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16-3
Spanning Tree Protocol (STP)
Overview
Spanning Tree Protocol
(STP)
As recommended in the IEEE 802.1Q VLAN standard, the Switch 4108GL uses
single-instance STP. (As a result, the switch generates untagged Bridge
Protocol Data Units—BPDUs.) This implementation creates a single spanning
tree to make sure there are no network loops associated with any of the
connections to the switch, regardless of whether multiple VLANs are config-
ured on the switch. Thus, these switches do not distinguish between VLANs
when identifying redundant physical links. If VLANs are configured on the
switch, see “STP Operation with 802.1Q VLANs” on page 16-12.
STP Fast Mode for Overcoming Server Access Failures. If an end node
is configured to automatically access a server, the duration of the STP startup
sequence can result in a “server access failure”. On ports where this is a
problem, configuring STP Fast Mode can eliminate the failure. For more
information, see “STP Fast Mode” on page 16-10. The next sections describe
how to configure STP on the switch. For more information on STP operation,
see “How STP Operates” on page 16-9.
In the factory default configuration, STP is off. If a redundant link (loop) exists
between nodes in your network, you should enable Spanning Tree.
Note STP retains its current parameter settings when disabled. Thus, if you
disable STP, then later re-enable it, the parameter settings will be the same
as before STP was disabled.
Caution Because the switch automatically gives faster links a higher priority, the
default STP parameter settings are usually adequate for spanning tree
operation. Also because incorrect STP settings can adversely affect network
performance, you should not make changes unless you have a strong under-
standing of how STP operates. For more on STP, see the IEEE 802.1D standard.