OSPF DR/BDR Election Manipulation- quick review

DR/BDR is very important part of Broadcast and Non-Broad cast multi-access OSPF network types and it is needed in an Area in order to get LSA1 from OSPF routers and rely LSA2 to the OSPF routers about the network information for all OSPF routers, please know that the below information is my notes that i have been writing down from my study for CCIE.

DR/BDR on OSPF is determined per interface level so you can have for each vlan/interface different DR/BDR from the other

You can increase priority for the DR by setting #ip ospf priority 255 [the maximum is 255] and [0] means the router or the

interface will not participate in election.

Choosing the DR is by priority, highest router id, highest loopback ip, highest physical interface ip

Choosing the DR/BDR is only on broadcast and non-broadcast network types on the OSPF

NOTE: preemption is not supported, so any device need to wait for the DR to fail until it can take over.

NOTE: if no router declared itself as DR then the router will say that I am the DR/BDR

You can check DR/BDR election process by #debug ip ospf adj    [and shutdown the DR router and monitor the debug messages]

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: Neighbor change event

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: DR/BDR election

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: Elect BDR 150.1.1.1

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: Elect DR 150.1.1.1

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: Elect BDR 0.0.0.0

*Jul  1 17:43:06.361: OSPF-1 ADJ   Et0/0: Elect DR 150.1.1.1

On the hub/spoke the Full state will be with the DR only, so it must be HUB configured as DR in order each spoke will form

 full adjacency with it.

Because spokes are not active to talk OSPF between each other so the spoke will form full adjacency with DR and the BDR only.

If spoke becomes DR, the OSPF database will be broken and the routing will be incomplete.

That’s why you need to make sure all spokes priority are set to 0

NOTE: both DR/BDR will receive LSA1 in Area but only DR will rely the information back to the rest of the network. So if R5 was BDR but not DR, the OSPF DB will be broken

Because for example: R2 will send LSA1 to R5 and R4 but only R4 will be able to reply the information to all the routers but it's not the HUB so the process will fail.

NOTE: when you here the OSPF DB is broken, think about the DR location in the Network.