To create or configure a VLAN trunk on a
Cisco IOS command-based switch, you must configure the port first as
a trunk and then specify the trunk encapsulation. To do this, issue
the commands:
Switch(config-if)#switchport
mode trunk
Switch(config-if)#switchport trunk encapsulation ?
dot1q Interface uses only 802.1q trunking encapsulation when
trunking
isl Interface uses only ISL trunking encapsulation when trunking
These commands are shown in Figure .
Before attempting to configure a VLAN
trunk on a port, it is wise to determine what encapsulation the port
can support. This can be done using the show
port capabilities command on
a set command-based switch, as shown in Figure . In this example,
you can see that Port 2/1 will support only the IEEE 802.1Q
encapsulation.
To create or configure a VLAN trunk
on a set command-based switch, enter the set
trunk command to configure
the port on each end of the link as a trunk port and to specify the
VLANs that will be transported on this trunk link. You can also use
the set trunk
command to change the mode of a trunk.
Switch> (enable) set
trunk mod_num/port_num
[on | off | desirable | auto | nonegotiate] vlan_range
[isl | dot1q | dot10 | lane | negotiate]
Fast Ethernet and Gigabit Ethernet
trunking modes are as follows:
- On - This mode puts the port into
permanent trunking. The port becomes a trunk port even if the
neighboring port does not agree to the change. The on state does
not allow for the negotiation of an encapsulation type. You
must, therefore, specify the encapsulation in the configuration.
- Off - This mode 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 - This mode 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 - This mode makes the port
willing 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. Notice that if the default setting is left on both sides
of the trunk link, it will never become a trunk; neither side
will be the first to ask to convert to a trunk.
- Nonegotiate - This mode puts the
port into permanent trunking mode but prevents the port from
generating Dynamic Trunking Protocol (DTP) frames. You must
configure the neighboring port manually as a trunk port to
establish a trunk link.
The example in Figure
configures Port 2/1 as a permanent trunk using the IEEE 802.1Q
encapsulation.
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