VLAN Trunks

At the access layer, end user devices connect to switch ports that provide simple connectivity to a single VLAN each. The attached devices are unaware of any VLAN structure and simply attach to what appears to be a normal physical network segment. Remember, sending information from an access link on one VLAN to another VLAN is not possible without the intervention of an additional device—either a Layer 3 router or an external Layer 2 bridge.

Note that a single switch port can support more than one IP subnet for the devices attached to it. For example, consider a shared Ethernet hub that is connected to a single Ethernet switch port. One user device on the hub might be configured for 192.168.1.1 255.255.255.0, whereas another is assigned 192.168.17.1 255.255.255.0. Although these subnets are discontiguous and unique, and both are communicating on one switch port, they cannot be considered separate VLANs. The switch port supports one VLAN, but multiple subnets can exist on that single VLAN.

A trunk link, however, can transport more than one VLAN through a single switch port. Trunk links are most beneficial when switches are connected to other switches or switches are connected to routers. A trunk link is not assigned to a specific VLAN. Instead, one, many, or all active VLANs can be transported between switches using a single physical trunk link.

Connecting two switches with separate physical links for each VLAN is possible. The top half of Figure 5-2 shows how two switches might be connected in this fashion.

Figure 5-2 Passing VLAN Traffic Using Single Links Versus Trunk Links

VLAN 1 VLAN 1

VLAN 1 VLAN 1

Figure 5-2 Passing VLAN Traffic Using Single Links Versus Trunk Links

VLAN 1 VLAN 1

As VLANs are added to a network, the number of links can grow quickly. A more efficient use of physical interfaces and cabling involves the use of trunking. The bottom half of the figure shows how one trunk link can replace many individual VLAN links.

Cisco supports trunking on both Fast Ethernet and Gigabit Ethernet switch links, as well as aggregated Fast and Gigabit EtherChannel links. To distinguish between traffic belonging to different VLANs on a trunk link, the switch must have a method of identifying each frame with the appropriate VLAN. In fact, the switches on each end of a trunk link both must have the same method for correlating frames with VLAN numbers. The next section covers several available identification methods.

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