If you
enable PortFast on a port connecting to another switch, you risk creating a spanning-tree
loop.
Trunk
Trunk
PortFast
Access Port
PortFast
Access Port
Trunk
Switch Z
Switch X Switch Y
54 Chapter 2: Medium-Sized Switched Network Construction
Table 2-10 lists the commands used to implement and verify PortFast on an interface.
Example: Spanning-Tree Operation
The best way to understand how spanning tree operates is to look at an operation example.
Figure 2-26 shows a sample network spanning tree topology and the relevant information
used by spanning tree.
Figure 2-26 Spanning Tree Topology
Table 2-10 PortFast Commands
Command Description
Switch(con?¬?g-if)#spanningtree
portfast
Enables PortFast on a Layer 2 access port and forces it to
enter the forwarding state immediately.
Switch(con?¬?g-if)#spanningtree
portfast bpdu-guard
Enables PortFast with BPDU guard. This disables the
switch port if a BPDU is ever received, preventing any
possibility of a loop.
Switch(con?¬?g-if)#no
spanning-tree portfast
Disables PortFast on a Layer 2 access port. PortFast is
disabled by default.
Switch(con?¬?g)#spanning-tree
portfast default
Globally enables the PortFast feature on all nontrunking
ports.
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