OSPF Network

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Jun 7th, 2007
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One LAB question. I have one switch and two router. both the routers are connected with switch. Initially i configure OSPF and it works fine. then i change the network type of one of the router interface to point to point. Now here both the router showing each other as their neighbor but in routing table i dont have the information of each other network. so what will be the cause of this thing?

they build the neighbor ship.... but not exchanges routes....

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Harold Ritter Thu, 06/07/2007 - 12:07
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This is normal behavior. The configuration mistake will not prevent the adjacency from coming up but the spf will not be able to calculate any path through this specific neighbor since its LSA and ours don't share a common link.

if you do a "show ip ospf da ro x.x.x.x" for the specific neighbor, you should see the following at the very beginning of the router LSA:

Adv Router is not-reachable

Hope this helps,

sundar.palaniappan Thu, 06/07/2007 - 12:21
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Just curious did the behavior change with the newer IOS or what? If my memory serves correct with a 12.2 code the neighbor wouldn't come up if the network type is point-to-point on one end and the other end is set to broadcast/non-broadcast.

I am just wondering why does the neighbor have to come up when they can't exchange any useful information between them when the network types are mismatched as it is in this case.



Harold Ritter Thu, 06/07/2007 - 12:37
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As far as I know, it has always been this way. I think the reason that the neighbor might not have come up at somepoint might have been due to timers difference and not to network type as such.

Th network type is not exchanged while the adjacency is coming up, so there is really no way to validate the proper setup on adjacent side of a link. Maybe an IETF proposal ;-)

Hope this helps,

Edison Ortiz Thu, 06/07/2007 - 12:44
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Broadcast and Point-to-Point OSPF network types will be adjacent since they have the same default hello and dead interval (10 and 40).

However they will not exchange LSAs since they are incompatible OSPF network types.

One OSPF network type (Broadcast) relies on the election of the DR (Designated Router) while the other network type (Point-to-Point) does not.

Harold Ritter Thu, 06/07/2007 - 12:47
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The LSAs will indeed be exchanged. It is the SPF that will fail to build a coherent topology given that these two neighbors don't share a common link from a logical standpoint.


dangal.43 Thu, 06/07/2007 - 13:16
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yes you are correct in debug you will see the same hello and dead interval as well as same multicast transmission of it.

but one thing i found is the router with the broadcast network type show that neighbor with "FULL/DR or BDR" status, where as router witht he point to point shows neighbor status with "FULL/-" status.

so what are the exact reason for not updating LSA? only incompatible OSPF network type.

If the one neighbor is Point to point then it should have atleast exchange of update with the one router which it discover very fist and all other routes should be reachable via that discovered router.....

any comments?

Harold Ritter Thu, 06/07/2007 - 13:22
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One sees its neighbor via a pt-to-pt link (without a LSA type 2) and the other through a broadcast link (with a LSA type 2). They each desperately try to find a valid neighboring link in each others router LSA but just can't.

The two routers absolutely can't construct a coherent topology given these circumstances.

Hope this helps,

Harold Ritter Thu, 06/07/2007 - 13:52
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Just as a precision, I should have said that they will be unable to create a coherent topology using that specific link. The topology can still be built but with this specific link being excluded.

My apology for the confusion,


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