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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 17 Configuring VLANs
Configuring VLAN Trunks
Configuring VLAN Trunks
These sections describe how VLAN trunks function on the switch:
• Trunking Overview, page 17-15
• 802.1Q Configuration Considerations, page 17-16
• Default Layer 2 Ethernet Interface VLAN Configuration, page 17-17
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.
The switch supports 802.1Q, the industry-standard trunking encapsulation.
Figure 17-2 shows a network of switches that are connected by 802.1Q trunks.
Figure 17-2 Switches in an 802.1Q Trunking Environment
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see Chapter 31, “Configuring EtherChannels.”
Ethernet trunk interfaces support different trunking modes (see Table 17-4). 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.
Catalyst 6500 series
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