Last Updated on August 7, 2021 by Admin 3
How is the designated router (DR) determined by OSPF on a multi-access network segment?
- The lowest interface priority, then the highest RID
- The highest interface priority, then the highest RID
- The lowest interface priority, then the highest OSPF process ID
- The highest interface priority, then the highest OSPF process ID
OSPF routers elect a designated router (DR) and backup designated router (BDR) on multi-access network segments in order to minimize the amount of update traffic sent between OSPF neighbors. All routers on multi-access network segment form adjacencies with the DR and BDR, but not with each other. Network events are communicated to the DR, and the DR distributes the event to the rest of the network.
The DR is determined by the router with the highest interface priority number. If the priority numbers tie (which will be the case if they are left to the default of 1), then the router with the highest router ID (RID) becomes the DR. The default priority number is 1, and can be configured as high as 255.
In many cases, it is desirable to intervene in this process and select the router you want to be the DR. If that is the case and the selected router is not becoming the DR for whatever reason, the following options are available to ensure that the selected router wins the election:
– Change the priority value of the router to a value higher than the other routers
– Set the priority value of the other routers to 0
– Create a loopback address on the selected router with an IP address higher than the IP addresses used on the other routers
Changing the priority to 0 makes the router ineligible to become the DR or BDR. The ip ospf priority # command is used to manually configure a priority on a specific interface.
It is also worth noting that a single OSPF area can have more than one DR. The election is NOT performed per area, but per network segment. So if you had six OSPF routers in area 0 with three in one IP subnet and three in another, there would be two elections, one for each segment.
The lowest interface priority does not determine the DR.
The OSPF process ID has no effect on DR elections.
Configure, verify, and troubleshoot single area and multi-area OSPFv2 for IPv4 (excluding authentication, filtering, manual summarization, redistribution, stub, virtual-link, and LSAs)