Concept Guide

● 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 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 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 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.
● Enable PortFast on an interface.
INTERFACE mode
spanning-tree stp-id portfast [bpduguard | [shutdown-on-violation]]
To verify that PortFast is enabled on a port, use the show spanning-tree command from EXEC Privilege mode or the show
config command from INTERFACE mode. Dell EMC Networking recommends using the show config command.
DellEMC#(conf-if-gi-1/1)#show conf
!
interface GigabitEthernet 1/1
no ip address
switchport
spanning-tree 0 portfast
no shutdown
DellEMC#(conf-if-gi-1/1)#
Prevent Network Disruptions with BPDU Guard
Configure the Portfast (and Edgeport, in the case of RSTP, PVST+, and MSTP) feature on ports that connect to end stations.
End stations do not generate BPDUs, so ports configured with Portfast/ Edgport (edgeports) do not expect to receive BDPUs.
If an edgeport does receive a BPDU, it likely means that it is connected to another part of the network, which can negatively
affect the STP topology. The BPDU Guard feature blocks an edgeport after receiving a BPDU to prevent network disruptions,
and Dell EMC Networking OS displays the following message.
Enable BPDU Guard using the bpduguard option when enabling PortFast or EdgePort. The bpduguard shutdown-on-
violation option causes the interface hardware to be shut down when it receives a BPDU. Otherwise, although the interface
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Spanning Tree Protocol (STP)