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, MaxAge, and Forward Delay. The root bridge dictates these timers in BPDUs.
| 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. |
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.
Max-Age Timer
As long as a switch receives a BPDU from its neighbors every 2 seconds, it operates normally. However, if it does not receive a BPDU from the neighbor within MaxAge, the switch takes 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 before reacting if it didn't hear a Hello BPDU.
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 takes another fifteen seconds to learn and add new entries to the CAM table. It uses the Learning state for this.
Viewing the STP timer
You can use the show spanning-tree command to view the STP timer values.
This tutorial is part of the tutorial series "STP, RSTP, PVST, RPVST, and MSTP Explained with Examples". Other parts of this series are the following.
Chapter 01 Introduction and overview of STP and RSTP
Chapter 02 STP, RSTP, PVST, RPVST, and MSTP
Chapter 03 Similarities and Differences between STP and RSTP
Chapter 04 STP and RSTP Terminology
Chapter 05 STP/RSTP Timers Explained
Chapter 06 The Bridge ID and System ID Extension Explained
Chapter 07 PVST/STP Root Bridge Election Process Explained
Chapter 08 STP Port and Path Costs Explained
Chapter 09 STP Port States:- Blocking, Listing, Learning, and Forwarding
Chapter 10 STP - Spanning Tree Protocol Explained With Examples
Chapter 11 RSTP Port States and Types Explained
Chapter 12 PVST/RPVST and EtherChannel Explained
Chapter 13 RSTP / RPVST Explained with Examples
Conclusion
There are three STP timers: Hello, MaxAge, and Forward Delay. Hello timer defines the interval between BPDUs. The MaxAge timer defines the time 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.
By ComputerNetworkingNotes Updated on 2026-05-12