Spanning Tree Protocol (STP) Timers

 Spanning Tree Protocol (STP) Timers

Spanning Tree Protocol (STP) operates as switches send BPDUs to each other in an effort to form a loop-free topology. The BPDUs take a finite amount of time to travel from switch to switch. In addition, news of a topology change (such as a link or root bridge failure) can suffer from propagation delays as the announcement travels from one side of a network to the other. Because of the possibility of these delays, keeping the spanning-tree topology from settling out or converging until all switches have had time to receive accurate information is important.

STP uses three timers to make sure that a network converges properly before a switching loop can form. The timers and their default values are as follows:

■ Hello timer : The time interval between Configuration BPDUs sent by the root bridge. The hello time value configured in the root bridge switch determines the hello time for all nonroot switches because they just relay the Configuration BPDUs as they are received from the root. However, all switches have a locally configured hello time that is used to time TCN BPDUs when they are retransmitted. The IEEE 802.1D standard specifies a default hello time value of 2 seconds.

■ Forward Delay timer : The time interval that a switch port spends in both the Listening and Learning states. The default value is 15 seconds.

■ Max (Maximum) Age timer : The time interval that a switch stores a BPDU before discarding it. While executing the STP, each switch port keeps a copy of the “best” BPDU that it has heard. If the switch port loses contact with the BPDU’s source (no more BPDUs are received from it), the switch assumes that a topology change must have occurred after the max age time elapsed and so the BPDU is aged out. The default Max Age timer value is 20 seconds.

The STP timers can be configured or adjusted from the switch command line. However, the timer values never should be changed from the defaults without careful consideration. Then the values should be changed only on the root bridge switch. Recall that the timer values are advertised in fields within the BPDU. The root bridge ensures that the timer values propagate to all other switches.

The network diameter can be configured on the root bridge switch to more accurately reflect the true size of the physical network. Making that value more accurate reduces the total STP convergence time during a topology change. Cisco also recommends that if changes need to be made, only the network diameter value should be modified on the root bridge switch. When the diameter is changed, the switch calculates new values for all three timers automatically.

Table 2.9 summarizes the STP timers, their functions, and their default values.

Table 2.9 STP Timers

Timer	Function	Default Value
Hello	Interval between configuration BPDUs.	2 seconds
Forward delay	Time spent in Listening and Learning states before transitioning toward Forwarding state.	15 seconds
Max age	Maximum length of time a BPDU can be stored without receiving an update. Timer expiration signals an indirect failure with designated or root bridge.	20 seconds