SwitchE is the root bridge for the network. The root bridge is the switch with the lowest bridge ID. The bridge ID is composed of a 2byte bridge priority and a 6byte Media Access Control (MAC) address. The bridge priority is considered first in the determination of the lowest bridge ID. The bridge priority can be set by issuing the spanning-tree priority value command, where value is a number from 0 through 65535? the default priority is 32768.

SwitchC and SwitchE both have a priority of 0. When two or more switches have the lowest priority, the switch with the lowest MAC address becomes the root bridge. MAC addresses are written in hexadecimal format. With MAC addresses, numbers are lower than letters and the hexadecimal value A is lower than the hexadecimal value F. Because SwitchE has a lower MAC address than SwitchC, SwitchE is the root bridge.

SwitchA is not the root bridge for the network, because it has the highest priority value, not the lowest priority value. Although link speed is somewhat relevant in determining the root port for a switch, link speed is irrelevant in determining the root bridge.

SwitchB is not the root bridge for the network; like SwitchA, SwitchB also has the highest priority value, not the lowest priority value. SwitchB contains redundant links to SwitchD, but redundant links are irrelevant in determining the root bridge. To avoid a switching loop, at least one of the redundant links between SwitchB and SwitchD will be blocked.

SwitchC is not the root bridge for the network. If the bridge priority of SwitchE were higher than 0, SwitchC would be the root bridge because a priority of 0 is the lowest configurable priority value.
SwitchD is not the root bridge for the network. Although SwitchD has the lowest MAC address on the network, the bridge priority is considered first in the determination of the root bridge. If all of the switches on the network had the same bridge priority values, SwitchD would be the root bridge because it has the lowest MAC address.