Trunking Overview - Cisco Catalyst 3560-X Software Configuration Manual

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Configuring VLANs

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.

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

trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate

trunking, the interfaces must be in the same VTP domain.

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point

Protocol. However, some internetworking devices might forward DTP frames improperly, which could

cause misconfigurations.

To avoid this, you should configure interfaces connected to devices that do not support DTP to not

forward DTP frames, that is, to turn off DTP.

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

IEEE 802.1Q— industry-standard trunking encapsulation.

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

Switches in an ISL Trunking Environment

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

configuration command to disable trunking.

To enable trunking to a device that does not support DTP, use the switchport mode trunk and

switchport nonegotiate interface configuration commands to cause the interface to become a trunk

but to not generate DTP frames. Use the switchport trunk encapsulation isl or switchport trunk

encapsulation dot1q interface to select the encapsulation type on the trunk port.

Catalyst 6500 series

Chapter 1, "Configuring EtherChannels and Link-State Tracking."

Catalyst 3750-X and 3560-X Switch Software Configuration Guide

Configuring VLAN Trunks

1-4). You can set an interface as

Catalyst 3750-x