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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 13 Configuring Optional Spanning-Tree Fea tures Configuring Optional Spanning-Tree Features
Note You can use the spanning-tree portfast default global con figuratio n command to glob ally en able th e
Port Fast featur e on all nontrun king por ts.
To disab le th e Por t Fast fe atur e, use t he spanning-tree portfast disable inter face con figura ti on
command.

Enabling BPDU Guard

When you globally enable BPDU guard on ports that are Port Fast-enabled (the ports are in a Port
Fast-operational state), sp anning tree shu ts down Port Fast-enabled ports th at receive BPDUs.
In a valid c on figurati on, Por t Fast- enab led po rts d o not re ceive BPDUs. Rec eiving a BPD U on a Port
Fast-enabled port signals an in valid configuration, such as the connection of an unauthorized device, and
the BPDU guard feature puts the port in the error-disabled state. The BPDU guard feature provides a
secure response to invalid configurations because you must manually put the port back in service. Use
the BPDU g uard featur e in a service- prov ider netwo rk to pre v ent an acce ss port from par ticipating in the
spannin g tr ee .
Caution Configure Port Fa st on ly o n p orts tha t c onn ect to en d sta tions; ot herw ise , an a ccide ntal topolo gy loo p
could ca use a data pa cket loop an d disrupt swi tch and net work operat ion.
You can also use th e spanning-tree bpduguard enable interface configuration command to enable
BPDU gu ar d on a ny po rt with o ut als o en ab lin g th e Po r t Fast fe atu r e. When th e po r t re ceiv es a BPD U, it
is put in the error-disabled state.
You can enable the BPDU guard feature if your switch is running PVST or MSTP. The MSTP is a vailable
only if you have the EI in stal led on y our swi tch .
Beginning in privileged EXEC mode, follow these steps to globally enable the BPDU guard feature:
Step 4 end Return to privileged EXEC mode.
Step 5 show spanning-tree interface interface-id
portfast Verify yo ur e ntri es.
Step 6 copy running-config startup-config (Optiona l) Save your entries in the co nfigurati on file.
Command Purpose
Command Purpose
Step 1 configure terminal Enter globa l configurati on mode.
Step 2 spanning-tree portfast bpduguard default Global ly en able B PDU guard.
By default, BPDU gu ard is disa bled.
Step 3 interface interface-id Enter interface configuration mode, and specify the interface
connect ed to an en d station.
Step 4 spanning-tree portfast Enable the Port Fast feature.
Step 5 end Return to privileged EXEC mode.