Sim-Ex™ Practice Exams for CCNP Switch (300-115) practice questions

Spanning Tree Algorithm, BDPU exchange

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Q4. During the exchange of BDPUs, to choose the ports that will be forwarding (and ports that will be blocked), the Spanning-Tree Algorithm on a switch may use which three components? [Choose 3]

A. Path cost

B. Subnet mask

C. Bridge ID

D. Port ID

Correct Answer: A,C,D


During the process of Spanning-Tree Algorithm execution, some redundant ports need to be blocked. This is required to avoid bridging loops. To choose which port to use for forwarding frames, and which port to block, the following three components are used by the Spanning-Tree Protocol:
1. Path Cost: The port with lowest path cost is placed in the forwarding mode. Other ports are placed in blocking mode.
2. Bridge ID: If the path costs are equal, then the bridge ID is used to determine which port should forward. The port with the lowest Bridge ID is elected to forward, and all other ports are blocked.
3. Port ID: If the path cost and bridge ID are equal, the Port ID is used to elect the forwarding port. The lowest port ID is chosen to forward. This type of situation may araise when there are parallel links, used for redundancy.

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