Understanding How Portfast Works; Understanding How Portfast Bpdu Guard Works; Understanding How Portfast Bpdu Filtering Works - Cisco WS-C2948G-GE-TX Configuration Manual

Understanding How PortFast Works

Understanding How PortFast Works
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.
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.
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 is transitioned to the forwarding or blocking state.
When you enable PortFast on a switch or trunk port, the port is immediately transitioned to the spanning
tree forwarding state.

Understanding How PortFast BPDU Guard Works

To prevent loops from occurring in a network, the PortFast mode is supported only on nontrunking
access ports because these ports typically do not transmit or receive BPDUs. The most secure
implementation of PortFast is to enable it only on ports that connect end stations to switches. Because
PortFast can be enabled on nontrunking ports connecting two switches, spanning tree loops can occur
because BPDUs are still being transmitted and received on those ports.
PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU
is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down
PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree
blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. If a
PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a
secure response to invalid configurations because the administrator must manually put the interface back
in service.
When you enable BPDU guard on the switch, spanning tree applies BPDU guard to all
Note
PortFast-configured interfaces.

Understanding How PortFast BPDU Filtering Works

BPDU filtering allows you to avoid transmitting BPDUs on PortFast-enabled ports that are connected to
an end system. When you enable PortFast on the switch, spanning tree places ports in the forwarding
state immediately, instead of going through the listening, learning, and forwarding states.
By default, spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled. BDPU
filtering is on a per-switch basis; after you enable BPDU filtering, it applies to all PortFast-enabled ports
on the switch.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
8-2
Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
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