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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 16 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. 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 IEEE 802.1Q, the industry-standard trunking encapsulation.
Figure 16-2 shows a network of switches that are connected by IEEE 802.1Q trunks.
Figure 16-2 Switches in an IEEE 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 30, “Configuring EtherChannels.”
Ethernet trunk interfaces support different trunking modes (see Table 16-4). 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.
• With GigaStack GBICs, dynamic trunking is only supported when two switches are connected by a
single GigaStack GBIC link. If trunking is required when more than two switches in a stack are
connected by GigaStack GBIC links, you must manually configure trunking in this manner:
Catalyst 6500 series
switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
Trunk
port
Trunk
port
Trunk
port
Trunk
port
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