What is the role of DR and BDR in OSPF protocol?

Created Mar 29, 2016 20:40:40Latest reply Mar 29, 2016 21:28:30 1046 1 0 0

What is the role of DR and BDR in OSPF protocol?

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TomJ     Created Mar 29, 2016 21:28:30 Helpful(0) Helpful(0)

In Broadcast and NBMA networks, the routing information is transmitted between any two routers. If there are n routers in the network, it need to establish n * (n-1) / 2 adjacency. Any route change will result in multiple transfer and a waste of bandwidth resources. To solve this problem, OSPF protocol defines the DR (Designated Router). All routers only send information to the DR.
If the DR fails, the network router must re-elect DR and synchronize with the new DR. It will take a long time to re-elect DR. In order to shorten the process, OSPF introduce the concept of BDR (Backup Designated Router).
In fact, BDR is a backup of DR and is elected at the same time with DR. BDR establish adjacency and exchange route information with all the other routers. When the DR fails, the BDR will become the DR. The process is very short-lived, because adjacencies have been established. Although it still need to take a long time to re-election a new BDR, but it does not affect the calculation of the route. DR other router will no longer establish adjacencies and can’t exchange any routing information. And adjacencies will be reduced among routers on broadcast and NBMA networks.
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