Allied Telesis SwitchBlade x3100 Series Manual page 385

Spanning Tree Protocol (STP and RSTP) Introduction
4.5.3.5 Rapid Spanning Tree (RSTP)
In the 802.1d Spanning Tree Algorithm and Protocol, timer driven processing controls how each port goes
through the STP state transitions before be placed into a "forwarding" mode where normal traffic flow is sup-
ported. In the Rapid Spanning Tree Algorithm and Protocol (RSTP), significant time savings are accomplished
using rapid STP port state transitions in many of the expected network topology change scenarios. The time
savings is accomplished through additional information exchange and new "hand shake" processing between the
ports of LAN connected bridges. The concept of a point-to-point connection is introduced to identify when a
port is connected to exactly one other bridge. This condition must exist for some of the above mentioned rapid
state transitions to take place. The concept of edge ports is also introduced to completely bypass the state tran-
sition process when a port is known to be connected to a single host.
The parameters that are associated with RSTP are included below.
4.5.3.6 Spanning Tree Parameters
The following subsection provides an overview of parameters that are of particular importance in setting up a
configuration.
4.5.3.7 Bridge Priority
Bridge IDs are used in root bridge elections. The root bridge is the switch in the extended LAN with the
numerically lowest bridge ID value. This is guaranteed to identify a single bridge due to the unique MAC address
component. The user is allowed to change the bridge priority component to override the arbitrary root selec-
tion that will result from only comparing MAC addresses when the default bridge priorities are in use.
Bridge IDs are also used in designated bridge elections. Normally the switch with the lowest root path cost is
the designated bridge for a physical LAN. If more than one switch ties has the same lowest root path cost, then
the designated bridge is the switch with the numerically lowest numbered bridge priority ID value.
The value of the PRIORITY parameter is used to set the writable portion of the bridge ID. The default bridge
priority is 32768. To change the STP priority value, use the SET STP PRIORITY command
The range is from 0 to 65535 (a limitation of RSTP) in increments of 4096.
Note:
4.5.3.8 Port Priority
Port IDs are used in root port elections. Normally, the port with the lowest root path cost is the root port for
the switch. If more than one port ties for the lowest root path cost, then the root port is the port with the low-
est numerical port ID (as assigned by the system).
The default port priority value is 128. The port priority values can be configured on a per-port basis, as a value
from zero to 240, in accordance with IEEE Std 802.1d, 1998 Edition. However, the storage space (number of
bits) allocated to the priority component of the port ID is reduced to support bridges with larger numbers of
ports, since this only left room for port numbers from 1-255.
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Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)