Unanswered Question
Jun 4th, 2007
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right now i have one SWITCH and 3 routers connected with switch using Fast Interface. All routers are assigned with same subnet and in same subnet and in same VLAN.

As per OSPF default interface Network type is Broadcast, up to that all router form adjacency with each other but when i change the network type in one of the interface then neighbor adjacency goes down. so what will be the reason for that... if i will change it to the nonboradcast then it is the hello interval is there reason but when i changed it to the point to point then i was not able to find out the exact reason for that... so will you please help me to understand it...

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Richard Burts Mon, 06/04/2007 - 17:39
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There have been other discussions in the forum recently that relate to your question. The basics are that OSPF has different behavior depending on the network type defined on the interface (does the router use multicast hello on the interface to discover neighbors or not, when a neighbor is discovered should OSPF proceed immediately to form adjacency or should it first deal with election of DR and BDR, among others). On a particular subnet all OSPF routers should configure the same network type. If network type is different on some routers then as you have discovered OSPF neighbor relationship and adjacency are probably not formed correctly.



bvsnarayana03 Wed, 06/06/2007 - 02:56
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FOllowing are the parameters carried by the HELLO pkt of OSPF: Router ID, Area ID, Hello Intrvl, Router Priority, NetMask, DR, BDR, Neighbor ID & Auth key.

I can only see a variation in Hello Intrvl of OSPF pkt when you try change the netwrk type on interface.

minumathur Wed, 06/06/2007 - 03:05
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OSPF will do adjacisy with multilcast method, this method is using by Mesh network ( Etherner ) or point to point network , but in point to point ospf will automatically discover neighbor.

While non-broadcast method , OSPF is using unicast method , so both are differnt method , that's why you are loosing adjancy.

also Hello timer also important it shoud be match both side.


dangal.43 Wed, 06/06/2007 - 05:03
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what is mean by Multicast method? and will point to point and broadcast form an adjacency?

and about non broadcast method i think hello is the problem for establishing and unicast method is causing the problem.

Ivan Villagomez Wed, 06/06/2007 - 07:47
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If i remeber correctly when the network type is nb you need to specify the neighbor in the ospf proccess, in nbma the hellos can not reach the neighbor and they need to be converted to unicast before sending them.


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