584 | Spanning Tree Protocol (STP)
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Modifying Interface STP Parameters
You can set the port cost and port priority values of interfaces in Layer 2 mode.
• Port cost is 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 influences 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 Table 33-2.
To change the port cost or priority of an interface, use the following commands:
To view the current values for interface parameters, use the
show spanning-tree 0 command from EXEC
privilege mode (Figure 33-5).
Enabling PortFast
The PortFast feature enables interfaces to begin forwarding traffic 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 shutdown 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 STP drops
packets in the hardware after a BPDU violation. BPDUs are
dropped in the software after receiving the BPDU violation.
BPDU Filtering enabled on an interface, stops sending and receiving BPDUs on the port fast enabled ports.
When BPDU guard and BPDU filter is enabled on the port, then BPDU filter takes the highest precedence.
By default bpdu filtering on an interface is disabled.
Task Command Syntax Command Mode
Change the port cost of an interface.
Range: 0 to 65535
Default: see Table 33-2.
spanning-tree 0 cost cost INTERFACE
Change the port priority of an interface.
Range: 0 to 15
Default: 8
spanning-tree 0 priority
priority-value
INTERFACE
Caution: Enable PortFast only on links connecting to an end station. PortFast can cause loops if you enable it on an
interface connected to a network.
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