Understanding Port Fast; Understanding Bpdu Guard - Cisco Catalyst 2950 Software Manual

Understanding Advanced STP Features

Understanding Port Fast

Port Fast immediately brings an interface configured as an access port to the forwarding state from a
blocking state, bypassing the listening and learning states. You can use Port Fast on access ports
connected to a single workstation or server, as shown in
immediately connect to the network, rather than waiting for STP to converge.
If the interface receives a BPDU, which should not happen if the interface is connected to a single
workstation or server, STP puts the port in the blocking state. An interface with Port Fast enabled goes
through the normal cycle of STP status changes when the switch is restarted.
Because the purpose of Port Fast is to minimize the time access ports must wait for STP to converge, it
Note
is effective only when used on access ports. If you enable Port Fast on a port connecting to another
switch, you risk creating a spanning-tree loop.
Figure 9-4
Catalyst 2950

Understanding BPDU Guard

When the BPDU guard feature is enabled on the switch, STP shuts down Port Fast-enabled interfaces
that receive BPDUs rather than putting them into the blocking state. In a valid configuration, Port
Fast-enabled interfaces do not receive BPDUs. Receipt of a BPDU by a Port Fast-enabled interface
means an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard
feature places the interface into the ErrDisable state. The BPDU guard feature provides a secure response
to invalid configurations because you must manually put the interface back in service.
When enabled on the switch, STP applies the BPDU guard feature to all Port Fast-enabled interfaces.
Note
Catalyst 2950 Desktop Switch Software Configuration Guide
9-10
Port Fast-Enabled Ports
Catalyst 3550
series switch
switch
Fast-enabled
Workstations
Figure
Catalyst 2950-T
switch
Port
ports
Workstations
Chapter 9 Configuring STP
9-4, to allow those devices to
Server
Port
Fast-enabled port
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