Exam JN0-351 topic 1 question 81 discussion

Root Port Election - Root Port (RP) is an upstream facing port towards the Root Bridge - Elected based on lowest root path cost ○ Cumulative cost of all links to get to the root - Cost based on inverse bandwidth ○ Higher bandwidth, lower cost Tie breakers: - Lowest upstream BID - Lowest Upstream Port ID

The correct answer is A. port ID. When there are equal-cost paths to the root bridge, the Spanning Tree Protocol (STP) uses the following tie-breakers in this order: Bridge ID: The switch with the lowest bridge ID is preferred. Port ID: The port with the lowest port ID is preferred. So, if the bridge IDs are the same, the port ID is used as the tie-breaker. Here's a brief explanation of the other options: B. Bridge ID Most Voted: This is not a valid tie-breaker criterion. C. Port priority: While port priority is used in STP, it's not the tie-breaker used when there are equal-cost paths. D. Interface number: This is not a standard term used in STP.

The phrase "if equal-cost paths exist between two or more switches to the root bridge" means that all about interfaces isn't compared anymore. So, Bridge ID is correct If the path costs are the same, STP compares the Bridge ID of the switches. The lower Bridge ID is preferred.

This question is ambiguous. If there are two switches with two equal paths, and one of them is the root bridge, then the port ID should be used to elect the root port. If the two switches are connected to the same root bridge, then the sender's bridge ID should be used.

Bridge ID takes precedence over Port ID in this case.

Correct Answer is B. Port ID is when you have several ports on a single switch to the root bridge

Correct Answer is : A

In the Spanning Tree Protocol (STP), if equal-cost paths exist to the root bridge, the protocol uses the port ID as a tie-breaker to select the best path. The port ID is part of the bridge port's information and helps in determining which port should be in the forwarding state.

"If two bridges have the same path cost to the root bridge, the bridge with the lowest bridge ID becomes the designated bridge."