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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 12 Configuring VLANs
Configuring VLAN Trunks
Configuring VLAN Trunks
These sections describe how VLAN trunks function on the switch:
• Trunking Overview, page 12-16
• Encapsulation Types, page 12-18
• Default Layer 2 Ethernet Interface VLAN Configuration, page 12-19
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. Fast Ethernet and Gigabit 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 12-2 shows a network of switches that are connected by ISL trunks.
Figure 12-2 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 30, “Configuring EtherChannels.”
Ethernet trunk interfaces support different trunking modes (see Table 12-4). 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.
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