OSPF Type 4 LSAs & NSSA

Unanswered Question
Aug 1st, 2007
User Badges:

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

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
Loading.
PETER EIJSBERG Wed, 08/01/2007 - 21:57
User Badges:

A type 4 LSA describes the route to an ASBR in another area and is created by the ABR. A stub area will not accept external routes from ASBRs in other areas (LSA type 5) so there is no need for any knowledge about ASBRs (type 4) either. So I would expect that a stub area only has type 1,2,3 LSAs.

A NSSA will allow ASBRs in it's own area, and when those type 7 LSAs cross the ABR into the backbone area, the ABR will create a Type 4 LSA as well, yes.

mohammedmahmoud Thu, 08/02/2007 - 05:34
User Badges:
  • Green, 3000 points or more

Hi,


I agree with Peter, NSSA shouldn't have Type4 LSAs, NSSAs block type 5 and accordingly type 4 LSAs, as Type4 LSA describes the route to an ASBR in another area, which NSSA blocks, however when the NSSA ABR converts the Type 7 to Type 5 before advertising it to the backbone, the ASBR summary LSAs (type4) are not needed in this case because the ABR originates the external LSA, and the ABR is reachable within area 0.


HTH,

Mohammed Mahmoud.

Harold Ritter Thu, 08/02/2007 - 06:14
User Badges:
  • Cisco Employee,

Peter,


You are absolutely correct about the NSSA not carrying type 4 LSAs as they are simply not needed.


Just one precision regarding your last comment. The ABR connected to the NSSA takes the type 7 LSAs and converts them into type 5 LSAs, which makes it an ASBR as well. Therefore this ABR doesn't generate a type 4 LSAs for itself. The type 4 LSA is rather generated by the other ABRs connected to other areas (not stubby or nssa).


Hope this helps,

MARK HEUZENROEDER Thu, 08/02/2007 - 23:15
User Badges:

Hi,


Thanks for your replies.


Some further clarification, please.


However, both hritter and peijsberg say the type 4 is generated by the ABR (in another area).

However this book,

CCIE Routing and Switching Off. Exam Cert. Guide_2nd Ed

says on P.284

?the ASBR creates both an LSA type 4 for itself and a type 5 LSA. Both types

of LSAs are flooded throughout the OSPF domain, including being forwarded by ABRs?

Also on P.284, Figure 10-9

- Step 1 clearly shows the Type 4 sourced from ASBR RID=7.7.7.7.

- The ABR can be seen *forwarding* the Type 4 (from ASBR) but changing the Metric to be the cost from ABR to ASBR.


On P.285,

R1 (the ABR) announces this cost in the *forwarded* LSA type 4 that describes a host route to reach ASBR with RID=7.7.7.7.


so it says the ABSR *creates* the Type 4 and the ABR *forwards* the Type 4.

Is this different to what hritter and peijsberg are saying?


As I understand that the purpose of the Type 4 is to enable a router in another Area to be able to calculate the Metric for an E1 external route. I understand that NSSAs have the concept of N1 (includes OSPF domain Costs) and N2 (does NOT include OSPF domain costs) external routes injected into an NSSA and that these are similar in metric contribution logic to E1 & E2.

Hence by the same logic, could it be possible that a NSSA ASBR generates both a Type 7 (which the ABR translates to Type 5 for injection into Area 0) and a Type 4, so that by the time the N1 from the NSSA ASBR gets converted to an E1 in Area 0, there is enough metric information in an area other than the NSSA, to know the Cost to the NSSA ASBR and be able to calculate the E1 metric for the external route?


Thanks for your help.


MH

mohammedmahmoud Thu, 08/02/2007 - 23:46
User Badges:
  • Green, 3000 points or more

Hi Mark,


IMHO, when the ASBR in a NSSA generates the Type7 LSA it doesn't need to generate a type4 LSA, as Type7 LSA is only significant inside the NSSA area itself, further on when the LSA reaches the ABR, and the ABR decides to convert it into Type5, now also there is no need for Type4 LSA as the ABR is working as if an ASBR and it is connected to the backbone Area and thus no Type4 LSA required.


HTH,

Mohammed Mahmoud.

Harold Ritter Fri, 08/03/2007 - 05:00
User Badges:
  • Cisco Employee,

Mark,


"the ASBR creates both an LSA type 4 for itself and a type 5 LSA. Both types"


There seems to be some confusion concerning this behavior as the ASBR should not generate a type 4 LSA for itself, no matter if the ASBR injects an E1 or E2 in the AS.


The fact that the type 4 LSA is generated by the ABR doesn't prevent the proper metric calculation to the ASBR, as the ABR is in the same area as the ASBR and it therefore has first hand information on the ASBR.


Hope this helps,


MARK HEUZENROEDER Fri, 08/03/2007 - 16:18
User Badges:

Hi hritter,


I can understand that the ASBR does not create a Type 4.


Following on from your comment,

"The fact that the type 4 LSA is generated by the ABR"

- so are you saying a NSSA ABR generates a Type 4 into Area 0.


Regards, MH

Harold Ritter Fri, 08/03/2007 - 17:51
User Badges:
  • Cisco Employee,

Mark,


What I meant the non-nssa ABR generates the type 4 LSA in non-backbone areas. The nssa ABR in this case really acts as an ASBR as it originates the type 5 LSA.


Hope this helps,

GillieLucent Fri, 08/03/2007 - 23:01
User Badges:

Hi,


So, when a nssa ABR converts type 7 LSA to type 5 LSA , it will become an ASBR, so it will advertise router LSAs with ASBR bit set into backbone area, isn't ?



Thanks,

Vijaybabu

Harold Ritter Sat, 08/04/2007 - 10:50
User Badges:
  • Cisco Employee,

Vijaybabu,


That is correct. That is why routers in area 0 (and other routers in non-backbone areas directly connected to this ABR/ASBR) do not need a type 4 LSA.


Regards,

mohammedmahmoud Sun, 08/05/2007 - 01:08
User Badges:
  • Green, 3000 points or more

Hi Harold,


Just to recap the whole discussion (and please correct me if i am wrong), in NSSA (Type 4 is of no use inside an NSSA) when the ABR converts the Type 7 LSA to Type 5 (and now the ABR is an ASBR itself), and since that the ABR of the NSSA belongs also to Area 0 then no Type 4 LSA is required in area 0, but however for this Type 5 LSA to enter to other Areas the ABRs of these certain Areas are responsible of creating the Type 4 LSA to be injected to these areas in order for the original ASBR to be reachable inside the other areas.


BR,

Mohammed Mahmoud.

Harold Ritter Sun, 08/05/2007 - 11:11
User Badges:
  • Cisco Employee,

Mohammed,


This is well summarized and describes the correct behavior.


Regards,

mohammedmahmoud Sun, 08/05/2007 - 11:22
User Badges:
  • Green, 3000 points or more

Harold,


Thank you very much for being a professional reference here :)


BR,

Mohammed Mahmoud.

balajee Mon, 08/06/2007 - 04:15
User Badges:

I have a small doubt here. If NSSA ABR generates type 5 LSA with its own ip address as forward address, how other routers calculate E1 and E2 metric?

Harold Ritter Mon, 08/06/2007 - 05:38
User Badges:
  • Cisco Employee,

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 Mon, 08/06/2007 - 21:58
User Badges:

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 Tue, 08/07/2007 - 00:39
User Badges:

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 Tue, 08/07/2007 - 01:53
User Badges:

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 Tue, 08/07/2007 - 02:51
User Badges:

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 Tue, 08/07/2007 - 06:24
User Badges:
  • Cisco Employee,

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 Thu, 04/10/2014 - 20:31
User Badges:
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: 
 

Actions

This Discussion