Which of the following switches will be selected as the root bridge? (Select the best answer.)
A. the switch with the lowest configured IP address
B. the switch with the highest configured IP address
C. the switch with the lowest MAC address
D. the switch with the highest MAC address
E. the switch with the lowest pathcost
F. the switch with the highest path cost
G. the switch with the lowest priority
H. the switch with the highest priority
Explanation:
The switch with the lowest priority will be selected as the root bridge. The bridge priority can be set by issuing the spanningtree priority value command, where value is a number from 0 through 65535; the default priority is 32768.
If multiple switches are configured with the same priority value, the switch with the lowest Media Access Control (MAC) addressis selected as the root bridge. MAC addresses are physical addresses that are written in hexadecimal format and are hardcoded into the switch. With MAC addresses, numbers are lower than letters and the hexadecimal value A is lower than the hexadecimal value F.
It can also be said that the switch with the lowest bridge ID is selected as the root bridge. The bridge ID consists of the 2byte bridge priority and the 6byte MAC address. Because the bridge priority is the first part of the bridge ID, a switch with the lowest bridge priority will also have the lowest bridge ID. For example, a switch with a bridge priority of 32768 and a MAC address of 1234.5678.9abc would have a bridge ID of 32768.1234.5678.9abc. If another switch had a lower priority value, it would become the root bridge, but if the priority values of the two switches were the same, the switch with the lower MAC address would become the root bridge.
Neither IP addresses nor path costs are used in the selection of the root bridge. The path cost, which is based on the bandwidth of a link, is used to determine the best path to the root bridge. The higher the bandwidth, the lower the cost. Spanning Tree Protocol (STP) uses the following path costs by default:
Reference:
Cisco: Configuring STP: SpanningTree Topology and BPDUs
Cisco: Spanning Tree Protocol Root Guard Enhancement