Concept Guide

• Change the max-age parameter (the refresh interval for conguration information that is generated by recomputing the spanning tree
topology).
PROTOCOL SPANNING TREE mode
max-age seconds
The range is from 6 to 40.
The default is 20 seconds.
To view the current values for global parameters, use the show spanning-tree 0 command from EXEC privilege mode. Refer to the
second example in Enabling Spanning Tree Protocol Globally.
Modifying Interface STP Parameters
You can set the port cost and port priority values of interfaces in Layer 2 mode.
• Port cost — a value that is based on the interface type. The greater the port cost, the less likely the port is selected to be a forwarding
port.
• Port priority — inuences the likelihood that a port is selected to be a forwarding port in case that several ports have the same port
cost.
The default values are listed in Modifying Global Parameters.
To change the port cost or priority of an interface, use the following commands.
• Change the port cost of an interface.
INTERFACE mode
spanning-tree 0 cost cost
The range is from 0 to 65535.
The default values are listed in Modifying Global Parameters.
• Change the port priority of an interface.
INTERFACE mode
spanning-tree 0 priority priority-value
The range is from 0 to 15.
The default is 8.
To view the current values for interface parameters, use the show spanning-tree 0 command from EXEC privilege mode. Refer to
the second example in Enabling Spanning Tree Protocol Globally.
Enabling PortFast
The PortFast feature enables interfaces to begin forwarding trac approximately 30 seconds sooner.
Interfaces forward frames by default until they receive a BPDU that indicates that they should behave otherwise; they do not go through
the Learning and Listening states. The bpduguard shutdown-on-violation option causes the interface hardware to be shut down
when it receives a BPDU. When you only implement bpduguard, although the interface is placed in an Error Disabled state when receiving
the BPDU, the physical interface remains up and spanning-tree drops packets in the hardware after a BPDU violation. BPDUs are dropped
in the software after receiving the BPDU violation.
CAUTION
: Enable PortFast only on links connecting to an end station. PortFast can cause loops if it is enabled on an interface
connected to a network.
To enable PortFast on an interface, use the following command.
Spanning Tree Protocol (STP)
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