Section 4.5: STP Timers

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, preventing the Spanning-Tree topology from converging until all switches have had time to receive accurate information is important. STP uses three timers for this purpose. There are three timers: Hello Time, Forward Delay, and Max Age.

• Hello Time is the time interval between Configuration BPDUs sent by the Root Bridge. The Hello Time value configured in the Root Bridge switch will determine the Hello Time for all non-root switches. However, all switches have a locally configured Hello Time that is used to time Topology Change Notification (TCN) BPDUs when they are retransmitted. The IEEE 802.1D standard specifies a default Hello Time value of two seconds.

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

• Max Age is 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 source of the BPDU loses contact with the switch port, the switch will notice that a topology change has occurred after the Max Age time elapses and the BPDU is aged out. The default Max Age value is 20 seconds.

To announce a change in the active network topology, switches send a Topology Change Notification (TCN) BPDU. This occurs when a switch either moves a port into the Forwarding state or moves a port from Forwarding or Learning into the Blocking state. The switch sends a TCN BPDU out its Designated Port. The TCN BPDU carries no data about the change, but only informs recipients that a change has occurred. However, the switch will not send TCN BPDUs if the port has been configured with PortFast enabled. The switch will continue sending TCN BPDUs every Hello Time interval until it gets an acknowledgement from an upstream neighbor. As the upstream neighbors receive the TCN BPDU, they will propagate it on toward the Root Bridge. When the Root Bridge receives the BPDU, the Root Bridge sends out an acknowledgement. The Root Bridge also sends out the Topology Change flag in a Configuration BPDU so that all other bridges will shorten their bridge table aging times down from the default 300 seconds to the Forward Delay value. This condition causes the learned locations of MAC addresses to be flushed out sooner than they normally would, easing the bridge table corruption that might occur due to the change in topology. However, any stations that are actively communicating during this time will be kept in the bridge table. This condition lasts for the sum of the Forward Delay and the Max Age.

The three STP timers can be adjusted. These timers need only be modified on the Root Bridge and any secondary or backup Root Bridges because the Root Bridge propagates all three timer values throughout the network in the Configuration BPDU.

To modify STP timers on a CLI-based switch, use the following commands:

Switch(enable) set spantree hello interval [ vlan ]

Switch(enable) set spantree fwddelay delay [ vlan ]

Switch(enable) set spantree maxage agingtime [ vlan ]

To modify STP Timers on an IOS-based switch, use the following commands:

Switch(config)# spanning-tree [vlan vlan_list] hello-time seconds Switch(config)# spanning-tree [vlan vlan_list] forward-time seconds Switch(config)# spanning-tree [vlan vlan_list] max-age seconds