VLAN tagging and trunking

[dminute]

I am trying to get a grapso on VLAN tagging and trunking. Namely, when a frame enters the switch, is it tagged with the VLAN ID at that interface? And if so, how does it know to tag it using ISL or 802.1q if you have both types of trunks on the switch?
Is this tag a different type of VLAN ID which is then encapsulated (ISL) or inserted (802.1q), and if so, where is the tag in relation to the incoming frame?
Any clarification is appreciated.

 

[l1carter]

Dminute,

There may be some confusion here. When a switch port is set to trunk vlans with a particular encapsulation method (ISL or Dot1q) VLAN's are tagged / inserted by that switch

(on the inbound but just before it's sent outbound to the remote switch) This is how the remote end knows what VLAN it is.

The switch would have to "assume too much" to tag or insert VLAN ID on input packets from other switches...

hope this helps.