PortFast, BPDU Guard, BPDU Filtering, and UplinkFast

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BPDU (Bridge Protocol Data Unit): A specialized frame to transfer the spanning tree information in 801.d STP (Spanning Tree Protocol). RSTP (IEEE 802.w, Rapid Spanning Tree Protocol): A standard protocol revised from STP (Spanning Tree Protocol).

RSTP and STP Port States

Operational State STP State (802.1d) RSTP State (802.1w) Forwards Data Frames?
Enabled Blocking Discarding No
Enabled Listening Discarding No
Enabled Learning Learning No
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No


PortFast is Cisco’s technology to speed up convergence for access ports when they become active. PortFast mode supports only nontrunking access ports which do not transmit or receive BPDUs for preventing loops occured in a network. The following shows how to enable portfast on Cisco switch:

Switch> enable Switch# configure terminal Switch (config)# interface fastEthernet 0/1 Switch (config-if)# spanning-tree portfast

BPDU Guard

Portfast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. By enabling BPDU guard, spanning tree shuts down Portfast-enabled interfaces instead of putting them into the spanning tree blocking state.

BPDU Filtering

BPDU filtering allows to avoid transmitting BPDUs on Portfast-enabled ports that are connected to end system. By enabling Portfast on the switch, spanning tree places ports in the forwarding state immediately, instead of going through the listening, learning, and forwarding states.


Uplinkfast provides fast convergence using uplink groups in the network access layer after a spanning tree topology change. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports (not including self-looped ports). The uplink group provides an alternate path in case the currently forwarding link fails.


Cisco Systems, Inc. “Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard.” In Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide, 8.1-8.20, 2007. http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html.

Posted by chlee


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