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Catalyst 3750 Switch Software Configuration Guide
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Chapter 13 Configuring VLANs
Configuring VLAN Trunks
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:
• Inter-Switch Link (ISL)—ISL is Cisco-proprietary trunking encapsulation.
• 802.1Q—802.1Q is industry-standard trunking encapsulation.
Figure 13-2 shows a network of switches that are connected by ISL trunks.
Figure 13-2 Switches in an ISL Trunking Environment
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see Chapter 33, “Configuring EtherChannels.”
Ethernet trunk interfaces support different trunking modes (see Table 13-4). You can set an interface as
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.
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