.
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    Sumit Sharma 4507-MPLS#sh ip ospf database router 10.144.1.4
    Number of Links: 5
    Link connected to: a Stub Network(Link ID) Network/subnet number: 10.144.1.4

    (Link Data) Network Mask: 255.255.255.255

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    Sumit Sharma From above output, we can see, we are getting the details of device with RID, 10.144.1.4 . Point to note is we are getting the details of all the OSPF enabled interfaces, that, to which area type they are connecting. In our example, this is, Link connected to: a Stub Network.
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    Shankar Masoun There is one thing, I would like to add… i.e. We can run ‘sh ip route’ command on any router, if it is showing ospf routes with ‘N1 or N2’, it means these routes are coming from NSSA area.Note: This output will be shown in all areas apart from NSSS itself.
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    Arpan Sarkar Stub does not means that it is network type stub. It means an end leaf node of the area. In Lsa 1 as well you will be seeing stub routes, and that does not means stub network. Now my question is, when you do not have any external routes in your network there us no question of asbr. Nor you can identify it’s an nssa. Bcoz neither you have any external routes, nor there going to be any database of any external prefix. Offcourse you can stop Lsa 7 in nssa by area nssa no-redistribute. But it does not make sense. With this you should also generate a default information originate from that router. There you will see a default route of Lsa 7. Which can be identified as nssa.
    4 hrs · Unlike · 1
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    Sumit Sharma Absolutely .. I think .. if there is ASBR in the NSSA area type, we can picturize the network.
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    Sumit Sharma In case of , no ASBR, no redistribution, we are not getting things clear. We need to come up with some logical approach, without any large scale analysis of all the devices.
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    Arpan Sarkar That only I mentioned.. There is no point of nssa if there is no n1or n2 routes. Atleast default route of n1 or n2 should be there.
    4 hrs · Unlike · 2
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    Sumit Sharma So, if there is any NSSA present, then default route must be present, which has to be manually configured, ultimately we will see either N1 or N2 routes. This can be checked by show ip route simply.
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    Arpan Sarkar Yes..other than that if there is no redistributions in that area, then being nssa makes no sense to me..
    3 hrs · Unlike · 1
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