Unanswered Question
Dec 7th, 2011


For OSPF, how the network from different area and autonomous system ID connect to each other?  Apart from tunnel, is there any other way?

How about EIGRP and BGP?

I have this problem too.
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smitesh kharecha Wed, 12/07/2011 - 20:37


You mean between different area and different proccess ID ??

Simply redistribue OSPF process into each other.



ssweehinlew Wed, 12/07/2011 - 22:29

Yes. May I know howv to redistribute the OSPF process into each other? Can show me example? I need to know that concept.

smitesh kharecha Wed, 12/07/2011 - 23:11


If your Topo is something like R1 -- Area 1 --- R2 --- Area 2 --- R3.

router ospf 10

redistribute ospf 20 ?

  match        Redistribution of OSPF routes

  metric       Metric for redistributed routes

  metric-type  OSPF/IS-IS exterior metric type for redistributed routes

  route-map    Route map reference

  subnets      Consider subnets for redistribution into OSPF

  tag          Set tag for routes redistributed into OSPF

  vrf          VPN Routing/Forwarding Instance




NickNac79 Thu, 12/08/2011 - 01:32

Hi Ssweehinlew,

If you want to redistribute from one process into another you can do something like:

router ospf 1

  network area 0

  redistribute ospf 2 subnets

router ospf 2

  network area 0

  redistribute ospf 1 subnets

Is this what you wanted or was it something else you were looking for?

You must be very  careful however if you have more than one router doing this,  as without careful filtering you could end up redistributing a  redistributed route and end up in a routing update loop, which could  cause major routing issues.

Hope that helps,


Latchum Naidu Thu, 12/08/2011 - 01:57


See the below link for the same scenario.

When reditributing between different OSPF processes in multiple points on the network, it is possible to get into situations of subobtimal routing or even worse, a routing loop. To prevent this you need to you need to make the routes learned via OSPF 1 will always be preferred over external routes redistributed from OSPF 1 into OSPF 2

Please rate the helpfull posts.

NickNac79 Thu, 12/08/2011 - 02:11

Hi Naidu,

Yes, one must be preferred over the other, but you also have to prevent re-redistribution...


router ospf 1

  network area 0

  redistribute ospf 2 subnets tag 2 cost 1000 route-map NOTAG_1

  distribute-list route-map NOTAG in


router ospf 2

  network area 0

  redistribute ospf 1 subnets tag 1 cost 1000 route-amp NOTAG_2

  distribute-list route-map NOTAG in

route-map NOTAG_1 deny 10

  match tag 1

route-map NOTAG_1 permit 20

route-map NOTAG_2 deny 10

  match tag 2

route-map NOTAG_2 permit 20

route-map NOTAG deny 10

  match tag 1 2

route-map NOTAG permit 20


Latchum Naidu Thu, 12/08/2011 - 02:50

Hi Nick,

I am not sure about that, what I did exactly is as per the link I have provided in my post and it is working as desired.

Ssweehinlew, implement as suggested and come back to us if you face any trouble.

Please rate the helpfull posts.

NickNac79 Thu, 12/08/2011 - 04:10

Hi Naidu,

It depends greatly upon the subnets that are in use, their source (external / internal etc) and also if you are doing any summarisation.  For example if you have a summary route from one AS which happens to mask a route in the second AS, you can end up in a right mess without distribute and redistribute filters.

If your network is nice and clean, you can probably get away with just prefering one over the other - then the re-redistribution protection comes from tha fact that a route that you redistributed wouldn't make its way into your RIB.

Every network is different of course :-)



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