Trunking Facts

Trunking Facts

Trunking is a term used to describe connecting two switches together. Trunking is important when you configure VLANs that span multiple switches as shown in the diagram.

Be aware of the following facts regarding trunking and VLANs:

• In the above graphic, each switch has two VLANs. One port on each switch has been assigned to each VLAN.
• Workstations in VLAN 1 can only communicate with workstations in VLAN 1. This means that the two workstations connected to the same switch cannot communicate with each other. Communications within the VLAN must pass through the trunk link to the other switch.
• Trunk ports identify which ports are connected to other switches.
• Trunk ports can automatically carry traffic for all VLANs defined on the switch. You can prevent traffic from a specific VLAN from being carried on the trunk through a specific configuration.
• Typically, Gigabit Ethernet ports are used for trunk ports, although any port can be a trunking port.

When trunking is used, frames that are sent over a trunk port are tagged with the VLAN ID number so that the receiving switch knows to which VLAN the frame belongs.

• Tags are appended by the first switch in the path, and removed by the last.
• Only VLAN-capable devices understand the frame tag.
• Tags must be removed before a frame is forwarded to a non-VLAN-capable device.

The trunking protocol describes the format that switches use for tagging frames with the VLAN ID. Cisco devices support two trunking protocols:

Trunking Protocol	Characteristics
Inter-Switch Link (ISL)	A Cisco-proprietary trunking protocol. ISL can only be used between Cisco devices. ISL tags each frame with the VLAN ID. Catalyst 2960 switches do not support ISL.
802.1Q	An IEEE standard for trunking and therefore supported by a wide range of devices. With 802.1Q trunking, frames from the default VLAN 1 are not tagged. Frames from all other VLANs are tagged.

Cisco switches have the ability to automatically detect ports that are trunk ports, and to negotiate the trunking protocol used between devices. Switches use the Dynamic Trunking Protocol (DTP) to detect and configure trunk ports. For example, when you connect two switches together, they will automatically recognize each other and select the trunking protocol to use.