VLAN Trunking

VLAN Trunking

A switch-to-switch link must be set up as a trunk link in order for the VLAN system to work properly. A trunk link is a special connection, the key difference between an ordinary connection (an Access port) and a Trunk port is that although an Access port is only in one VLAN at a time, a Trunk port has the job of carrying traffic for all VLANs from one switch to another. Any time you connect a switch to another switch, you want to make it a trunk.

There are two methods of identifying VLANs over trunk links are:

 ISL :

The Inter-Switch Link (ISL) protocol is a Cisco-proprietary Layer 2 protocol. ISL operates by re-encapsulating host frames as they are received by the switch port. The ISL encapsulation adds a 26-byte header and a 4-byte trailer to the original host frame.

802.1Q

The IEEE-standard 802.1Q trunk encapsulation has the advantage of being an industry standard, so inter-vendor operation is much less of a problem .This protocol does not re-encapsulate the original frame, but instead inserts a 4-byte tag into the original header.

Important Note : Switch-to-Switch trunk links always require the use of a crossover cable, never a straight-through cable.

Configuring a Trunk Link

Switch1(config-if)#switchport mode {dynamic {auto | desirable} | trunk}

Switch1(config-if)#switchport trunk encapsulation {isl | dot1q | negotiate}

 

Access:The port is a user port in a single VLAN.

Trunk:The port negotiates trunking with the port on the other end of the link.

Non-negotiate:The port is a trunk and does not do DTP negotiation with the other side of the link.

Dynamic Desirable:Actively negotiates trunking with the other side of the link. It becomes a trunk if the port on the other switch is set to trunk, dynamic desirable, or dynamic auto mode.

Dynamic Auto:Passively waits to be contacted by the other switch. It becomes a trunk if the other end is set to trunk or dynamic desirable mode.