Feb 132012
 

So by it’s definition an access port can only belong to one VLAN while a trunk port can belong to multiple VLANs. It’s important to distinguish that we’re talking about single ports. A trunk group or trunk port group is made up of multiple ports which are combined into a single virtual port. Protocols such as MultiLink Trunking (Avaya), EtherChannel (Cisco) and LACP provide the ability to combine multiple trunk ports into a single virtual interface providing redundancy and additional bandwidth.

via untagAll vs tagAll on Avaya Ethernet Routing Switches | Michael McNamara.

The above is an oversimplification, but I thought Mr. McNamara did a reasonable job of explaining the four 802.1q tagging methods (untagAll, untagPvidOnly, tagPvidOnly, tagAll).

My attribution of oversimplification comes when you combine the above claim that access ports can only belong to one VLAN, with the definition of untagPvidOnly then combine it with the instruction to use untagPvidOnly with converged VoIP/Desktop solutions (plug the PC in to the phone, which plugs in to the LAN). Since converged VoIP solutions that place the VoIP and desktop traffic in different VLANs recommend untagPvidOnly and because doing so causes the port to belong to more than one VLAN which creates a conflict with his summry of access vs trunk.

If you skip over this one little self contradiction, the rest of the information is quite good. Once I get through my CS1000 IP Phone registration process, I might do a series on the Ethernet Routing Switches.