STP Timers
The following parameters affect the STP port states and convergence.
- Hello timer
The Hello timer determines the interval at which an STP-capable device sends configuration BPDUs. The device sends BPDUs at an interval of the Hello timer to check whether any link has failed.
When the Hello timer is changed, the new value takes effect only after a new root bridge is elected. The new root bridge includes the new Hello timer value in BPDUs it sends to non-root bridges. If the network topology changes, TCN BPDUs are immediately transmitted regardless of the Hello timer.
- Forward Delay timer
The Forward Delay timer specifies the delay in a port state transition. If a link fails, STP calculation is triggered and the spanning tree structure changes accordingly. However, new configuration BPDUs cannot be flooded immediately across the entire network. If the new root port and designated port forward data immediately, transient loops may occur. Therefore, STP defines a delay mechanism for port state transition. The newly selected root port and designated port must wait for two Forward Delay intervals before transitioning to the Forwarding state. During this period, the new configuration BPDUs can be transmitted over the network, preventing transient loops.
The Forward Delay timer defines the time that is spent in Listening or Learning state. The default Forward Delay value is 15 seconds. This means that the port stays in Listening state for 15 seconds and then stays in Learning state for another 15 seconds before transitioning to the Forwarding state. The port in Listening or Learning state does not forward user traffic, effectively preventing transient loops.
- Max Age timer
The Max Age timer specifies the aging time of BPDUs. This parameter is configurable on the root bridge.
The Max Age value is encapsulated in configuration BPDUs and transmitted on the entire network to ensure consistency. Upon receipt of a configuration BPDU, a non-root bridge 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 greater than the Max Age value, the non-root bridge discards the configuration BPDU. In this case, the network 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 Message Age value is 0. Otherwise, the Message Age value is the total time required to transmit the BPDU from the root bridge to the local bridge, including the transmission delay. The Message Age value of a configuration BPDU is incremented by 1 each time the configuration BPDU passes through a bridge.
In Figure 1:
- S2 and S3 receive configuration BPDUs from S1 with the Message Age value of 0. On the port that goes to S1, the information ages out in (Max Age value - 0) seconds.
- S4 and S5 receive configuration BPDUs from S2 with the Message Age value of 1. On the port that goes to S1, the information ages out in (Max Age value - 1) seconds.
- S6 receives configuration BPDUs from S5 with the Message Age value of 2. On the port that goes to S1, the information ages out in (Max Age value - 2) seconds.
