Configuring Portfast; Enabling Portfast On An Access Port - Cisco WS-C2948G-GE-TX Configuration Manual

Configuring PortFast

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Configuring PortFast
The following sections describe how to configure PortFast on the switch.

Enabling PortFast on an Access Port

You can use PortFast to connect a single end station or a switch port to a switch port. If you enable
Caution
PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network
loops.
To enable PortFast on a switch port, perform this task in privileged mode:
Task
Step 1
Enable PortFast on a switch port that is connected
to a single workstation, switch, or server.
Step 2
Verify the PortFast setting on a switch port.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
8-8
Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
PVID-inconsistent ports. If the port is already blocked by loop guard, misconfigured BPDUs that
are received on the port make loop guard recover, but the port is moved into the type-inconsistent
state or PVID-inconsistent state.
In high-availability switch configurations, if a port is put into the blocked state by loop guard, it
remains blocked even after a switchover to the redundant supervisor engine. The newly activated
supervisor engine recovers the port only after receiving a BPDU on that port.
Loop guard uses the ports known to spanning tree. Loop guard can take advantage of logical ports
provided by the Port Aggregation Protocol (PAgP). However, to form a channel, all the physical
ports grouped in the channel must have compatible configurations. PAgP enforces uniform
configurations of root guard or loop guard on all the physical ports to form a channel.
These caveats apply to loop guard:
Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that
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link becomes unidirectional, loop guard blocks the channel, even if other links in the channel
are functioning properly.
– If a set of ports that are already blocked by loop guard are grouped together to form a channel,
spanning tree loses all the state information for those ports and the new channel port may obtain
the forwarding state with a designated role.
– If a channel is blocked by loop guard and the channel breaks, spanning tree loses all the state
information. The individual physical ports may obtain the forwarding state with the designated
role, even if one or more of the links that formed the channel are unidirectional.
You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure. A loop may
occur until UDLD detects the failure, but loop guard will not be able to detect it.
Loop guard has no effect on a disabled spanning tree instance or a VLAN.
Command
set spantree portfast mod_num/port_num enable
| disable
show spantree [mod_num/port_num] [vlan]
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