OSPF DR

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Oct 1st, 2008
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Would this be a problem...

All routers area 0

All network types broadcast

There is FR so there are Serial links set at betworktype Non-broadcast


There is a DR for the Broadcast network type

and there is a DR for the Non-broadcast type. Thus there are 2 DRs in area 0. Is this a problem. I can get around it via making the network types Point-to-Multipoint thus no second DR is elected. But do i even have to do that?

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Richard Burts Wed, 10/01/2008 - 08:43
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Michael


You seem to assume that having multiple DR in an area is a problem. That is not the case.


DR is elected per segment (per subnet) and is not per area. So it is normal to have multiple DR in area 0 and it is not a problem at all.


HTH


Rick

mikegrous Wed, 10/01/2008 - 09:29
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DR is elected per segment (per subnet) and is not per area.


That one statement Cleared up everything. Thank You.

Giuseppe Larosa Wed, 10/01/2008 - 08:47
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Hello Mike,

the DR concept is link specific: on different links L3 subnets you have different DRs.

No problem in having multiple DRs in the same area.

The DR is the OSPF router responsible to represent the multi-access segment.

What really counts is that all devices connected to the same ip subnet / multiaccess segment must agree on the ip ospf network type to avoid problems.

So you have a DR on each LAN segment and a DR on each FR link that uses main interface or a point-to-mulipoint subifs.

Serial interfaces with HDLC or PPP encapsulation and FR point-to-point subif are seen as point-to-point OSPF networks and no DR is elected


So you don't need to worry about this is normal OSPF behaviour.


Hope to help

Giuseppe


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