OSPF — Designated Router and Backup Designated Router

The DR and BDR election process

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Why should a DR and BDR be elected

This election is done ONLY in multi-access networks.

( Multi-access networks are networks that have multiple routers connected through a switch — Shown below )

A sample multi-access network

Without DR and BDR

Without having a Designated Router and a Backup Designated Router

• Every router will be flooding LSA’s to the connected routers
• This leads to unnecessary LSA traffic and overhead in the netowrk and routers

Therefore, a DR and BDR are elected.

The Election Process

• DR and BDR are elected according to the OSPF priority of the router.
• The router with the highest priority will be the DR and the second-highest priority will be the BDR.
• By default, the priority of all routers is 1.
• The priority can be changed according to the network admin’s requirement, but if priority is not changed, (which means that all the router will have the same priority) the following criteria will be used to get different priority values for routers.

1. Router ID
2. Loopback interface
3. IP address

• The router with the higest priority will be the DR
• The router with the 2nd highest priority will be the BDR

Example Scenario 1:

• Configured priorities (Not default)
• Router IDs configured
• IP addresses configured
• Loopback address configured

Example Scenario 1

According to scenario 1, the priorities are set and the election will be done based on the priorities.

Router 3 has the highest priority and Router 2 has the 2nd highest priority.

Thus, Router3 = DR and Router2 = BDR

Example Scenario 2:

• Default priorities
• Router IDs configured
• IP addresses configured
• Loopback address configured

Example Scenario 2

According to scenario 2, the priorities are not set therefor the election process will look for the Router IDs.

Router2 has the highest Router ID and Router1 has the 2nd highest Router ID.

Thus, Router2 = DR and Router1 = BDR

Example Scenario 3:

• Default priorities
• No Router IDs configured
• IP addresses configured
• Loopback address configured

Example Scenario 3

According to scenario 3, the priorities are not set and the Router IDs are also not set. Therefore the election process will look for the IP addresses.

Router1 has the highest IP address and Router0 has the 2nd highest IP address.

Thus, Router1 = DR and Router0 = BDR

Example Scenario 4:

• Default priorities
• No Router IDs configured
• No IP addresses configured
• Loopback address configured

Example Scenario 4

According to scenario 4, the priorities are not set , the Router IDs are not set and also the IP addresses are not assigned. Therefore the election process will look for the Loopback addresses.

Router0 has the highest Loopback address and Router3 has the 2nd highest Loopback address.

Thus, Router0 = DR and Router3 = BDR

With DR and BDR

After DR and BDR election

• All the routers in the multi-access network will only send LSA’s to the DR.
• The DR will then update the other router about the information in the LSA’s sent.
• By using this method, LSA’s will not be flooded to everyone but only to the Designated Router (DR).
• This reduces the LSA flooding and the Overhead.

In case the DR fails, the elected BDR will be the new DR of the topology and a new BDR will be elected.

Just Like that the OSPF LSA flooding problem in the Multi-access network is solved.