OSPF NSSA confusion !

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Jan 10th, 2008
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Hi, suppose i have a router RA that is connected to area 1 and area 0, RB is in area 1 and is redistributing rip routes in area 1 which i have configured to be area 1 nssa. On RB in ospf database i can see TYpe 7 LSA, where as on RA i see type 7 as well as type 5 LSA, why is this so ?? if i want to prevent type 7 LSA to be propagated to other areas then what i need to do ?

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s.arunkumar Thu, 01/10/2008 - 21:35
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Now thats the property of NSSA.Type 7 lsa is seen or is for only NSSA area only.At you abr between nssa and normal area this type 7 is converted into type 5 and propagted on other area of ospf domain..


the convertion will happen and lsa will be flooded,only u can prevent on other area router routing table not to install this routes via "distribution-list in" cmd..

Now there is a P bit in options of type 7 lsa .If this is set then convertion of type 7 to 5 happens.I dont know if that can be manipulated any way to stop the convertion..


lets see if anyother way for this 4rm others.. :)

arun



shrikar.dange Thu, 01/10/2008 - 22:03
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Hi,

In your senario Ra is an ABR connected to a NSSA.The property of the ABR connected to NSSA is to convert the type 7 LSAs genarated by an ASBR (RB in this case) to type 5.

Thats why you can see both LSA types (5 & 7 )on RA.Now by default type 7 LSA is only permitted in NSSA so it will not go beyond area 1 it will be translated into type 5.

Now in your case the ABR is connected to area 0 which is the back bone area so type 5 will be present in that area.You can use filtering between areas at ABR using prefix-list.But it is applicable to type 3 LSAs only.I dont have any idea how to prevent type 5 LSAS.


HTH,


regards,


shri :)

s.arunkumar Thu, 01/10/2008 - 22:07
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Hi jorge

i think the command would cause to suppress the forwarding address in type 7 lsa from specific address to 0.0.0.0 . This makes the forwarding address to the advertising address ,that is the converting abr itself.Mostly used when the forwarding address in type 7 lsa is not reachable..


lets hear what others says..

shrikar.dange Thu, 01/10/2008 - 22:19
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Hi,

I think the same as arun said.It will change the fwding address to the NSSA abr. the type 7 lsas will still be there in the form of type 5 lsas in other areas.

you can use area range command with not-advertise key word I am not sure though.


Any comments xperts?


Regards,


shri :)

s.arunkumar Thu, 01/10/2008 - 22:40
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gr8 shri..

u lead to the solution,but u messed up with command.Can use "summary-address " with "no-advertise" keyword.

area-range is for interarea routers ,ie type 3.

Hence anyway convertion will happen at abr,but we can stop propagating that type 5 into ospf domain..

shrikar.dange Thu, 01/10/2008 - 22:48
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Thankx arun.... :) Ur post deserves ratings so here they are. :)

Yes it should be summary address comand instead of area range.


But i wonder the type 5 lsas will be there in backbone area?It wont be present in other areas?Or we have to configure this on the NSSA area ABR then it wont be present anywhere. But what will be use of such configuration?


regards,


shri :)

s.arunkumar Thu, 01/10/2008 - 23:06
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Tks.. :),

As i understand External redistributed routes will not be advertise into the backbone area in this example,ie,no lsa 5 on backbone area after issuing the cmd at nssa abr. Note type 5 lsa is not classified under an area in ospf database..


Use is that just if u dont want external network to be reachable from area other than nssa..

but i dont know if u ask then whats the use of making nssa here then,as normal area is enough here.. ;).

shrikar.dange Fri, 01/11/2008 - 00:19
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Yes i kno that there will not be LSA type 5 present in BB area after issuing cmd @ NSSA abr.Anyways it was nice little conversation with you.


regards,


shri :)

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