Which values are used by an RSTP bridge to remove stale BPDU information?

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Rapid Spanning Tree Protocol (RSTP) is an improvement over the standard Spanning Tree Protocol (STP) that helps to reduce the convergence time of the spanning tree, which is the time it takes for the network to recognize a topology change and reconfigure the forwarding paths. One of the ways RSTP accomplishes this is by reducing the time it takes to remove stale information about the network topology.

When a bridge receives a BPDU (Bridge Protocol Data Unit) from a neighboring bridge, it stores information about that BPDU, such as the source bridge's ID, the port it received the BPDU on, and the BPDU's age. This information is used to build the spanning tree and to detect changes in the network topology. However, if a bridge doesn't receive any BPDUs from a neighbor for a certain period of time, it assumes that the neighbor is no longer there and removes the stale information about it.

RSTP uses two values to determine when to remove stale BPDU information: Max Age and Forward Delay.

1. Max Age: This is the maximum age a BPDU can have before it is considered stale. The default value for Max Age is 20 seconds. If a bridge doesn't receive a BPDU from a neighbor for Max Age seconds, it assumes that the neighbor is no longer there and removes the stale BPDU information. This is also the time it takes for a BPDU to expire and be removed from the network.

2. Forward Delay: This is the time it takes for a bridge to transition from the blocking state to the forwarding state. The default value for Forward Delay is 15 seconds. When a bridge transitions from the blocking state to the forwarding state, it sends BPDUs on all its ports to inform its neighbors of the change. Any BPDUs received on these ports that are older than Forward Delay seconds are considered stale and are discarded.

Therefore, the two values used by an RSTP bridge to remove stale BPDU information are Max Age and Forward Delay.