CCIE Pursuit Blog

January 20, 2009

Enabling Portfast On Trunks

I was working through one of the IE volume I labs and ran across a very easy task.  It simply asked you to configure portfast on all ports connected to routers.  Easy enough, just slap ‘spanning-tree portfast’ on the interfaces connected to the routers.  Easy points.  Next task.

Unfortunately I overlooked the fact that one or the ports connected to a router was trunking (part of a router-on-a-stick configuration).  I blew by the BIG FREAKING CLUE that IOS gave me:

SW2(config)#int f0/6
SW2(config-if)#spann 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 FastEthernet0/6 but will only
have effect when the interface is in a non-trunking mode.

I don’t think that I’ve ever used the ‘trunk’ option with portfast before:

SW2(config-if)#spann portf ?
disable  Disable portfast for this interface
trunk    Enable portfast on the interface even in trunk mode
<cr>

spanning-tree portfast (interface configuration)
trunk – (Optional) Enable the Port Fast feature on a trunking interface.

Sure enough, even though the interface configuration showed that portfast was configured, it was not working on the trunk interface:

interface FastEthernet0/6
switchport trunk encapsulation dot1q
switchport mode trunk
switchport nonegotiate
spanning-tree portfast

SW2#sh spanning-tree int f0/6 portfast
VLAN0001         disabled
VLAN0005         disabled
VLAN0007         disabled
VLAN0008         disabled
VLAN0009         disabled
VLAN0010         disabled
VLAN0022         disabled
VLAN0043         disabled
VLAN0058         disabled
VLAN0067         disabled
VLAN0079         disabled
VLAN0146         disabled

So much for those ‘easy points’.

Here’s what happens with the ‘trunk’ option enabled:

SW2(config)#int fa0/6
SW2(config-if)#spann portfast trunk
%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

SW2#sh spanning-tree int f0/6 portfast
VLAN0001         enabled
VLAN0005         enabled
VLAN0007         enabled
VLAN0008         enabled
VLAN0009         enabled
VLAN0010         enabled
VLAN0022         enabled
VLAN0043         enabled
VLAN0058         enabled
VLAN0067         enabled
VLAN0079         enabled
VLAN0146         enabled

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2 Comments »

  1. Nice on e.

    Comment by Greg Ferro — January 20, 2009 @ 2:42 pm | Reply

  2. I’m not near any lab equipment at the moment, so I’ll just ask the question – what happens if the interface is configured with:

    switchport mode dynamic desirable

    I wonder if the command causes the switch to check the state of the interface or the configuration of the interface.

    Comment by nickwalt — January 29, 2009 @ 10:51 pm | Reply


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