OSPF Type 4 LSAs & NSSA

[MARK HEUZENROEDER]

Hi,

1) Does NSSA allow Type4 LSAs inside the NSSA?

a. Doyle vol.1 2nd Ed. says Yes (P.389)

b. Cisco OSPF Not-So-Stubby Area (NSSA)_DocID 6208.pdf says no "There are NSSAs that block type 5 and type 4 LSAs,

but allow type 3 LSAs"

2) Does the NSSA ABR inject Type4 into Area 0?

a. I can?t find this in Doyle vol.1 2nd Ed. If someone has a page number I?d appreciate it.

Thanks, MH

[Harold Ritter]

Muggalla,

The forward adress on the type 5 LSA should point to the router generating the type 7 LSA, not the one translating the type 7 into a type 5. This is how the E1 can be calculated properly.

Regards,

[balajee]

Sorry for extending the discussion. But there is a certain gap in my view. If *forward address* in type 5 LSA is original NSSA ASBR address(which might be right), how routers in area 0(other than NSSA ABR) know the reachablity of NSSA ASBR? Dont they need type4 LSA?

Thanks,

Balajee

[GillieLucent]

Hi,

For a Type-5 LSA the forwarding address must specify an intra-area or inter-area path in the routing table. Since, NSSA ABR would have summarized NSSA networks into backbone area, definitely there will be an inter-area route to reach ASBR in the routing table.

HTH,

Thanks,

Vijaybabu

[balajee]

Hi Vijay,

Then what is the need of type 4 LSA in normal areas. Anyway, we have type 3 LSAs generated by ABR. right!

Thanks,

Balajee

[GillieLucent]

Hi,

Type 4 LSA is used to reach the ASBR. It is not for forwarding address, to reach forwarding address there should be an intra-area or inter-area path in the routing table.

Consider the below topology :

R5--R1--R2--R3--R4

R5,R1 and R2 in area 1

R2 and R3 in area 0

R3 and R4 in area 2

R5 is ASBR, R2 is ABR for area 1 and area 0, R2 is ABR for area 0 and area 2.

In above case R2( ABR) generates Type 4 LSA into backbone area and R3 (ABR) in turn generates type 4 LSA into area 2, so that R4 can reach the R5 (ASBR).

Now consider area 1 as nssa, in that case R2 (nssa-ABR) translates type 7 LSAs orginated by R5(nssa-ASBR) to type 5 LSA and advertises into backbone area. Since R2 advertises type 5 LSA, it then becomes an ASBR, there is no need of type 4 LSA in backbone area, since R2 and R3 are in same area. Now, for R4 to reach R2(newly become ASBR), R3 (ABR) originates type 4 LSA into area 2, so then R4 can reach R2.

But to reach forwarding address we just need ospf intra or inter-area route, which are calculated using type 1,2,3 LSAs.

Hope this helps.

Thanks,

Vijaybabu

[Harold Ritter]

Balajee,

The purpose of the type 4 LSA is to ensure all routers in the AS that are not directly connected to the ASBR area know whether the ASBR is available or not. If the ASBR goes down, the ABR for the area where the ASBR resides let the other areas know about it by sending a new type 4 LSA with MAX AGE value (3600) and all routers can therefore invalidate the type 5 LSAs generated by the ASBR.

Hope this helps,

[keiran_harris]

Hi all. Interesting thread. Somewhat old, but still interestingly still completely current (good to know a protocol like OSPF doesn't change that much). 

I too questioned the table on pg389 of Jeff Doyles 'Routing tcp/ip ed2 vol1' (Im going for my CCIE so have to dive deep), and now, thanks to this thread understand it to be wrong. So thanks everyone!

Regarding the numerous posts above, i believe all the confusion can be cleared up by a single rule to remember:

"Type4 LSA will never be advertised into the areas directly attached to the ASBR"  -me :) 

(As others have said, there is no need as that connected area already has 1st hand information direct from that ASBR)

- the rule is true for both

    1/ a NSSA area (where the ASBR within is issuing t7 LSAs, which the ABR, now also a ASBR converts to t5).... and

    2/ a backbone/nonstub area (where t5 is sourced by the ASBR directly)

- it is only downstream (non stub) areas connected off area0 that will need it, so the ABRs fronting those downstream areas will source t4 LSAs at this point to provide reachability to the remote ASBR. 

Hope this helps.

Keiran. 

PS- my scribles over the pg389 Doyles 'Routing tcp/ip ed2 vol1'  textbook are included. The below is the correct version for anyone who was wondering. He also left out a row, the "totally NSSA" option, accessed via the "no-summary" keyword in the area statement. Here it is: 

[williams-thomas]

Hope this helps. Technically a total Stub area will not allow TYP-3 external routes from other areas key word other areas. Because of the nature of this area, the true STUB routes in the area gets a default route from their ABR in it shows up in the form of OIA routes which are external correct but the default route was generated within the Area so technically it is not an external Type 3