Re: OSPF understanding

Hi Nadeem,

I don't think you should look at this as a 224.0.0.6 thing... But if it
helps you understand...

OSPF hellos are sent with a TTL of 1. R5 would not forward any hello to R4.
So therefore you wont have more than one neighbor on all the routers except
for the hub which would have 4 neighbors, right?

This explains why R5 can chose R4 as the DR, all other routers which are not
even aware of R4 as a nieghbor in the first instance cannot possibly use it
(R4) as the DR or even BDR. As far as R1 - R4 are concerned, the only ospf
neighbor they have is R5.

I'm sure other GS members would be able to explain better though...

HTH

Bobola

On Thu, Feb 11, 2010 at 9:14 PM, Nadeem Rafi <nrafia_at_gmail.com> wrote:

> Thanks for input...
>
> I know, this is bad design and the reason is also obvious as you stated.
> But i am interested in how this is happening in back end. As i stated
> before, DR/BDR is property of a interface, an interface can be only DR/BDR
> for an interface/segment. Then how it come can be DR/BDR at the same time?
> Please can you shed some light on how 224.0.0.6 address is
> announced/selected?
>
>
>
> On Thu, Feb 11, 2010 at 11:51 PM, Bobola Oke <okebobola_at_gmail.com> wrote:
>
>> Hi Nadeem,
>> This is a frame relay thing....
>>
>> The other routers cannot connect to R4 on layer2 so they use the only
>> available mapping they've got which is R5
>>
>> They can only get to R4 when ospf is already running and they need a DR to
>> do so which has to be R5. But R5 has a mapping to R4 so it can use that as
>> its own DR. Hope you understand this.
>>
>> In real life, this is bad network design though. When running hub and
>> spoke, you only want your hub to be DR.
>>
>> HTH
>>
>> Bobola Oke
>>
>>
>>
>> On Thu, Feb 11, 2010 at 7:49 PM, Nadeem Rafi <nrafia_at_gmail.com> wrote:
>>
>>> Hi GS,
>>>
>>> working with OSPF HUB and spoke, R5 being hub, R1 - R4 being spokes
>>> (using
>>> non-broadcast network type, all routers using physical interfaces).
>>> Spokes
>>> dont have direct frame-relay mappings with each other, but through hub.
>>> Made
>>> some experimental changes in this way.
>>>
>>> 1- make R5 DR by using priority
>>> 2- R1-R3 set ip ospf priority 0
>>> 3- R4 set priority lower than R5 but non 0.
>>>
>>> R5 becomes DR and R4 BDR, every thing works fine, other three spokes get
>>> R5
>>> as DR.
>>>
>>> Now cleared the R5 OSPF Process and R4 Become DR and R5 elected as BDR.
>>> Vriefied both on R4 and R5 by show ip ospf int command.
>>>
>>> BUT , other three spokes, R1-3 are still identifying R5 ad DR. Why? As
>>> for
>>> as i know, DR/BDR is proprty of interface not router. When R5 is BDR with
>>> R4
>>> on same interface then how come it can be DR for other routers on same
>>> interface?
>>> Interestingly, Database identifies correct DR, but show ip ospf neighour
>>> command don't reflect it correctly.
>>>
>>> Any input will be of great value.
>>>
>>> Best Regards,
>>>
>>> Nadeem Rafi
>>>
>>>
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Received on Thu Feb 11 2010 - 21:32:36 ART