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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
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Release 8.1
78-15486-01
Chapter 11 Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports
Understanding How VLAN Trunks Work
Fast Ethernet and Gigabit Ethernet trunk ports support five different trunking modes (see
Table 11-1
).
In addition, on certain Fast Ethernet and Gigabit Ethernet ports, you can specify whether the trunk uses
ISL encapsulation, 802.1Q encapsulation, or whether the encapsulation type is autonegotiated.
For autonegotiated trunking on Fast Ethernet and Gigabit Ethernet ports, the ports must be in the same
VTP domain. However, you can use the on or nonegotiate mode to force a port to become a trunk, even
if it is in a different domain. For more information on VTP domains, see
Chapter 9, “Configuring VTP.”
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP). DTP supports autonegotiation
of both ISL and 802.1Q trunks.
Note
Trunking capabilities are hardware dependent. For example, the Catalyst 4500 series switch modules
support only 802.1Q encapsulation. To determine whether your hardware supports trunking, and to
determine which trunking encapsulations are supported, see your hardware documentation or use the
show port capabilities command.
Trunking Modes and Encapsulation Types
Table 11-1
lists the trunking modes used with the set trunk command and describes how they function
on Fast Ethernet and Gigabit Ethernet ports.
Table 11-2
lists the encapsulation type used with the set trunk command and describes how it functions
on Fast Ethernet and Gigabit Ethernet ports. You can use the show port capabilities command to
determine which encapsulation types a particular port supports.
Table 11-1
Fast Ethernet and Gigabit Ethernet Trunking Modes
Mode
Function
on
Puts the port into permanent trunking mode and negotiates to convert the link into a trunk link. The port
becomes a trunk port even if the neighboring port does not agree to the change.
off
Puts the port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link.
The port becomes a nontrunk port even if the neighboring port does not agree to the change.
desirable
Makes the port actively attempt to convert the link to a trunk link. The port becomes a trunk port if the
neighboring port is set to on, desirable, or auto mode.
auto
Enables the port to convert the link to a trunk link. The port becomes a trunk port if the neighboring port
is set to on or desirable mode. This is the default mode for Fast and Gigabit Ethernet ports.
nonegotiate
Puts the port into permanent trunking mode but prevents the port from generating DTP frames. You must
configure the neighboring port manually as a trunk port to establish a trunk link.
Table 11-2
Fast Ethernet and Gigabit Ethernet Trunk Encapsulation Type
Mode
Function
dot1q
Specifies 802.1Q encapsulation on the trunk link. 802.1Q trunks are supported in the Catalyst 4500 series
switch with 802.1Q-capable hardware. Automatic negotiation of 802.1Q trunks is supported in software
release 4.2 and later.
