Allied Telesis AT-GS950/8 manual Hello Time and Bridge Protocol Data Units BPDUs

Chapter 8: Spanning Tree Protocol (STP)

change has occurred, especially if it is a large network. If a topology change is made before all bridges have been notified, a temporary data loop could occur, and that could adversely impact network performance.

To forestall the formation of temporary data loops during topology changes, a port designated to change from blocking to forwarding passes through two additional states—listening and learning—before it begins to forward frames. The amount of time a port spends in these states is set by the forwarding delay value. This value states the amount of time that a port spends in the listening and learning states prior to changing to the forwarding state.

The forwarding delay value is adjustable in the AT-S82 management software. The appropriate value for this parameter depends on a number of variables; the size of your network is a primary factor. For large networks, you should specify a value large enough to allow the root bridge sufficient time to propagate a topology change throughout the entire network. For small networks, you should not specify a value so large that a topology change is unnecessarily delayed, which could result in the delay or loss of some data packets.

Note

The forwarding delay parameter applies only to ports on the switch that are operating STP-compatible mode.

Hello Time and Bridge Protocol Data Units (BPDUs)

The bridges that are part of a spanning tree domain communicate with each other using a bridge broadcast frame that contains a special section devoted to carrying STP or RSTP information. This portion of the frame is referred to as the bridge protocol data unit (BPDU). When a bridge is brought online, it issues a BPDU in order to determine whether a root bridge has already been selected on the network, and if not, whether it has the lowest bridge priority number of all the bridges and should therefore become the root bridge.

The root bridge periodically transmits a BPDU to determine whether there have been any changes to the network topology and to inform other bridges of topology changes. The frequency with which the root bridge sends out a BPDU is called the hello time. This is a value that you can set in the AT-S82 management software. The interval is measured in seconds and the default is two seconds. Consequently, if an AT-GS950/8 WebSmart switch is selected as the root bridge of a spanning tree domain, it transmits a BPDU every two seconds.

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