Root Bridge Election in VLAN 1

A

In a Cisco network, the Spanning Tree Protocol (STP) is used to prevent loops in the network by establishing a single path for data to travel between switches. One of the most critical functions of STP is the election of a root bridge, which serves as the central point for the network topology.

In the scenario described in the question, there are three switches connected to one another via trunk ports, and we need to determine which switch is elected as the root bridge for the spanning-tree instance of VLAN 1, assuming the default switch configuration.

The default STP algorithm used by Cisco switches is IEEE 802.1D, which operates based on the Bridge Protocol Data Units (BPDUs) exchanged between switches. Each switch in the network sends BPDUs to its neighboring switches to exchange information about its identity, configuration, and topology.

To elect a root bridge for a particular VLAN, switches exchange BPDUs containing a priority value and a MAC address. The switch with the lowest priority value and the lowest MAC address is elected as the root bridge. By default, all switches have a priority value of 32,768, so the MAC address becomes the tiebreaker.

Therefore, the answer to the question is A: the switch with the lowest MAC address is elected as the root bridge for the spanning-tree instance of VLAN 1.

It is worth noting that if the priority value has been manually configured on a switch, that switch will have a better chance of being elected as the root bridge, regardless of its MAC address. In addition, the root bridge can be manually configured to ensure that it is always the same switch, which can be useful in certain network designs.