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LSA 7 – NSSA External LSA

LSA 7 is same as LSA 5 (External LSA) but for NSSA area. When we make any area in OSPF as NSSA, following changes will happen in that area :-
1. LSA 5 will be not be allowed in that area.
2. Since LSA 5 is not allowed, NSSA area will not need LSA 4 also. SO as a result LSA 4 is also not allowed.
But if LSA 5 is not allowed, how do ASBR in NSSA area passes information about outside routes (outside of OSPF domain routes), The answer to this hurdle is LSA 7. LSA 7 does the same thing as LSA 5 but it is limited to NSSA area. SO LSA 5 and LSA 7 has their similarity and differences.
How about checking LSA 7 in action. Remember our LAB topology – did you notice that we have not used R4 till now in our lab. Its time to spin up the all 4 routers of our topology.

So Area 3 will be our NSSA area. Below steps are perfomed in the lab :-
1. Make the AREA 3 as NSSA [use command area nssa 3 under router ospf 100] –
What will happen – as soon as both side NSSA area configured, R2 and R4 will be neighbors.
2. Adv and in area 3
R2 will send LSA 1 with information of its links to R2. R2 will populate them to R1 and R3 using LSA 3. 
3. Now lets make the R4 as ASBR , We configured Loopback 4 on R4 with IP add as and we redistributed connected routes.

R4(config)#int lo4
*Nov 21 17:51:37.403: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback4, changed state to up
R4(config-if)#ip add
R4(config-if)#router ospf 100
R4(config-router)#redistribute connected

What happens now :-
R4 will be ASBR in NSSA area 3. It will send the LSA 1 with E bit on to R2.
Since R4 has communicated the ASBR info to R2, it will send LSA 7 for route)
Now How R2 will treat this LSA 7? It will do a LSA 7 to LSA 5 translation as the P bit was set in LSA 7. Let check on R2

    Area 3
        Number of interfaces in this area is 1
        It is a NSSA area
        Perform type-7/type-5 LSA translation
        SPF algorithm last executed 00:14:05.985 ago
        SPF algorithm executed 9 times
        Area ranges are
        Number of LSA 9. Checksum Sum 0x052D79
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0

So now R2 will preform a LSA 7 to LSA 5 translation and will send LSA 5 to R1 and R3
So that was all about LSA 7.
What will happen if we disable the P bit on NSSA ASBR router R4? Let me know in comments

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