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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Chapter 6 Configuring the System
Configuring STP
switches in the VLAN; however, if you are running STP only on a minimal set
of switches, an incautious change to the network that introduces another loop
into the VLAN can result in a broadcast storm.
Note
If you have the default allowed list on the trunk ports of that switch, the new
VLAN is carried on all trunk ports. Depending on the topology of the network,
this could create a loop in the new VLAN that will not be broken, particularly
if there are several adjacent switches that all have run out of STP instances.
You can prevent this by setting allowed lists on the trunk ports of switches that
have used up their allocation of STP instances. Setting up allowed lists is not
necessary in many cases and makes it more labor-intensive to add another
VLAN to the network.
Using STP to Support Redundant Connectivity
You can create a redundant backbone with STP by connecting two of the switch
ports to another device or to two different devices. STP automatically disables one
port but enables it if the other port is lost. If one link is high-speed and the other
low-speed, the low-speed link is originally disabled. If the two link speeds are the
same, the port priority and the port ID are added together, and STP disables the
link with the lowest value.
You can also create redundant links between switches by using EtherChannel port
groups. For more information about creating port groups, see the
“Creating
EtherChannel Port Groups” section on page 7-10
.
Disabling STP
STP is enabled by default. Disable STP only if you are sure there are no loops in
the network topology.
Caution
When STP is disabled and loops are present in the topology, excessive traffic
and indefinite packet duplication can severely reduce network performance.