Which switch will become the root bridge?

View the following network diagram:

Which switch will become the root bridge?

A.
SwitchA

B.
SwitchB

C.
SwitchC

D.
The root bridge cannot be determined from the given information.

Explanation:
SwitchA will become the root bridge. The bridge ID, also known as the switch ID, is used to elect the root bridge
in a redundant network topology. The bridge ID has two components:
Switch’s priority number: Configured as 32768 on Cisco switches by default
Switch’s Media Access Control (MAC) address: The burnt-in hardware address of the network interface
card
The switch with the lowest bridge ID is selected as the root bridge. If the same priority number is configured on
two or more switches in the network, the switch with the lowest MAC address will become the root. Bridge
Protocol Data Units (BPDUs) communicate the details of the switch with the lowest bridge ID in the network.
The election process for the root bridge takes place every time there is a topology change in the network. A
topology change may occur due to the failure of a root bridge or the addition of a new switch in the network.
The root bridge originates BPDUs every two seconds, which are propagated by other switches throughout the
network. BPDUs are used as keepalives between switches, and if a switch stops receiving BPDUs from a
neighboring switch for ten intervals (20 seconds), it will assume a designated role for the network segment.
Neither SwitchB nor SwitchC will become the root bridge. Although both have an equal priority value to SwitchA
(32768), the MAC addresses of SwitchB and SwitchC are higher than that of SwitchA.The root bridge can be determined with the information given. If the diagram did not indicate MAC addresses,
then the root bridge would not be able to be determined, since the priorities are equal.
Objective:
LAN Switching Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot STP protocols

Cisco Documentation > Cisco 7600 Series Cisco IOS Software Configuration Guide, 12.2SX > Configuring STP
and IEEE 802.1s MST > Understanding the Bridge ID
Support > Technology Support > LAN Switching > Spanning Tree Protocol > Configure > Configuration
Examples and TechNotes > Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst
Switches