RSTP and Port Fast

How much PortFast is actually fast? Well, let’s see.

For this purpose I will use debug for spanning-tree to see how much it is take to make interface in forwarding state.

SW_1#debug spanning-tree events

I choose interface FastEthernet 1/10 on my SW_1, and I shutdown interface.

That interface is in VLAN 1 and it is access port, because PortFast have to be configured on access port only, not trunk port.

So let’s make this interface UP/UP…

SW_1(config)#interface fastethernet 1/10

SW_1(config-if)#no shutdown

SW_1(config-if)#

*Mar  1 00:33:20.215: STP: VLAN1 Fa1/10 -> listening -- 15 sec forward delay

*Mar  1 00:33:35.231: STP: VLAN1 Fa1/10 -> learning -- 15 sec  forward delay

*Mar  1 00:33:50.255: STP: VLAN1 Fa1/10 -> forwarding

SW_1(config-if)#

As you can see its need 30 sec to goes from blocking to forwarding state.

So let’s configure interface with PortFast command, port fast command goes under interface mode.

SW_1(config-if)#spanning-tree portfast

%Warning: portfast should only be enabled on ports connected to a single host.

 Connecting hubs, concentrators, switches, bridges, etc.to this interface

 when portfast is enabled, can cause temporary spanning tree loops.

 Use with CAUTION

%Portfast has been configured on FastEthernet1/10 but will only

 have effect when the interface is in a non-trunking mode.

SW_1(config-if)#shutdown

SW_1(config-if)#

*Mar  1 00:41:54.219: STP: VLAN1 Fa1/10 -> blocking

SW_1(config-if)#no shutdown

SW_1(config-if)#

*Mar  1 00:42:11.563: STP: VLAN1 Fa1/10 ->jump to forwarding from blocking

*Mar  1 00:42:12.555: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up

*Mar  1 00:42:14.311: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/10, changed state to up

SW_1(config-if)#

And as you can see its need, well it doesn’t need a second it’s just jump from blocking state to forwarding state. Conclusion, PortFast is fast J

To check PortFast on interface do show command…

SW_1#show spanning-tree interface fastEthernet 1/10

 Port 51 (FastEthernet1/10) of VLAN1 is forwarding

   Port path cost 19, Port priority 128, Port Identifier 128.51.

   Designated root has priority 32768, address c203.0f60.0000

   Designated bridge has priority 32768, address c203.0f60.0000

   Designated port id is 128.51, designated path cost 0

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   BPDU: sent 1273, received 0

   The port is in the portfast mode

To enable PortFast on all non-trunk port issue command under global configuration mode

 SW_1(config)#spanning-tree portfast default