Port Selection in Spanning-Tree Topology

# Determining the Root Port in a Spanning-Tree Topology

### Question

What value is primarily used to determine which port becomes the root port on each nonroot switch in a spanning-tree topology?

### Explanations

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A. B. C. D. E.

E

The correct answer is E. path cost.

In a spanning tree topology, each switch selects a root port and designated ports for each segment in order to avoid loops and create a loop-free network. The root bridge is the central point of the spanning tree, and all non-root bridges use the Spanning Tree Protocol (STP) to determine the shortest path to reach the root bridge.

When a non-root switch receives BPDUs (Bridge Protocol Data Units) from multiple ports, it needs to select one of them as the root port. The root port is the port with the lowest cost path to the root bridge. The cost of each path is calculated based on the speed of the link and the amount of congestion on the link.

Therefore, the value primarily used to determine which port becomes the root port on each non-root switch is the path cost, which is a combination of the link speed and the delay. The lowest path cost indicates the shortest and fastest path to the root bridge.

Option A, lowest port MAC address, is not relevant in determining the root port. It is only used in tiebreaker situations when two ports have the same path cost.

Option B, port priority number and MAC address, is used to determine the designated port. The designated port is the port that provides the best path to reach the root bridge for a specific segment. The port priority number is used to break ties when two ports have the same path cost.

Option C, VTP revision number, is not relevant in determining the root port. VTP (VLAN Trunking Protocol) is used to manage VLAN configurations across a network.

Option D, highest port priority number, is used to determine the root port only when two ports have the same path cost. The port with the highest priority is selected as the root port.

In summary, the path cost is the primary value used to determine the root port on each non-root switch in a spanning-tree topology.