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NSSA and default route

R2-(ospf10/NSSAArea10)-Pix-(ospf1/NSSAArea1)-R5-(ospf1/Area0)-R3

PIX is sending "area 5 nssa defaul-information-originate" as well as redistributing the connected routes.

Problem - I am recieving N2 Type-7 routes in R5 but R5 is not sending any NSSA routes to R3 as either type7 or type5. There are no filters on R3.

3 REPLIES
Cisco Employee

Re: NSSA and default route

Anthony,

This command is only to be used on an ABR. When this command is used on the ABR, it generates the default route in the NSSA area with the P-bit cleared so that the type 7 LSA is not translated by other ABRs, hence R5 not translating the default route into a type 5.

If you do a "show ip ospf da nssa-external 0.0.0.0", the options field should say "No Type 7/5 translation".

For more information on the usage of the P-bit, please refer to RFC3101.

http://www.ietf.org/rfc/rfc3101.txt?number=3101

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
Blue

Re: NSSA and default route

Anthony:

Of course Mr Ritter is 100% correct, but just to add a little something....

Not only will you see the No Type 7/5 translation, but you will also see a 0.0.0.0 forwarding address, which violates another requirement that type 7 LSAs need a non-zero forwarding address if they are to be translated into a type 5 LSA.

Moreover, that non-zero address must be internal to the OSPF domain, otherwise OSPF will not allow an external network with an external forwarding address to be placed in the routing tables of routers that sit behind the ABR. It does this to prevent routing loops.

I am sharing this with you because your problem reminded me of an issue I had like this that kicked my ass for a few hours before I figured it out using a Ciscopress book called "Troubleshooting IP Routing Prtocols." :-)

HTH

Victor

Cisco Employee

Re: NSSA and default route

Just some clarification. The "area nssa deafult-information originate" command can also be used on a non-ABR router but requires the router to already have a default route installed in the RIB from a source other than ospf.

Also, the type 7 LSA will not be translated to a type 5 LSA by the ABR even if the P-bit unless the fowarding address is also set.

See the following document for some additional rules and restrictions related to NSSA areas:

http://www.cisco.com/warp/customer/104/nssa.html

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
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