9-22
Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 9 Creating and Maintaining VLANs
Understanding VLAN Trunks
Understanding VLAN Trunks
These sections describe how VLAN trunks function on the switch:
•
Trunking Overview, page 9-22
•
Encapsulation Types, page 9-23
•
Default Layer 2 Ethernet Interface VLAN Configuration, page 9-24
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. Trunks carry the traffic of multiple VLANs over a single link, and you can extend
VLANs across an entire network. The 100BASE-T and Gigabit Ethernet trunks carry traffic for multiple
VLANs over a single link.
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 9-4
shows a network of switches that are connected by ISL trunks.
Figure 9-4
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 21, “Configuring EtherChannel.”
Ethernet trunk interfaces support different trunking modes (see
Table 9-6
). You can specify whether the
trunk uses ISL or 802.1Q encapsulation or if the encapsulation type is autonegotiated. To autonegotiate
trunking, the interfaces must be in the same VTP domain. Use the trunk or nonegotiate keywords to
force interfaces in different domains to trunk. Trunk negotiation is managed by the Dynamic Trunking
Protocol (DTP), which supports autonegotiation of both ISL and 802.1Q trunks.
Catalyst 5000 series
switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3
VLAN1
VLAN1
VLAN2
VLAN3
ISL
trunk
ISL
trunk
ISL
trunk
ISL
trunk
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