CISCO CCNA 200-301 Q12

In the network exhibit, the routers are running OSPF and are set to the default configurations.

What would be the effect of configuring a loopback interface on Router A with an address of

A. Router B would become the DR
B. Router A would become the DR
C. Router C would become the DR
D. Router A would become the BDR

Correct Answers: B

Configuring a loopback interface on RouterA with an address of would cause Router A to become the designated router (DR). The designated router (DR) is determined by the router with the highest interface priority number. If the priority numbers are tied, 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. Changing the priority to 0 would make the router ineligible to become the DR or the backup designated router (BDR). The ip ospf priority # command is used to manually configure a priority on a specific interface.

Router IDs are determined first by the highest loopback IP address, followed by the highest IP address on an active physical interface. Thus, in the case of a priority tie, the router with the highest loopback IP address will have the highest RID, and will become the DR for the network segment.

The current Router ID for a router can be determined by executing the show ip interface brief command. In the sample output of the show ip interface brief command below, the RID will be

Router# show ip interface brief

Interface IP-Address OK? Method Status Protocol Ethernet0 YES NVRAM up up
Ethernet1 unassigned YES unset administratively down down Loopback0 YES NVRAM up up
Serial0 YES NVRAM up up Serial1 YES NVRAM up up Serial2 YES manual up up
Serial3 unassigned YES unset administratively down down

Neither Router B nor C will be the DR because the IP addresses on their physical interfaces are lower than Router A will not be the backup designated router. Since it is the DR, it cannot also be the BDR.
Router C will not be the BDR because its IP address is lower than that of Router B. Router B will be the BDR.