DR & BDR communication

Unanswered Question
Feb 5th, 2009
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Dear all,

As DR & BDR listen on 224.0.0.6 & All other routers listen on 224.0.0.5.

If this is correct , That means DR send hellos on 224.0.0.5 , and as BDR listen on 224.0.0.6 ,then how BDR knows DR is Down when Dr goes down.


Thanks in Advance.

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lejoe.thomas Thu, 02/05/2009 - 04:08
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Hi suryakant ,




All OSPF routers continue to multicast HELLOS to 224.0.0.5, so they can keep track of their neighbors.


DROTHERs will send multicast updates to 224.0.0.6 (DR/BDR listen). DR will flood multicast updates to DROthers on 224.0.0.5.


Please note multicast hellos are always send to 224.0.0.5 and all OSPF routers [DR/BDR/DRother] listen to this address for hellos. So since everybody's listening it is easy to detect failure of any router, including your question, failure of DR by BDR.


HTH


Lejoe

suryakant.chavan Thu, 02/05/2009 - 04:58
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Hi Lejoe,

As any link on DROTHER goes down or comes up , DROTHER send multicast update to 224.0.0.6 , so DR & BDR gets update.

but if any links on DR (or BDR) goes down or comes up , it will send multicast update to 224.0.0.5, but BDR (or DR )listen for multicast update on 224.0.0.6 ,then how they get update


Thanks & Regards,

suryakant

Giuseppe Larosa Thu, 02/05/2009 - 05:59
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Hello Suryakant,


if you do


sh ip interface type x/y on DR and BDR


at the line Multicast reserved groups joined: you will see

224.0.0.5, 224.0.0.6 (and others)


DR and BDR can listen 224.0.0.5 not only to 244.0.0.6.


if you do it on a DRother you will see


224.0.0.5 (and others) but not 224.0.0.6


these are multicast addresses so BDR can listen to DR update sent with destination 224.0.0.5


Hope to help

Giuseppe



lejoe.thomas Thu, 02/05/2009 - 14:28
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Hi suryakant,


As I pointed out previously every ospf router listens on 224.0.0.5, regardless of whether is DR/BDR/DRother. So when DR floods an update to 224.0.0.5, even the BDR listens to that one.


HTH


Lejoe

Mohamed Sobair Thu, 02/05/2009 - 04:39
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Suryakant,


In OSPF, all routers when initially build the link states database they exchanged full routing updates and all routers have all information about OSPF topology based on the SPF tree algorithm.


Partial updates are sent when there is achange of the Network like a router failure,adding new router .. etc.


Full updates are also send every 30 minutes if no changes occured in the Network.


HTH

Mohamed

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