OSPF Designated Router and Backup Designated Router

This video describes the OSPF Designated Router and Backup Designated Router. To fully understand why the Designated Router and Backup Designated Router are needed, let’s think about how OSPF would form adjacencies in a multiaccess network. In a broadcast network of 5 routers, there would be 10 adjacencies between the routers. There is actually a formula to calculate this. For n number of routers, the number of adjacencies is n(n-1)/2. The number of adjacencies quickly goes up as more routers are added to the network. Each router would flood LSAs to its adjacent router, which means there would be several copies of the same LSAs circulating in the network. This is unnecessary and a waste of resources.

Here’s where the Designated Router comes in. The DR has two jobs: 1) to represent the multiaccess network and its attached routers to the rest of the OSPF area, and 2) to manage the flooding process on the network. One DR is elected per network segment. A router might be the DR on one of its broadcast networks but not for another. In other words, the DR is a property of a router’s interface, not the entire router.

What if the DR fails? Simply, a new DR must be elected. But while this is happening, the network is unavailable to transit packets. To prevent this problem, a BDR is also elected. All routers form adjacencies not only with the DR but also with the BDR. If the DR fails, the BDR becomes the new DR. Because the other routers are already adjacent with the BDR, network unavailability is minimized.

How is the DR elected? First, each router interface on the network has a router priority between 0 and 255. The default priority on Cisco routers is 1. The router with the highest priority becomes the DR. Routers with a priority of 0 are not eligible to become DR or BDR. Second, if the priority values are the same, which by default they are, the router with the highest router ID becomes the DR. Note: the priority can influence an election, but will not override an active DR or BDR. That is, if a router with a higher priority becomes active after the DR and BDR have been elected, the new router will not replace either of them. Therefore, the first two DR-eligible router to initialize will become the DR and BDR.

Note that if only one eligible router is attached to a multiaccess network, that router will become the DR and there will be no BDR. Any other routers will form adjacencies only with the DR. If none of the routers attached to a multiaccess network are eligible, there will be no DR or BDR and no adjacencies will form.

Thank you for watching!

[teddyadonia]

The clearest explanation I've ever seen about DR/BDR relationship !
Thanks. Watching from ISRAEL.

[ahmode]

Thank you for this clear and easy to understand explination.

which app did you use in this video?

[jeromefeb]

Great content this was... Can you please upload video about OSPF stuck between different states.
Thanks :)

[enriquegabriel]

Amazing video, it helps a lot!. Thank you very much.

[РоманКлимовицкий]

How does the initial DR discover that it is a DR if it is the only one router in the network? I mean, is there any timeout that defines how much time we should wait for the answer to our hello message until we become a DR in our network?

Also, the way the DBR is re-elected is not quite clear.

Thank you

[JohnCena-vrrg]

Will loopback address always win over ip address for Router ID?. For instance, if Router 1 has a loopback address of 5.5.5.5 and an ip address of 192.168.91.2, will the Router ID be 5.5.5.5?

[mattjoe]

That's why you have 0 comments because your explanation is very confusing.