Basics and Concepts Flashcards

Which IEEE standard introduced Rapid Spanning Tree Protocol (RSTP)?

Answer: IEEE 802.1w

IEEE 802.1w defined RSTP, which provides much faster convergence than the original 802.1D STP.

What is the purpose of the Topology Change Notification (TCN) BPDU in STP?

Answer: To signal that the network topology has changed

A TCN BPDU is sent upstream toward the root bridge to notify all switches that a topology change has occurred, prompting MAC table flushes.

How many seconds does it take for a classic STP port to transition from Blocking to Forwarding by default?

Answer: 50 seconds

A port spends 15 seconds in Listening and 15 seconds in Learning for a total of 30 seconds before entering Forwarding, but max age (20s) is added when recovering from indirect failures, totaling up to 50 seconds.

Which STP port role is used to block traffic and prevent switching loops on non-root, non-designated ports?

Answer: Alternate port

In RSTP, the alternate port is the blocking port that provides a standby path to the root bridge.

What does STP use path cost to determine?

Answer: The best path to the root bridge

STP calculates cumulative path costs to identify the lowest-cost path to the root bridge, which becomes the root port.

Which condition causes a non-root switch to generate a TCN BPDU?

Answer: A port transitions from Blocking to Forwarding or from Forwarding to Blocking

A switch sends a TCN BPDU when a port that was forwarding goes down, or when a port transitions to Forwarding while another port is already forwarding.

What is the maximum number of switches (hops) STP supports by default before a BPDU is considered stale?

Answer: 20

The max age default of 20 seconds limits the diameter of the spanning tree; by default this corresponds to a maximum of 7 switches in the path (with the default hello time of 2 seconds).