9-2
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 9 Configuring Spanning-Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
Understanding How PortFast Works
Understanding How PortFast Works
Spanning-tree PortFast causes a switch or trunk port to enter the spanning-tree forwarding state
immediately, bypassing the listening and learning states.
You can use PortFast on switch or trunk ports that are connected to a single workstation, switch, or server
to allow those devices to connect to the network immediately, instead of waiting for the port to transition
from the listening and learning states to the forwarding state.
Caution You can use PortFast to connect a single end station or a switch port to a switch port. If you enable
PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network
loops.
When the switch powers up, or when a device is connected to a port, the port enters the spanning-tree
listening state. When the Forward Delay timer expires, the port enters the learning state. When the
Forward Delay timer expires a second time, the port transitions to the forwarding or blocking state.
When you enable PortFast on a switch or trunk port, the port transitions immediately to the spanning-tree
forwarding state.
Understanding How PortFast BPDU Guard Works
BPDU guard prevents spanning-tree loops by moving a port into the errdisable state when a BPDU is
received on that port. When you enable BPDU guard on the switch, spanning tree shuts down the
interfaces that receive BPDUs instead of putting the interfaces into the spanning-tree blocking state.
When you enable BPDU guard globally and set the port configuration as the default for BPDU guard
(see the “Configuring PortFast BPDU Guard on the Switch” section on page 9-11), then the PortFast
configuration enables or disables BPDU guard.
If the port configuration is not set to default, then PortFast will not affect BPDU guard. Table 9-1 lists
all the possible BPDU guard port configurations. BPDU guard can prevent invalid configurations,
because you must manually put the interface back in service.
Table 9-1 BPDU Guard Port Configurations
Per-Port Configuration Global Configuration PortFast Operational Value Operational BPDU Guard
Default Enable Enable Enable
Default Enable Disable Disable
Default Disable X Disable
Disable X X Disable
Enable X X Enable
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