DR & BDR communication

Unanswered Question
Feb 5th, 2009
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Dear all,

As DR & BDR listen on & All other routers listen on

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

Thanks in Advance.

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Overall Rating: 4 (3 ratings)
lejoe.thomas Thu, 02/05/2009 - 04:08
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Hi suryakant ,

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

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

Please note multicast hellos are always send to 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.



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 , so DR & BDR gets update.

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

Thanks & Regards,


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, (and others)

DR and BDR can listen not only to

if you do it on a DRother you will see (and others) but not

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

Hope to help


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

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



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




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