Allied Telesis AT-S63 - Manual (Page 255)

AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Features 255
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 (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-S63 management software. The interval is measured in
seconds and the default is two seconds. Consequently, if an AT-9400
Series 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
Note
This section applies only to RSTP.
Part of the task of configuring RSTP is defining the port types on the
bridge. This relates 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 operating in full-duplex mode, than the port is
functioning as a point-to-point port. Figure 74 illustrates two AT-9400