Spanning-Tree Portfast Command on CISCO Router/Switch

Command

Spanning-Tree Portfast

Use

This command essentially disables spanning-tree on an interface and brings it immediately into the forwarding state. Only use this command on access ports

Syntax

Switch(config-if)#spanning-tree portfast

Example

In the below example we will configure SW1’s Fa0/2 to use port-fast to immediately skip to the forwarding state. First, we will flap Fa0/2 and observe how spanning-tree normally treats the interface.

SW1(config)#int fa0/2
SW1(config-if)#shut

The port goes through all the spanning tree timers. Here is the listening phase.

SW1(config-if)#do sh spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg LIS 19 128.2 P2p
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/19 Desg FWD 19 128.19 P2p

Here is the learning phase.

SW1(config-if)#do sh spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg LRN 19 128.2 P2p 
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/19 Desg FWD 19 128.19 P2p

Finally the port is forwarding.

SW1(config-if)#do sh spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p 
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/19 Desg FWD 19 128.19 P2p

Now we will configure portfast on Fa0/2.

SW1(config)#int fa0/2
SW1(config-if)#spanning 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 bridging loops.
Use with CAUTION

%Portfast has been configured on FastEthernet0/2 but will only
have effect when the interface is in a non-trunking mode.

Notice how Fa0/2 is no longer seen in spanning tree.

SW1(config-if)#do sh spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/19 Desg FWD 19 128.19 P2p
Fa0/20 Desg FWD 19 128.20 P2p

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