OSPF DR & BDR question??

[bava_ccna]

Hi ,

I have a doubt about ospf behaviour in Broadcast multiaccess network. Why do we require DR & BDR in ospf broadcase multiaccess network???

I read a CCIE book says that DR/BDR are used to avoid flooding of LSA. Is that true? OSPF is using muticast for sending/receving LSA. Is there any other valid reason behind the DR/BDR concept?

Thanks

Bava

[cadet alain]

Hi,

I read a CCIE book says that DR/BDR are used to avoid flooding of LSA

Yes that is the role of the DR.The BDR is there in case the DR fails so it will take over the DR role and a new BDR is elected.

Regards.

Alain.

[Thotsaphon Lueangwattanaphong]

Hi,

   I read a CCIE book says that DR/BDR are used to avoid flooding of LSA.  Is that true?

   A: It's true

   Ref: http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t20

  OSPF is using muticast for sending/receving LSA.

   A: It's not true. OSPF uses both unicast and multicast to send "hello packets" and link state updates(LSAs).

   Ref: http://en.wikipedia.org/wiki/Open_Shortest_Path_First

HTH,

Toshi

[Richard Burts]

Bava

DR and BDR were introduced into OSPF as a way to increase efficiency and to reduce flooding of LSAs. Perhaps it will help if we think a bit about how the DR reduces flooding of LSAs. Let us start by reviewing a fundamental concept of OSPF which is that in establishing a neighbor relationship OSPF neighbors must be sure that their link state data bases are in synch. They do this by exchanging the content of their link state data base.

So consider the example of a broadcast multiaccess segment with 10 routers running OSPF. Now each of 10 routers has 9 neighbors to which it must send LSAs. If 10 routers exchange with 9 neighbors that is 90 exchanges (or maybe you may say 45 adjacencies if you consider an adjacency as a 2 way relationship). Still 45 is a fairly large number.

Now consider what happens with DR in OSPF. Each router on the segment must exchange with the DR and must exchange with the BDR, but no longer must exchange with all neighbors.

So with DR/BDR 8 routers do only 2 exchanges while 2 routers do 9 exchanges. So with DR/BDR there are 34 exchanges instead of 90 (or 17 instead of 45 if you divide by 2).

In the case of a multi access broadcast segment with only 2 or 3 neighbors it does not change much. But as the number of neighbors gets larger the use of DR provides a very good efficiency.

HTH

Rick