Allied Telesis AT-S39 - Manual (Page 83)

AT-S39 User’s Guide
83
STP Overview
The AT-8024, AT-8024GB, and AT-8026FC Fast Ethernet Switches
support the Spanning Tree Protocol, as specified in the IEEE 802.1D
standard. STP can be an important part of large networks where loops,
either planned or unplanned, exist in the network topology.
A loop exists when two or more nodes on your network can transmit
data to each other over more than one data link. A network loop can
pose a danger to network performance and operability. Data packets
can become caught in repeating cycles, referred to as broadcast storms,
that needlessly consume network bandwidth and often significantly
reduce network performance.
STP prevents data loops from forming in your network by ensuring that
only one path exists between the end nodes in your network. Where
multiple paths exist, STP places the extra paths in a standby or blocking
mode, leaving only one main active path.
The redundant paths can be activated by STP if the main path goes
down. So not only does STP guard against multiple links between end
nodes, but it can also activate backup redundant paths in case a main
link fails.
Selecting a Root
Bridge
The first task that bridges perform when STP is activated on a network is
the selection of a root bridge. The root bridge is used by the other
bridges to determine if there are redundant paths in the network. The
root bridge also distributes network topology information to the other
network bridges.
A root bridge is selected by a combination of a bridge’s priority number,
also referred to as the bridge identifier, and sometimes its MAC address.
The bridge with the lowest bridge priority number in the network is
selected as the root bridge. If two or more bridges have the same bridge
priority number, of those bridges the one with the lowest MAC address
is designated as the root bridge.
The bridge priority number is adjustable on the AT-8024 and AT-8024GB
Switches. By adjusting the value, you can designate which switch on
your network you want as the root bridge by giving it the lowest bridge
priority number. You should probably also consider which bridge should
function as a backup in the event you need to take the primary root
bridge off-line, and assign that bridge the second lowest bridge
identifier number.