Allied Telesis AT-S63 - Manual (Page 481)

AT-S63 Management Software Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 481
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:
ˆ Point-to-point port
ˆ Edge port
If a bridge port is operating in full-duplex mode, than the port is functioning
as a point-to-point port. Figure 165 illustrates two AT-9400 Series switches
that have been connected with one data link. With the link operating in full-
duplex, the ports are point-to-point ports.