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
Nice on e.
Comment by Greg Ferro — January 20, 2009 @ 2:42 pm |
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 |