role of DR

Unanswered Question
Oct 2nd, 2008

Hi all

Am i right in saying the role of the DR in ospf is so that all the routers send updates to the DR, the DR then forwards them out to all routers? can anyone explain this a little further?

I have this problem too.
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Jon Marshall Thu, 10/02/2008 - 14:11

Carl

Yes you are correct. On an multi-access network such as ethernet there is a DR and a BDR. All other routers that have an interface on that sgement form an adjacency with both the DR and the BDR.

Any changes in the topology are then communicated to the DR and the DR then ensures that the LSA is then sent to all routers on that segment.

The concept of a DR/BDR cut down on the number of adjacencies and number of LSA's that need to be transmitted on a segment.

Jon

carl_townshend Fri, 10/03/2008 - 02:39

Hi There

So on an ethernet segment

do routers only form an adjacency with the DR and BDR?

But are they all neighbours of each other ?

please explain

Giuseppe Larosa Fri, 10/03/2008 - 09:43

Hello Carl,

OSPF terminology and concepts distinguish between neighborship and adjacency

neighborship:

all routers see each other OSPF RID listed in respective OSPF hellos:

they are in the same lan segment and see each other. this is called two way state in the OSPF neighbor state machine.

if none of the two routers is DR or BDR this is the final state

if one of them is DR or BDR : the OSPF hello packet has a field for the current DR RID and one for the current BDR RID so no doubt on this.

In this case the neighbor state machine moves forward other states until they reach the full state (db is in sync)

DR and BDR help to reduce the number of OSPF adjacencies to be built on the lan segment and optimize/minimize the flow of OSPF updates

OSPF DR and BDR are addressed using 224.0.0.6, while the all OSPF routers address is 224.0.0.5

Hope to help

Giuseppe

carl_townshend Sun, 10/05/2008 - 09:30

hello there

I am a little unsure of how you explained it here,

do all routers form neighbours, but are not adjacent, are they only adjacent with the dr/bdr? please explain

Giuseppe Larosa Sun, 10/05/2008 - 12:03

Hello Carl,

yes it is so.

OSPF two way state = OSPF neighbors

details in previous post: means they see each other in hellos

OSPF full = OSPF Fully adjacent

details in previous post: DB in sync

you can see this on the

sh ip ospf neighbors

in a lan segment where there are 4 OSPF routers.

Without DR/BDR concept you need a full mesh of OSPF adjacencies:

N*(N-1)/2 = 4(4-1)/2 = 6 adjacencies

With DR/DBR you need two adjacencies for every router that is not DR or BDR and the adjacency between DR and BDR

2*(N-2) + 1 = 5

you can see that as N the number of routers increases the advantage of the DR/BDR approach becomes bigger and provides scalability.

Hope to help

Giuseppe

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