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New Member

Doubt in LSA translated from type 7 to type 5

Hi all,

Below is my topo:

R5--area1--R1--area0--R2---area0-R3

Area 1 is a NSSA.

R5 is an ASBR which advertises 100.0.0.0/8 and I am redistributing into ospf.

R3 is also an ASBR which advertises 100.0.0.0/8 and I am redistributing into ospf.

Since, R5 advertises type 7 LSA, it will be converted into type 5 LSA into area 0 by R1. My doubt is whether area0's database has both routes for 100.0.0.0/8 in its database ? (One translated by R1 and other advertised by R3)

Thanks,

Vijaybabu

10 REPLIES
Cisco Employee

Re: Doubt in LSA translated from type 7 to type 5

Vijaybabu,

The answer is yes. All routers in area0 will have both type 5 LSAs in their LSDB. One advertised by R1 and another by R3.

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
New Member

Re: Doubt in LSA translated from type 7 to type 5

Hi,

But in RFC2328 section 12.4.4.1,they have mentioned "if two routers, both reachable from one another, originate functionally equivalent AS-external-LSAs (i.e., same destination, cost and non-zero forwarding address), then the LSA originated by the router having the highest OSPF Router ID is used. The router having the lower OSPF Router ID can then flush its LSA. "

In my secnario, I am advertising 100.0.0.0/8 with same metric from R1 and R3, but the forwarding address is different. In that case, both LSAs should be in the database, rt ?

Thanks,

Vijaybabu

Cisco Employee

Re: Doubt in LSA translated from type 7 to type 5

Vijaybabu,

That is correct. RFC2328 clearly states that the destination, cost and non-zero forwarding address have to be the same for this rule to apply. So in your case the two type 5 LSAs should be present in the LSDB as the FAs are different.

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
Silver

Re: Doubt in LSA translated from type 7 to type 5

If you have different networks behind R3 and R5, you should use longer prefixes.

New Member

Re: Doubt in LSA translated from type 7 to type 5

Good explaination. But, I have a doubt. What is the significance of forwarding address in type 5 LSA?To reach all external networks, every router (in the topology given by Vijay) has to reach ASBR only. Then, what is there to do with forwarding adress? Does it play any key role?

Thanks,

Balajee

Cisco Employee

Re: Doubt in LSA translated from type 7 to type 5

Balajee,

The forwarding address does play a key role indeed. If it is set in the type 5 LSA then the reachibility of the external route is based on the reachibility of this forwarding address via an OSPF internal route.

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
New Member

Re: Doubt in LSA translated from type 7 to type 5

Hi ritter,

As per you explaination, I got this info ie OSPF routers use Forwarding address only to reach external networks, not ASBRs. right!

Thanks for the valuable info.

Thanks,

Balajee

New Member

Re: Doubt in LSA translated from type 7 to type 5

Hi ritter,

Why forward address is 0.0.0.0 in type LSA when the routes are originated from non-OSPF networks? Why it shouldnt be ASBR address itself?

http://cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml

Cisco Employee

Re: Doubt in LSA translated from type 7 to type 5

Balajee,

RFC2328 states the following:

"If the forwarding address is set to 0.0.0.0, packets should be sent to the ASBR itself."

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
Cisco Employee

Re: Doubt in LSA translated from type 7 to type 5

Correct.

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
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