Content

| 3.2 |  | STP Processes |
| 3.2.7 |  | STP timers |

STP operation is controlled by three timers. The Hello Time is the amount of time between the sending of Configuration BPDUs. The 802.1D standard specifies a default value of 2 seconds. This value controls Configuration BPDUs as the Root Bridge generates them. Other bridges propagate BPDUs from the Root Bridge as they are received.

If BPDUs stop arriving for the time interval ranging from 2 to 20 seconds because of a network disturbance, or if the Root Bridges stop sending periodic BPDUs during this time, the timer will expire. 2 to 20 seconds is the range between the expected receipt of a BPDU and the expiration of the Max Age time. If the outage lasts for more than 20 seconds, the default Max Age time, the bridge invalidates the saved BPDUs and begins looking for a new Root Port.

Forward Delay is the amount of time the bridge spends in the Listening and Learning states. This is a single value that controls both states. The default value of 15 seconds was originally derived assuming a maximum network size of seven bridge hops, a maximum of three lost BPDUs, and a Hello Time of 2 seconds. The Forward Delay timer also controls the bridge table age-out period after a change in the active topology.

Max Age is the STP timer that controls how long a bridge stores a BPDU before discarding it. Max Age is only an issue when the link failure is not on a directly connected link. When a failure occurs on a directly connected link, the switch knows there will not be any BPDUs coming in on that link, so Max Age is not considered in transitioning the port to Forwarding mode. Recall that each port saves a copy of the best BPDU it has seen. As long as the bridge receives a continuous stream of BPDUs every 2 seconds, the receiving bridge maintains a continuous copy of the BPDU values. However, if the device sending this best BPDU fails, a mechanism must exist to allow other bridges to take over.

For example, assume that the Segment 3 link in Figure uses a hub and that the Port-1/2 on Cat-B transceiver fails. Cat-C has no immediate notification of the failure, because it still has an active Ethernet link to the hub. The only thing Cat-C knows is that BPDUs stop arriving. Twenty seconds (Max Age) after the failure, Port-1/2 on Cat-C ages out the BPDU information that lists Cat-B as having the best Designated Port for Segment 3. This forces Port-1/2 on Cat-C to transition to the Listening state in an effort to become the Designated Port. Because Port-1/2 on Cat-C now offers the most attractive access from the Root Bridge to this link, it eventually transitions all the way into Forwarding mode. In practice, it takes approximately 50 seconds (20 Max Age + 15 Listening + 15 Learning) for Cat-C to take over after the failure of Port 1/2 on Cat-B. In some situations, switches can detect topology changes on directly connected links and immediately transition into the Listening state without waiting Max Age seconds.

For example, in Figure , Port-1/1 on Cat-C fails. Because the failure results in a loss of link on the Root Port, there is no need to wait 20 seconds for the old information to age out. Note the difference between this scenario and the preceding one, in which link integrity was maintained due to the presence of a hub. When Port-1/1 on Cat-C fails, Port-1/2 on Cat-C immediately goes into Listening mode in an attempt to become the new Root Port. The STP convergence time reduced from 50 seconds to 30 seconds (15 Listening + 15 Learning).

There are two key points to remember about using the STP timers. First, default timer values should not be changed without careful consideration. Second, if timer tuning is attempted, modify the STP timers only from the Root Bridge, because the BPDUs contain three fields with timer values that can be passed from the Root Bridge to all other bridges in the network. If every bridge were locally configured, some bridges could work their way up to the Forwarding state before other bridges ever left the Listening state. This chaotic approach could quickly lead to an unstable network. By providing timer fields in the BPDUs, the single bridge acting as the Root Bridge can dictate the timing parameters for the entire bridged network.