Portfast

When a new switch port joins STP network, it starts at blocking state, stays as blocking for 20 seconds, then transition to listening state, stay there for 15 seconds, then transition to learning state, stay there for 15 seconds, finally transition to forwarding state and start to forwarding frame.

Portfast is a cisco preparatory feature which allows a port participating STP to skip the first 3 states, namely blocking, listen, learning, and directly go to forwarding state. This allow the port start to work 50 seconds earlier than usual, thus the name portfast.

You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops.

The cisco commands for enable portfast on a switch port is as the follows.

Console> (enable) set spantree portfast 4/1 enable

Warning:Connecting Layer 2 devices to a fast start port can cause

temporary spanning tree loops. Use with caution.

Spantree port  4/1 fast start enabled.

Console> (enable) show spantree 4/1

Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method

--------- ----  -------------  -----  --------  ----------  ------------

 4/1      1     blocking          19        20   enabled              

 4/1      100   forwarding        10        20   enabled              

 4/1      521   blocking          19        20   enabled              

 4/1      522   blocking          19        20   enabled              

 4/1      523   blocking          19        20   enabled              

 4/1      524   blocking          19        20   enabled              

 4/1      1003  not-connected     19        20   enabled              

 4/1      1005  not-connected     19         4   enabled              

Console> (enable)

You can disable portfast on a switch port as well.

Console> (enable) set spantree portfast bpdu-guard disable

Spantree portfast bpdu-guard disabled on this switch.

Console> (enable) show spantree summary

Summary of connected spanning tree ports by vlan

Portfast bpdu-guard disabled for bridge.

Uplinkfast disabled for bridge.

Backbonefast disabled for bridge.

Vlan  Blocking Listening Learning Forwarding STP Active

----- -------- --------- -------- ---------- ----------

   1         0         0        0          4          4

   2         0         0        0          4          4

   3         0         0        0          4          4

   4         0         0        0          4          4

.

.

.

1003         0         0        0          0          0

1005         0         0        0          0          0

      Blocking Listening Learning Forwarding STP Active

----- -------- --------- -------- ---------- ----------

Total        0         0        0         85         85

Console> (enable) 

There are other portfast command variations, such as enable/disable them in a bulk or set portfast globally, these variations can be retrieved from cisco command line help.


[ICND1 and ICND2 break down]