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OSPF Type 4 LSAs & NSSA

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

22 Replies 22

PETER EIJSBERG
Level 1
Level 1

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.

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.

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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.

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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.

Mohammed,

This is well summarized and describes the correct behavior.

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Harold,

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

BR,

Mohammed Mahmoud.

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?

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