Cisco Catalyst 3750-X Software Configuration Manual page 461

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Configuring VLANs

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.

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.

You can also specify on DTP interfaces whether the trunk uses ISL or IEEE 802.1Q encapsulation or if

the encapsulation type is autonegotiated. The DTP supports autonegotiation of both ISL and

IEEE 802.1Q trunks.

DTP is not supported on private-VLAN ports or tunnel ports.

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

Switches in an ISL Trunking Environment

Catalyst 6500 series

Chapter 40, "Configuring EtherChannels and Link-State

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

Configuring VLAN Trunks

15-4). You can set an interface as

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Troubleshooting

Catalyst 3560-x