Understanding Vlan Trunks; Trunking Overview - Cisco WS-C3550-12G Software Configuration Manual

Multilayer switch

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Understanding VLAN Trunks

These sections describe how VLAN trunks function on the switch:

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. Trunks carry the traffic of multiple VLANs over a single link, and you can extend

VLANs across an entire network. The 100BASE-T and Gigabit Ethernet trunks carry traffic for multiple

VLANs over a single link.

Two trunking encapsulations are available on all Ethernet interfaces:

You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more

information about EtherChannel, see

Ethernet trunk interfaces support different trunking modes (see

trunk uses ISL or 802.1Q encapsulation or if the encapsulation type is autonegotiated. To autonegotiate

trunking, the interfaces must be in the same VTP domain. Use the trunk or nonegotiate keywords to

force interfaces in different domains to trunk. Trunk negotiation is managed by the Dynamic Trunking

Protocol (DTP), which supports autonegotiation of both ISL and 802.1Q trunks.

Catalyst 3550 Multilayer Switch Software Configuration Guide

Trunking Overview, page 9-22

Encapsulation Types, page 9-23

Default Layer 2 Ethernet Interface VLAN Configuration, page 9-24

Inter-Switch Link (ISL)—ISL is Cisco-proprietary trunking encapsulation.

802.1Q—802.1Q is industry-standard trunking encapsulation.

shows a network of switches that are connected by ISL trunks.

Switches in an ISL Trunking Environment

Catalyst 5000 series

Chapter 21, "Configuring EtherChannel."

Creating and Maintaining VLANs

9-6). You can specify whether the

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