DR and BDR election

In the previous lesson, we have discussed the challenges of OSPF Multi-access network. The two challenges of the OSPF Multi-access network are unwanted adjacencies and the flooding of LSAs. The solution is the Designated Router (DR). DRs are designated to coordinate topology updates.

On multiaccess networks, At initialization, a Designated Router (DR), and a backup designated router (BDR) are elected. It is the collection and distribution point for LSAs sent and received. The BDR listens passively to this exchange and maintains a relationship with all the routers. If the DR stops sending Hello packets, the BDR promotes itself and assumes the role of Designated Router (DR).

When a topology change occurs, the update is sent to the Designated Router (DR). Then, the DR will update all other routers in the area. If the Designated Router (DR) not exists, each router would have to update its neighbour. If the routers are interconnected, this can direct to multiple unnecessary updates. Having a Designated Router (DR) will assist to eliminate this problem.

In the OSPF network router that is non-DR or non-BDR becomes DROTHER. DROTHERs only form full adjacencies with the DR and BDR in the network. The DROTHERs send their LSAs to the DR and BDR using the multicast address 224.0.0.6. 224.0.0.6 is the address of all Designated Router (DR) routers.

Now look at Figures 1, R1 has been elected as the designated router for the LAN. The number of adjacencies has been reduced to 3, which is 6 adjacencies without a Designated Router (DR).

DR/BDR elections only occur in multiaccess networks and do not occur in point-to-point networks.

Default DR/BDR Election Process

There are some criteria for a Designated Router (DR) and Backup Designated Router (BDR) election:

1. The first criteria for Designated Router (DR) and backup designated router (BDR) election are the interface priority. All OSPF enabled routers in the network elect the router with the highest interface priority as the DR and the router with the second-highest interface priority is elected as the BDR. The priority range is between 0 – 255. If the interface priority is set to 0, the router is not competent of becoming the DR. Serial interfaces have default priorities set to 0 therefore, they do not elect Designated Router (DR)and BDRs. The default interface priority of multiaccess broadcast interfaces is 1. Therefore, without configuring the interface priority, all routers have an equal priority value, so this method must be finished on a tie.

2. So If the first method is finished on tie due to same interface priority value. Then the router with the highest router ID is elected the DR and the router with the second-highest router ID is the BDR. We can configure the router ID manually. If we router ID is not configured, the heist loopback IP address has become the router ID of the router. If no loopback interfaces are configured, the router ID is determined by the highest active IPv4 address.
3. If we have running IPv6 network and there are no IPv4 addresses configured on the router, then the router ID must be manually configured, otherwise, the OSPFv3 does not start. The command for router ID configuration is “router-id rid”.

The DR and BDR election process starts as soon as the first router with an OSPF-enabled interface is active on the multiaccess network.  When the OSPF “network” command for the interface command is configured the process of the DR and BDR election takes place. If the OSPF enabled router powered on it also start the election process. The election process of DR and BDR only takes a few seconds.

Now lookit figure 2, all Ethernet router interfaces have a default priority of 1. Based on the above selection criteria, the OSPF router ID is used to elect the DR and BDR. R1 with the highest router ID becomes the DR; and R2, with the second-highest router ID, becomes the BDR.

DR/BDR Election Process

When a new router with a higher priority or higher router ID is added to the network after the DR and BDR election, this router does not take over the DR or the BDR role because those roles have already assigned and the new router does not initiate a new election process. After the DR is elected, it remains the DR until the DR fails or the OSPF process on the DR fails or is stopped and if the multiaccess interface on the DR fails or is shutdown.

When DR fails, the BDR is automatically got to the role of DR even if another DROTHER with a higher priority or router ID is added to the network after the initial DR/BDR election. So, when BDR is promoted to DR, a new BDR election process occurs and the DROTHER with the higher priority or router ID is elected as the new BDR. In Figure 2, the R3 with higher RID is selected as the DR and router with the second-highest RID is selected as the BDR.

In Figure 3 above, the current DR (R1) fails; therefore, the pre-elected BDR (R2) assumes the role of DR. Then, an election is held to choose a new BDR. R3 and R4 are the DROTHER, but the RID of R3 is higher than the RID of R4, so R3 is elected as the BDR.

In Figure 4, R1 and R2 both going fail, in this situation the existing BDR takes the role of DR and the only DROTHER R4 selected as the BDR.

In Figure 5, the R1 rejoined the network but it does not take over either role of DR and BDR instead, it becomes a DROTHER.DR (R3) and BDR (R4) retains the DR and BDR roles. R1 automatically becomes a DROTHER.

Verifying DR/BDR Roles

When routers are connected over a common multiaccess broadcast network, OSPF automatically elects a DR and BDR. In the above example in Figure1 the R1 is elected is DR and R2 is elected as BDR. Figure 6 to 9 illustrates the  “show ip ospf interface” command output for all routers.

In the example, R1 has been elected as the DR because the router ID of the R1 is the highest in this network. RW is the BDR because it has the second-highest router ID in the topology. The show ip ospf interface command can verify the OSPF router role.

Verifying DR/BDR Adjacencies

We can verify the OSPF neighbour adjacencies using the “show ip ospf neighbor” command. The state for neighbours adjacencies in the multi-access network are:

• FULL/DROTHER- This is a DR or BDR router that is fully adjacent with a non-DR or BDR router. Both neighbours can replace Hello packets, updates, queries, replies, and acknowledgements.
• FULL/DR- The router is fully adjacent with the indicated DR neighbour. The router can exchange Hello packets, updates, queries, replies, and acknowledgements with the DR.
• FULL/BDR- The router is fully adjacent with the indicated BDR router. Both can exchange Hello packets, updates, queries, replies, and acknowledgements.
• 2-WAY/DROTHER- The non-DR or BDR router has a neighbour adjacency with another non-DR or BDR router. Both can exchange Hello packets.

Usually FULL and 2-way both are the normal stat of OSPF router. If a router displays another state then FULL and 2-way, it is mean that there are some problems in forming neighbours adjacencies.

In multi-access networks, only DROTHERs form FULL adjacencies with the DR and BDR. It forms a 2-WAY neighbour adjacency with any other DROTHERs that join the network. When two DROTHER routers form a neighbour adjacency, the state displays as 2-WAY/DROTHER. Figure 10 illustrates the output of the “show ip ospf neighbours” command on all four routers.