Learn about the spanning tree algorithm tie-breaker that is used when multiple equal-cost paths exist between switches in a LAN segment. Understand how the port ID is used to determine which path to use.
Table of Contents
Question
In a LAN segment, which tie-breaker would spanning tree consider if equal-cost paths exist between two or more switches to the root bridge?
A. port ID
B. bridge ID
C. port priority
D. interface number
Answer
A. port ID
Explanation
When the spanning tree algorithm is determining the best path in a switched LAN environment, it may encounter a situation where there are two or more equal-cost paths between switches back to the root bridge. In this case, spanning tree uses the following tie-breaker criteria, in order:
- Lowest root bridge ID
- Lowest path cost to the root
- Lowest sender bridge ID
- Lowest sender port ID
So if there are multiple equal-cost paths, the path that goes through the switch port with the lowest port ID will be chosen. The port ID is a unique identifier for each port on a switch, composed of a configurable 4-bit priority and a 12-bit interface number. By default, all ports have the same priority, so the interface number is used as the tie-breaker.
Therefore, the correct answer is A – spanning tree will consider the port ID as the final tie-breaker if multiple equal-cost paths exist between switches back to the root bridge. The bridge ID is not considered at this stage since it was already used earlier to choose the root bridge. The port priority is part of the port ID.
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