Networking

Virtual LAN trunking on Cisco Catalyst switches

Confused about trunking between two switches? Get connected as Warren Heaton explains trunking basics and configuration options.


More and more networks administrators are using Layer 2 switching as the data transport method of choice. As these networks grow, the network is eventually divided into multiple virtual LANs or VLANS. (For more information on VLANs, check out our recent article “Microsegment your network with a virtual LAN.”) In addition, there are only a finite number of physical ports available on the Layer 2 switches. This means that in order for a network to grow, multiple Layer 2 switches must be interconnected. The process of connecting one switch to another switch, router, or server, is known as trunking.

Trunking basics
A trunk can be a link between two switches, a switch and a router, or a switch and a server. Trunks support traffic for multiple VLANs and allow the administrator to extend VLANs across multiple switches. Additionally, a trunk line connecting a switch to a router allows inter-VLAN routing (remember—hosts belonging to different VLANs can communicate with each other only through a router). Lastly, if there are multiple VLANs that must communicate with a single server, a trunk can be established between the switch and the server. The most popular method of trunking is between two switches, and this is the method we will be discussing.

Because a trunk link carries traffic, or frames, from multiple VLANs, the switch must have a method of identifying which VLAN a frame belongs to. Cisco supports four methods of frame identification:
  1. Cisco Inter-Switch Link (ISL)—The Cisco proprietary trunking method used over Fast Ethernet, Gigabit Ethernet, and EtherChannel
  2. IEEE 802.1Q—The IEEE industry standard trunking method, also used over Fast Ethernet, Gigabit Ethernet, and EtherChannel
  3. 802.10—The Cisco proprietary method of trunking over Fiber Distributed Data Interface (FDDI)
  4. LAN Emulation (LANE)—The IEEE standard for trunking over Asynchronous Transfer Mode (ATM) networks

Configuring a trunk
When configuring a trunk link, there are five different switch port modes:
  1. auto—This is the default mode for Fast Ethernet and Gigabit Ethernet ports. In this mode, a port will become a trunk port if the device the port is connected to is set to the on or desirable mode.
  2. on—This mode sets the port to permanent trunking mode.
  3. off—This mode sets the port to permanent non-trunking mode.
  4. desirable—This mode allows the port to become a trunk port if the device the port is connected to is set to the on, desirable, or auto mode.
  5. nonegotiate—This mode sets the port to permanent trunking mode. (Note: this mode prevents the port from generating Dynamic Trunking Protocol (DTP) frames. DTP frames can be used to negotiate trunking on Fast Ethernet or Gigabit Ethernet using ISL or IEEE 802.1Q.)

To create the VLAN trunk, the set trunk command is used on both ends of the trunk link. This command establishes the trunk, sets the trunking mode, specifies the range of VLANs transmitted over the trunk, and chooses the trunking method. Here is the command syntax:
Console> (enable) set trunk mod_num/port_num [on | off | desirable | auto | nonegotiate] vlan_range [isl | dot1q | dot10 | lane | negotiate]

More to come
VLAN trunking is an intricate task requiring complex configuration and troubleshooting. This has been an introduction to the basics. In the next few weeks, I will explain some of the more complex issues with VLAN trunking and present a few of the solutions Cisco offers to make VLAN trunking easier to support and configure.

Warren Heaton Jr., MCSE+I, CCNP, CCDP, is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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