Kiran, Delia,

The reason that this setup is not described anywhere is that it is
non-standard. It is uncommon for a router to support multiple
interfaces on the same subnet. If a router does support multiple
interfaces on the same subnet and the topology is broadcast then
the network (not R2) would need to assure that a broadcast/multicast
packet sent by R1 would be received by R3. The forwarding discussed
below is not expected (unless R2 is bridging).

Thanks,
Acee

Kiran S wrote:

> Hi Delia,
>
> Your network setup is R1, R2, R3 are in same broadcast domain (eg. in same
> VLAN and same subnet) and R1 and R3 are connected to the two differnet
> interfaces/ports of R2 and all belong to the same subnet a.b.c.0 right ?

>
>>Should DR and BDR mechanism be also applied to this scenario, that is R1,
>>R2, R3 would elect a DR and BDR for that subnet a.b.c.0/24.
>>
>
> A common DR and BDR election can be done only if R2 takes the multicast OSPF
> packets (eg. hello , DBD, LS packets ) from R1 and forwards to R3 and send
> to itself. Similiarly all the multicast packets from R3 are send to R1 and
> to itself. It means to make this a complete broadcast domain of same subnet
> for the OSPF routers you need to duplicate all the mulitcast OSPF packets at
> R2. If this is done, a common DR and BDR election is possible.
> we neednot duplicate the unicast OSPF routing packets It will get routed by
> L2 or L3 to the correct interface. right ?
> If we donot do this R3 will have only one neighbour (R2) and R1 will also
> have only one neighbour (R2). R2 will have two neighbours R1 and R3. And
> there will be either "two DRs" or "one DR" in this set up. (If R2 advertises
> as DR then R3 and R1 will be BDR. and if R2 is BDR then R1 and R3 will be
> DRs.)..
>
>
>>Therefore if R1
>>and R3 are the DR and BDR, then R2 would duplicate sending of PDUs to both
>>links, in order to reach both DR and BDR? In my opinion it should not. I
>>
> am
>
>>told that it should, as this would be a particular case of OSPF support
>>
> for
>
>>layer 2 switching.
>>
>
> how can R1 and R3 can become a common DR and BDR without even R2 duplicating
> the packets from R1 and from R3. ? R2 needs to duplicate the incoming OSPF
> mulicast routing packets from its interfaces/ports and forward to the other
> interfaces/ports of same subnet and send a copy to itself. The OSPF routing
> PDUs from R2 will naturally get forwarded to all the interfaces on the same
> subnet if it a multicast packet. So there is no need for any new duplication
> of the packets from R2 itself.
> Am I right ?
>
>
>>In my opinion it should not. I am
>>told that it should, as this would be a particular case of OSPF support
>>
> for
>
>>layer 2 switching.
>>
>
> Please let me know if you have a method to accomplish the common DR/BDR
> election without duplicating the packets at R2. Or do you feel there is no
> need for it ?
>
>
>>Is there some specification in an RFC about this?
>>
>
> I also don't find any RFC saying about this. If some body knows, it will be
> helpful.
>
> Thanks
>
> Cheers
> Kiran
>
>
> ----- Original Message -----
> From: "Delia Kecskemeti" <delia.kecskemetiWINDRIVER.COM>
> To: <OSPFDISCUSS.MICROSOFT.COM>
> Sent: Wednesday, October 23, 2002 3:51 AM
> Subject: DR / BDR election on different links configured as on same subnet
>
>
>
>>Hi,
>>
>>I need to find out how is OSPF supposed to act in this particular case:
>>Link between routers R1 and R2 and link between R2 and R3 are configured
>>
> to
>
>>be on the same subnet, say:
>>a.b.c.x / 24
>>In my understanding, the DR / BDR mechanism is applied on one broadcast
>>
> link
>
>>to which several routers attach, to minimize number of adjacencies and
>>network burden on that link.
>>
>>Should DR and BDR mechanism be also applied to this scenario, that is R1,
>>R2, R3 would elect a DR and BDR for that subnet a.b.c.0/24. Therefore if
>>
> R1
>
>>and R3 are the DR and BDR, then R2 would duplicate sending of PDUs to both
>>links, in order to reach both DR and BDR? In my opinion it should not. I
>>
> am
>
>>told that it should, as this would be a particular case of OSPF support
>>
> for
>
>>layer 2 switching. Is there some specification in an RFC about this?
>>
>>R1-----------R2-----------R3
>>
>>Thank you very much and have a nice day,
>>Delia
>>
>

--
Acee

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