User's Manual

13-16
Catalyst 3750 Switch Software Configuration Guide
OL-8550-09
Chapter 13 Configuring VLANs
Configuring VLAN Trunks
Configuring VLAN Trunks
These sections contain this conceptual information:
• Trunking Overview, page 13-16
• Encapsulation Types, page 13-18
• Default Layer 2 Ethernet Interface VLAN Configuration, page 13-19
• Configuring an Ethernet Interface as a Trunk Port, page 13-19
• Configuring Trunk Ports for Load Sharing, page 13-24
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.
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 36, “Configuring EtherChannels and Link-State
Tracking.”
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.
Catalyst 6500 series
switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
ISL
trunk
ISL
trunk
ISL
trunk
ISL
trunk
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