In OSPF, Why BDR is Elected First ? with a Video Tutorial.

1. Initial State: When OSPF becomes active on a network, the Designated Router (DR) and Backup Designated Router (BDR) are initially unknown, set as 0.0.0.0. A wait timer starts based on the dead interval.
2. Neighbor Discovery: Routers send Hello packets with DR/BDR values of 0.0.0.0 until they learn about the DR/BDR from others. If a Hello packet is received with valid DR/BDR values, these are accepted, and the wait timer stops.
3. Wait Timer Expiry: If no DR/BDR information is learned before the wait timer expires, DR election begins.
4. Eligible Neighbors List: Create a list of neighbors eligible to be DR/BDR. This includes routers with a priority greater than zero.
5. Subset for BDR Selection: Exclude any routers that claim to be the DR from this list. From the remaining routers, select the one with the highest priority to be the BDR. If priorities are equal, use the highest Router ID (RID), loopback address, or IP address as tiebreakers.
6. DR Selection: From the original list (including routers claiming to be DR), select the router with the highest priority as the DR. Use RID, loopback, or IP address as tiebreakers if needed.
7. Promotion and Election: If no routers initially claimed to be the DR, promote the chosen BDR to DR. Elect a new BDR using the same selection method.

These steps ensure a stable and efficient election process for DR and BDR in an OSPF multi-access network setup.

The Backup Designated Router (BDR) is selected first to ensure a seamless transition in case the current DR fails. Here’s why:

1. Smooth Transition: By selecting the BDR first, there’s already a standby ready to take over as DR if the current DR goes down, minimizing network disruption.
2. Election Efficiency: Selecting the BDR first ensures that if no router initially claims to be the DR, the BDR can be promoted immediately, and a new BDR can be elected without restarting the election process.
3. Redundancy: Having a BDR ready provides redundancy, which is crucial for maintaining network stability and reliability.

This process helps maintain continuous communication and routing efficiency within the network.

The election process is structured to select the BDR first through these steps:

1. Neighbor List Creation: A list of neighbors with a priority greater than zero is created. These are eligible candidates for DR/BDR roles.
2. Subset for BDR: From this list, exclude any routers that claim to be the DR. This subset is used to elect the BDR.
3. BDR Selection: Within this subset, choose the router with the highest priority to become the BDR. If priorities are equal, use the highest Router ID (RID), loopback address, or IP address as tiebreakers.
4. DR Selection: After the BDR is chosen, use the original list of neighbors (including those claiming to be DR) to select the DR using the same criteria.

This method ensures that a BDR is always ready to step in, enhancing network resilience.

1.1.1.1

1.1.1.2

1.1.1.3 - BDR

1.1.1.4 - BDR > DR

1.1.1.1

1.1.1.2

1.1.1.3 - BDR

1.1.1.4 - DR

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