Allied Telesis AT-S81 - Manual (Page 141)

AT-S81 Management Software User’s Guide
Section II: Menus Interface 141
Table 6 lists the values. The default value is 0.
If two paths have the same port cost and the same priority, then the ports
with the lowest port MAC addresses become the root ports of their
respective bridges.
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-S81 management software. The interval is measured in seconds
and the default is two seconds. Consequently, if an AT-8000/8POE Fast
Ethernet 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
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 quickly reconfigure a network when a change in
network topology is detected.
Table 6. Port Priority Value Increments
Port
Priority
Port
Priority
0 128
16 144
32 160
48 176
64 192
80 208
96 224
112 240