DR and BDR in ospf process

[shadowman724]

hi,

what i understood about DR electing in a broadcast multiaccess network is:

-the router that has the highest priority is elected DR, the next is elected BDR,

-if the routers' priorities tie, the election is based on the router ID.

-the router ID is:

* the highest IP address of a physical interface(with no loopback set)

*the loopback IP address( if loopback set).

is what i understood true?

[farkascsgy]

If you have multiple Loopback interface the highest will be the RID and participate in the DR election.

Anyway you are right...

bye

FCS

Please rate me if I helped.

[shadowman724]

ok,

but what made me hesitate is that in a cisco academy e-lab, there are 3 routers in a broadcast multiaccess network. and their interfaces participating in OSPF have these IP:

192.168.1.1

192.168.1.2

192.168.1.3

the "show ip ospf neighbor detail" command on one router showed that RID 192.168.1.1 was the DR. However it is not the highest IP address( there are no loopback configured)

[sundar.palaniappan]

Hi,

It's the highest OSPF router ID and not the interface IP address that serves as the tie-breaker if the priorities are the same.

Here's the quote from CCO:

This is the OSPF priority that helps determine the DR and BDR on the network to which this interface is connected. Priority is an 8-bit field based on which DRs and BDRs are elected. The router with the highest priority becomes the DR. If the priorities are the same, the router with the highest router ID becomes the DR. By default, priorities are set to 1.

Hope that helps!

Regards,

Sundar

[globalnettech · Accepted solution]

Hello,

your findings are exactly right. In addition, have a look at the following explanations as well:

The OSPF protocol provides for automatic election of Designated Router (DR) and Backup Designated Router (BDR) on Broadcast and NBMA segments. There is no DR/BDR election on Point-to-Point and Point-to-Multipoint networks.

The OSPF interface priority is the parameter that controls DR election. Its default value is ?1?. When OSPF interface priority is ?0?, the router is not eligible to become a DR. If a router comes up on a network segment and there are no other routers there already, it will become a DR. If there is an existing DR (or BDR), the new router will NOT attempt to preempt them. If several routers come up at roughly the same time (or the DR fails), a new DR election process will be triggered. The router with the highest priority value then will become a DR.

In short, the DR/BDR election process is not deterministic and depends on the sequence of events. Therefore it is important to be able to prevent routers from EVER becoming DR/BDR, when that is appropriate for the topology in which they are connected. This is accomplished by setting their OSPF interface priority to ?0?:

ospf set interface priority 0

Regards,

GNT

[carl_townshend]

but if the prioritys are set to default, I was told it goes on the highest active ip address ?

[shadowman724]

if the ospf interface priorities tie, you then consider the routers' IDs. as i mentioned above, when the router ID is the criterion to choose a DR, you can to distinguish two cases:

1. if you did not configure any loopback address on the router, then his ID is the greatest IP address value of all of his interfaces(e.g between 10.1.1.1 and 10.1.2.2, the IP 10.1.2.2 is greater)

2.if you did configure any loopback address, then this address will be the router ID in the ospf process.

3.another case is, if you configure more than one loopback address, then the higher loopback IP value is the router ID.

hope this helps.

[Alex12]

1. Router priority

2. Router ID

3. LOOPBACK

4.ACTIVE Physical int.