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DR/BDR election process

hello to all,

in DR/BDR election process each and every router which are eligible take part in election process...and they selecet one calculation router what is the property or crietarea to select that perticular router????

and other thing is that each and every router in that network send their RID in DR/BDR field of HELLO protocol or some router will put their ID for DR and some will put their id of BDR???

please clear my confusion ....



Hall of Fame Super Silver

Re: DR/BDR election process


When an OSPF router becomes active on an interface it sends out hello messages which contain its RID and its priority (assuming that it is an interface which will elect DR/BDR) and with the DR and BDR fields empty. As it receives hello messages from other routers and begins to form neighbor relationships, the routers compare priority. If there is a single router with highest priority then it is elected. If there is more than one router with highest priority then the routers compare RID among the routers with highest priority and decide the election based on the value of the RID.



Re: DR/BDR election process

As Jeff would say:

The election of the DR and BDR is triggered by the interface state machine. For the election process to function properly, the following preconditions must exist:

Each multi-access interface of each router has a Router Priority, which is an 8-bit unsigned integer ranging from 0 to 255. The default priority on Cisco routers is 1 and can be changed on a per multi-access-interface basis with the command ip ospf priority. Routers with a priority of 0 are ineligible to become the DR or BDR.

Hello packets include fields for the originating router to specify its Router Priority and for the IP addresses of the connected interfaces of the routers it considers the DR and BDR.

When an interface first becomes active on a multi-access network, it sets the DR and BDR to It also sets a wait timer with a value equal to the RouterDeadInterval.

Existing interfaces on a multi-access network record the addresses of the DR and the BDR in the interface data structure.

The election procedure of the DR and BDR is as follows:

1. After two-way communication has been established with one or more neighbors, examine the Priority, DR, and BDR fields of each neighbor's Hello. List all routers eligible for election (that is, routers with priority greater than 0 and whose neighbor state is at least two-way); all routers declaring themselves to be the DR (their own interface address is in the DR field of the Hello packet); and all routers declaring themselves to be the BDR (their own interface address is in the BDR field of the Hello packet). The calculating router will include itself on this list unless it is ineligible.

2. From the list of eligible routers, create a subset of all routers not claiming to be the DR (routers declaring themselves to be the DR cannot be elected BDR).

3. If one or more neighbors in this subset include its own interface address in the BDR field, the neighbor with the highest priority will be declared the BDR. In a tie, the neighbor with the highest Router ID will be chosen.

4. If no router in the subset claims to be the BDR, the neighbor with the highest priority will become the BDR. In a tie, the neighbor with the highest Router ID will be chosen.

5. If one or more of the eligible routers include their own address in the DR field, the neighbor with the highest priority will be declared the DR. In a tie, the neighbor with the highest Router ID will be chosen.

6. If no router has declared itself the DR, the newly elected BDR will become the DR.

7. If the router performing the calculation is the newly elected DR or BDR, or if it is no longer the DR or BDR, repeat steps 2 through 6.

Re: DR/BDR election process


first of all thanks for such a good reply...

sir but one more query....there is one calculating router is there??? so how that router is selected for the calculation....

and as per your reply all router put their interface id in DR and BDR both the how u creat the subset some router as they are participating in DR and other are for BDR???

please clear my that doubt...



Re: DR/BDR election process

Hello Devangs, I think you take a better look in how ospf works , there are some good examples on

But ok...

the election is done for every multi-access network (frame-relay , or ethernet , fddi, token).

it all begins with the hello packets being transmitted in the interfaces connected to those networks.

So you can even be a DR for 1 network and a BDR for another one!

the DR election is done automatically and unless you set the priority on the interface to 0, every router connected on the multi-access network (etherthet) will participate. The rest is up to the router ID, the higher wins the election , its quite simple.

Anyone I recommend Jeff Doyle book great resource on OSPF.