STP/RSTP Timers Explained

STP/RSTP uses three timers for its functions. These timers control and define the time STP needs to reach convergence. These timers are Hello, Max age, and Forward delay. The root bridge dictates these timers in BPDUs. The following table lists these timers.

Timer Default Value Description
Hello 2 seconds This is the interval between two consecutive BPDUs.
MaxAge 10 times Hello This is the maximum time a switch waits for a BPDU before changing the STP topology.
Forward delay 15 seconds This is the time the switch keeps ports in learning and listing states.

Let us understand the above timers in detail.

Hello Timer

Switches use BPDU messages to communicate and share STP information. A BPDU message includes all the information that STP needs to build and maintain STP topology. All STP-running switches broadcast a BPDU at a fixed interval. The Hello-timer defines this interval. The default value of this timer is two seconds. Therefore, all STP-running switches broadcast a BPDU every two seconds.

Hello timer

Max-Age Timer

As long as a switch receives a BPDU every two seconds from its neighbors, it operates normally. However, if it does not receive a BPDU from the neighbor within MaxAge time, the switch reacts by taking steps to change the STP topology. The default value of this timer is ten times the Hello timer. Since the default value of the hello timer is 2 seconds, the default value of it will be 20 seconds (2 * 10 = 20). So, a switch would wait 20 seconds without hearing a Hello BPDU before reacting.

max age timer

Forward delay

Switches use this timer to age out old entries from the CAM table and add new entries. The switch uses CAM table entries to make forwarding decisions. When a network change occurs, CAM table entries become invalid. The switch can not use the existing entries to make forwarding decisions. It must flush the existing entries and learn them again to make forwarding decisions. The switch completes these tasks in the Forward delay time. The default forward delay time is fifteen seconds. STP uses fifteen seconds to flush old entries. It uses the Listening state for this. It uses another fifteen seconds to learn and add new entries to the CAM table. It uses the Learning state for this.

forward delay

Viewing STP timer

You can use the show spanning-tree command to view the STP timer values.

show spanning tree command

Conclusion

There are three STP timers: Hello, Max age, and forward delay. Hello timer defines the interval between BPDUs. The Max age timer defines the switch waits before rebuilding the STP topology when it does not receive BPDUs from its neighbors. STP uses the forward delay timer to rebuild the CAM table entries. It uses this timer for two states: Listening and Learning. In the Listening state, it ages out old entries. In the Learning state, it adds new entries to make forwarding decisions.

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