Parameters affect the STP port states and convergence

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Hello Time

The Hello Time specifies the interval at which an STP device sends configuration BPDUs and Hello packets to detect link failures.

When the Hello Time is changed, the new value takes effect only after a new root bridge is elected. The new root bridge adds the new Hello Time value in BPDUs it sends to non-root bridges. When the network topology changes, TCN BPDUs are transmitted immediately, independent of the Hello Time.

Forward Delay

The Forward Delay timer specifies the length of delay before a port state transition. When a link fails, STP calculation is triggered and the spanning tree structure changes. However, new configuration BPDUs cannot be immediately spread over the entire network. If the new root port and designated port forward data immediately, transient loops may occur. Therefore, STP defines a port state transition delay mechanism. The newly selected root port and designated port must wait for two Forward Delay intervals before transitioning to the Forwarding state. Within this period, the new configuration BPDUs can be transmitted over the network, preventing transient loops.

The default Forward Delay timer value is 15 seconds. This means that the port stays in the Listening state for 15 seconds and then stays in the Learning state for another 15 seconds before transitioning to the Forwarding state. The port is blocked when it is in the Listening or Learning state, which is key to preventing transient loops.

Max Age

The Max Age specifies the aging time of BPDUs. This parameter is configurable on the root bridge.

The Max Age is spread to the entire network with configuration BPDUs. After a non-root bridge receives a configuration BPDU, it compares the Message Age value with the Max Age value in the received configuration BPDU.
If the Message Age value is smaller than or equal to the Max Age value, the non-root bridge forwards the configuration BPDU.
If the Message Age value is larger than the Max Age value, the non-root bridge discards the configuration BPDU. When this happens, the network size is considered too large and the non-root bridge disconnects from the root bridge.
If the configuration BPDU is sent from the root bridge, the value of Message Age is 0. Otherwise, the value of Message Age is the total time spent to transmit the BPDU from the root bridge to the local bridge, including the transmission delay. In real world situations, the Message Age value of a configuration BPDU increases by 1 each time the configuration BPDU passes through a bridge.
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