OSPF DR and BDR Elections
As a CCNA / CCNP candidate you are expected to understand the purpose and function of the OSPF protocol.
The CCNA / CCNP’s exam will ask of you to determine the purpose of the DR and BDR along with the election procedure.
OSPF Designated Quality Cisco Switches and Routers and Backup Designated Quality Cisco Switches and Routers elections
OSPF routing algorithm is CPU intensive. Each and everytime the OSPF process runs the Dikstras shortest path first algorithm it will have an impact on the Quality Cisco Switches and Routers. In a point to point network OSPF events are limited to the periodic Hello packet (Default 10secs on a PPP, HDLC, 30sec Frame Relay NBMA), 30 mins for each route entry in the Link state database and triggered events.
In a Multi-access broadcast network where Quality Cisco Switches and Routerss are connected to a common broadcast segment i.e many Quality Cisco Switches and Routerss in one vlan) all Quality Cisco Switches and Routerss would need to create an adjacency with all other Quality Cisco Switches and Routerss in the same broadcast domain. Also event which causes an LSA to be generated would require the advertising Quality Cisco Switches and Routers to create one LSA to be multicasted to all neighbours.
The more Quality Cisco Switches and Routerss there are in the broadcast multicast network the more adjacencies would be required:
Two Quality Cisco Switches and Routerss require the following number of adjacencies:
2(2–1) / 2 = 1
Four Quality Cisco Switches and Routerss require the following number of adjacencies:
4(4–1)/2=6
Ten Quality Cisco Switches and Routerss require the following number of adjacencies:
10(10–1)/2=45
Note: DR’s and BDR’s are not elected on point-to-point links because there can be only one adjacency and the point of electing the DR and BDR is for effiecienty
The more Quality Cisco Switches and Routerss you have on a segment will increase the number of adjacencies consume more bandwidth and use more processing power on the Quality Cisco Switches and Routerss
We need to reduce the amount of work which each Quality Cisco Switches and Routers has to do on the next by electing one Quality Cisco Switches and Routers as a Designated Quality Cisco Switches and Routers which all other Quality Cisco Switches and Routerss with form adjacencies with.
When you use a Designated Quality Cisco Switches and Routers in your Multiaccess broadcast network the number of adjacencies is reduced along with the reduction in processing and bandwidth.
All Quality Cisco Switches and Routerss will form full adjacencies with the Designated Quality Cisco Switches and Routers, when you run the command on a cisco Quality Cisco Switches and Routers which is not the DR:
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
You will see that your Quality Cisco Switches and Routers has formed a “FULL” neighbour adjacency with the DR and the output also defines which neighbor is the DR. The Quality Cisco Switches and Routers ID of the neighbour in the output above is 192.168.1.3 and that this neighbour is the DR
Any events which occur are advertised to the DR using the Multicast address of 224.0.0.6
When the DR advertises any events to the Non-DR devices it is done using the address of 224.0.0.5
Designated and Backup Designated Quality Cisco Switches and Routers Election
The DR is elected based on the Quality Cisco Switches and Routers with the highest Quality Cisco Switches and Routers priority. The
