Allied Telesis AT-S112, AT-GS950/16PS manual Hello Time and Bridge Protocol Data Units Bpdu

Chapter 4: STP and RSTP

The forwarding delay value is adjustable in the AT-S112 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 specify a smaller value so that the time for a topology change is optimized for minimum data loss.

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 (BPDU)

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-S112 Management software. The interval is measured in seconds. Consequently, if the switch is selected as the root bridge of a spanning tree domain, it transmits a BPDU every two seconds.

Point-to-Point and Edge Ports

This section applies only to RSTP. Part of the task of configuring RSTP is defining the port types on the bridge, which is directly related to the device(s) connected to the port. With the port types defined, RSTP can reconfigure a network much quicker than STP when a change in network topology is detected.

There are two possible selections:

Point-to-point port

Edge port

If a bridge port is connected to another bridge or router port, it normally operates in full-duplex mode and is functioning as a point-to-point port. Figure 17 on page 67 illustrates two switches that are connected with one data link. This link is operating between two point-to-point ports.

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