In a multi-access Community, OSPF wants to pick out a DR (Designated Router) and BDR (Backup designated Router). LAN is the perfect instance of Multi-access Community kind when OSPF DR and BDR routers are chosen so that every one OSPF specking Routers in that LAN section have full OSPF Database.
To ship Routing data to a DR or BDR the multicast tackle of 224.0.0.6 is utilized by different Routers in the identical LAN. DR sends routing updates to the multicast tackle of 224.0.0.5. If DR fails, BDR takes over its function of distributing routing data.
Each router on a community section establishes a full neighbor relationship with the DR and BDR. Non-DR and non-BDR routers set up a 2-way neighbor relationship between themselves.
DR and BDR Choice Guidelines
- Router with the best OSPF precedence will change into a DR. Router with the second highest OSPF precedence or router ID will change into a BDR (By default, all routers have a precedence of 1)
- In case of a tie, Router with the best router ID wins the election.
Precedence performs a pivotal function in OSPF DR and BDR choice. Some Key takeaways on Router OSPF precedence are –
- The Precedence may be set on the Router Interface utilizing “ip ospf precedence” command.
- The default precedence is 1.
- A precedence of 0 means you’ll by no means be elected as DR or BDR.
- You must use “clear ip ospf course of” earlier than this transformation takes impact.
Additionally, it’s essential to grasp that OSPF DR BDR election is non-preemptive exercise i.e. even when a brand new OSPF Router with higher precedence or RouterID comes up on the section, nonetheless the brand new Router is not going to take DR function till the OSPF course of is cleared.
OSPF DR BDR Election: Instance State of affairs
Let’s perceive by way of the beneath situation intimately how OSPF DR BDR election course of takes place –
OSPF is configured on interface Fa0/0 of all of the Routers (R1 – R4) as per the beneath desk –
It’s assumed that every one OSPF configured Routers come up at identical time, the output on R1 and R4 will probably be as per snapshot beneath –
Router R1 output of “present ip ospf neighbor” exhibits that R4 is the DR
Router R4 output of “present ip ospf neighbor” exhibits that R1 because the DRother.
Now , lets see the change is OSPF DR election as soon as R1 precedence is modified to 255 as beneath –
As soon as the OSPF course of is cleared on all of the 4 Routers , now we see the output of “present ip ospf neighbor” command on R4 .
The above output of Router R4 exhibits that new DR is Router R1 since it’s got highest precedence of 255.
Proceed Studying:
OSPF N1 and N2 Routes: Configuration State of affairs
