Re: OSPF understanding

Bobola, yours is a great explanation - it gets straight to the point which
is that if the TTL is 1 and the hops needed are 2 then the OSPF packet won't
make it to the supposed DR. Well done.

-- 
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Jared Scrivener
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On Fri, Feb 12, 2010 at 7:32 AM, Bobola Oke <okebobola_at_gmail.com> wrote:
> 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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