Specifications

11-15
Catalyst 2970 Switch Software Configuration Guide
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Chapter 11 Configuring VLANs
Configuring VLAN Trunks
Configuring VLAN Trunks
These sections describe how VLAN trunks function on the switch:
• Trunking Overview, page 11-15
• Encapsulation Types, page 11-16
• Default Layer 2 Ethernet Interface VLAN Configuration, page 11-17
• Configuring an Ethernet Interface as a Trunk Port, page 11-18
• Configuring Trunk Ports for Load Sharing, page 11-22
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)—ISL is Cisco-proprietary trunking encapsulation.
• 802.1Q—802.1Q is industry-standard trunking encapsulation.
Figure 11-2 shows a network of switches that are connected by ISL trunks.
Figure 11-2 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 28, “Configuring EtherChannels.”
Ethernet trunk interfaces support different trunking modes (see Table 11-4). 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 6000 series
switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
ISL
trunk
ISL
trunk
ISL
trunk
ISL
trunk
45828