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What is 802.1Q Trunking?


802.1Q or VLAN Tagging, is a networking standard written by the IEEE 802.1 workgroup allowing multiple bridged networks to transparently share the same physical network link without leakage of information between networks. IEEE 802.1Q - along with its shortened form dot1q - is commonly used to refer to the encapsulation protocol used to implement this mechanism of Ethernet networks. This protocol allows for individual VLANs to communicate with one another with the use of a switch with layer-3 capabilities, or a router.




Switches with factory defaults work fine with more then one VLAN configured. If you do use VLANs, you should add some configuration. Trunking configuratin on Cisco switches involves two important configuation choices:

      (1) - The type of trunking: IEEE 802.1Q, ISL, or negotiate which one to use

             configure using:
                   "interface type mod/port" (global command)
                   "switchport trunk encapsulation [negotiate | isl | dot1Q]" (interface subcommand)



      (2) - The administrative mode: Whether to trunk, not trunk, or negotiate

             configure using:
                   "switchport mode" (interface subcommand)


Cisco switches can either negotiate or configure the type of trunking to use (ISL or 802.1Q). By default, Cisco switches negotiate the type of trunking with the switch on the other end of the trunk, using the Dynamic Trunk Protocol (DTP). When negotiating, if both switches support both ISL and 802.1Q, they choose ISL. If one switch is willing to use either type, and the other switch is only willing to use one type of trunking, the two swiches agree to use that one type of trunking supported by both switches. The type of trunking preferred on an interface, for switches that support both types, is configured using the "switchport trunk encapsulation {dot1q | isl | negotiate}" interface subcommand. (Many of the most recently developed Cisco switches, including 2960s, only support the IEEE-standard 802.1Q trunking today, so these switches simply default to a setting of "switchport trunk encapsulation dot1q".)




The administrative mode refers to the configuation setting for whether trunking should be used on an interface. The term administrative refers to what is configured, whereas an interface's operational mode refers to what is currently happening on the interface. Cisco switches use an interface's administrative mode, as configured with the "switchport mode" interface subcommand, to determine whether to use trunking.


Trunking Administrative Mode Options with the "switchport mode" Command:

access - Prevents the use of trunking, making the port always act as an access (nontrunk) port
trunk - Always uses trunking
dynamic desirable - *Initiates* negotiation messages and responds to negotiation message to dynamically choose whether to start using trunking, and defines the trunking encapsulation
dynamic auto - Passively *waits* to receive trunk negotiation messages, at which point the switch will respond and negotiate whether to use trunking, and if so, the type of trunking







References

Odom, Wendell (2008) CCNA ICND2 Official Exam Certification Guide, Second Edition. Indianapolis: Cisco Press.