.Which switch will be selected as the root bridge by Spanning Tree Protocol (STP)?
A. switch with lowest bridge ID
B. switch with lowest IP address
C. switch with lowest Media Access Control (MAC) address
D. switch with lowest number of root ports
Correct Answer: A
STP will use elections to arrive at a fully converged state that will ensure a switching loop free network. It will select:
The root bridge
The root port on each non-root bridge
Designated ports on any shared segments with no direct connection to the root bridge.
The switch with the lowest bridge ID will be selected as the root bridge by STP. A bridge ID has two components: the priority number and the MAC address. On Cisco devices, the
priority number may range from 0 to 65535. The priority number constitutes the most significant bits of the bridge ID. If you want to ensure that a particular switch in a topology always
becomes a root bridge, regardless of the MAC address, you can set the priority number of that switch to the lowest value among all switches in the topology.
Since the selection of the root bridge influences all other decisions and thus the single loop free path for each VLAN, the selection and location of the root bridge is important and best
not left to chance. Once you have determined the best switch for the role of root bridge, you can ensure its election by lowering its bridge priority.
It is best for the root bridge to be centrally located with respect to the clients and the servers that generate the most traffic on the VLAN. For example, in the diagram below, if most of
the traffic travels between the clients and the servers on VLAN 20, the best choice for the root bridge for VLAN 20 would be SwitchD. SwitchD is centrally located between the clients
on VLAN 20 and the servers on VLAN 20.
To illustrate the type of inefficient traffic that could occur when care is not given to the location of the root bridge, consider the diagram above and assume that Switch B was chosen
the root bridge. Next, assume that traffic needs to go from VLAN 10 connected to Switch C to VLAN 10 connected to Switch A. The shortest path would be from Switch C to Switch A.
However, because the only port that is forwarding on Switch C is the port that leads to the root bridge (Switch B), then the actual path would be from Switch C, to Switch B, to Switch
E, and then to Switch A.
By default, the priority number of all Cisco switches is configured to a value of 32768. For example, consider three switches in network topology with the following MAC addresses and
the same default priority number:
The switch with the lowest MAC address, 0000.0B02.AAAA, will become the root bridge.
The switch with the lowest IP address will not be selected as the root bridge by STP because the IP address of the switch does not influence the selection of the root bridge.
The switch with the lowest MAC address will not be selected as the root bridge by STP. A combination of priority number and MAC address determines the selection of the root bridge.
The MAC address will determine the root bridge only if there is a tie for the switch with the lowest priority number.
The switch with the lowest number of root ports will not be selected as the root bridge by STP. Root ports are the interfaces on non-root bridges. On a non-root bridge, the least-rootcost
interface is known as a root port. Therefore, the switch having the fewest root ports is not the root bridge.
LAN Switching Fundamentals
Configure, verify, and troubleshoot STP protocols
Cisco > Support > Configuring Spanning Tree Protocol > How STP Works > How a Switch or Port Becomes the Root Switch or Port
Cisco Documentation > Cisco 7600 Series Cisco IOS Software Configuration Guide, 12.2SX > Configuring STP and IEEE 802.1s MST > Understanding the Bridge ID