Specifications

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Chapter 7 Configuring VLANs

Configuring VLAN Trunks

• Trunking Overview, page 7-12

• Default Layer 2 Ethernet Interface VLAN Configuration, page 7-13

• Configuring an Ethernet Interface as a Trunk Port, page 7-13

• Configuring Trunk Ports for Load Sharing, page 7-16

Trunking Overview

A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device

such as a router or a switch. Ethernet trunks carry the traffic of multiple VLANs over a single link, and you

can extend the VLANs across an entire network. The router supports the IEEE 802.1Q industry-standard

trunking encapsulation.

Ethernet interfaces support different trunking modes (see Table 7-4). You can set an interface as trunking

or nontrunking.

• If you do not intend to trunk across links, use the switchport mode access interface configuration

command to disable trunking.

• To enable trunking, use the switchport mode trunk interface configuration command to change the

interface to a trunk.

IEEE 802.1Q Configuration Considerations

The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:

• In a network of Cisco switches connected through IEEE 802.1Q trunks, the switches maintain one

spanning-tree instance for each VLAN allowed on the trunks. Non-Cisco devices might support one

spanning-tree instance for all VLANs.

When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco

switch combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree

instance of the non-Cisco IEEE 802.1Q switch. However, spanning-tree information for each VLAN

Table 7-4 Layer 2 Interface Modes

Mode Function

switchport mode access Puts the interface (access port) into permanent nontrunking mode and negotiates to

convert the link into a nontrunk link. The interface becomes a nontrunk interface

regardless of whether or not the neighboring interface is a trunk interface. This is the

default mode.

switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the

neighboring link into a trunk link. The interface becomes a trunk interface even if the

neighboring interface is not a trunk interface.

switchport mode dot1q-tunnel Configures the interface as a tunnel (nontrunking) port to be connected in an asymmetric

link with an IEEE 802.1Q trunk port. The IEEE 802.1Q tunneling is used to maintain

customer VLAN integrity across a service provider network. See Chapter 8, “Configuring

IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling,” for more information on tunnel

ports.