trunk funk...

to make a trunk two commands are necessary (see below for ISL)

switchport trunk encap dot1q
and
sw mode trunk

in that order for IEEE

sw3550_01(config-if)#do sh run int fa0/17
Building configuration...

Current configuration : 82 bytes
!
interface FastEthernet0/17
 switchport mode access
 speed 100
 duplex full
end

sw3550_01(config-if)#sw mode trunk
Command rejected: An interface whose trunk encapsulation is "Auto" can not be co
nfigured to "trunk" mode.
(that's a hell of an error message, it was access)
sw3550_01(config-if)#sw trunk encap dot1q
sw3550_01(config-if)#sw mode trunk
sw3550_01(config-if)#^Z
sw3550_01#sh run int fa0/17
Building configuration...

Current configuration : 119 bytes
!
interface FastEthernet0/17
 switchport trunk encapsulation dot1q
 switchport mode trunk
 speed 100
 duplex full
end

sw3550_01#

and cisco's ISL

sw3550_01(config-if)#sw trunk encap isl
sw3550_01(config-if)#sw mode trunk

sw3550_01(config-if)#do sh run int fa0/17
Building configuration...

Current configuration : 117 bytes
!
interface FastEthernet0/17
 switchport trunk encapsulation isl
 switchport mode trunk
 speed 100
 duplex full
end

sw3550_01(config-if)#

to dump the trunk, the opposite is true...

sw3550_01(config-if)#no sw trunk encap isl
Command rejected: A port which is configured to "trunk" mode can not be configur
ed to negotiate the encapsulation.
sw3550_01(config-if)#no sw mode trunk
sw3550_01(config-if)#no sw trunk encap isl
sw3550_01(config-if)#do sh run int fa0/17
Building configuration...

Current configuration : 93 bytes
!
interface FastEthernet0/17
 switchport mode dynamic desirable
 speed 100
 duplex full
end

sw3550_01(config-if)#sw mode access
sw3550_01(config-if)#do sh run int fa0/17
Building configuration...

Current configuration : 82 bytes
!
interface FastEthernet0/17
 switchport mode access
 speed 100
 duplex full
end

sw3550_01(config-if)#

funk trunk...