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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 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)—Cisco-proprietary trunking encapsulation.
•
IEEE 802.1Q— industry-standard trunking encapsulation.
shows a network of switches that are connected by ISL trunks.
Figure 1-2
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 1, “Configuring EtherChannels and Link-State Tracking.”
Ethernet trunk interfaces support different trunking modes (see
). 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.
Catalyst 6500 series
switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3
VLAN1
VLAN1
VLAN2
VLAN3
ISL
trunk
ISL
trunk
ISL
trunk
ISL
trunk
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