Cisco Catalyst 2000 Configuration Handbook page 115

94 Cisco LAN Switching Configuration Handbook
Trunking is a function that must be enabled on both sides of a link. If two switches con-
nect together, for example, both switch ports must be configured for trunking, and they
must both be configured with the same tagging mechanism (ISL or 802.1Q).
To enable trunking between the switches, use the following steps:
Enable trunking on a port.
1.
a. Enable the trunk:
Note Some switches do not support Dynamic Trunking Protocol. For these switches, the
only command that you can use to configure trunking is switchport mode trunk, which
essentially turns trunking on.
Tip Remember that not all switches support DTP and might not establish a trunk without
intervention. Also remember that DTP offers no benefit when you are trunking with a non-
Cisco switch. To eliminate any overhead associated with DTP, it is useful to use the
nonegotiate option when DTP is not supported.
Note When enabling trunking, you cannot specify a range of ports.
(global) interface type mod/port
(interface) switchport mode dynamic [auto | desirable]
(interface) switchport mode trunk
(interface) switchport nonegotiate
The most basic way to configure a trunk link is to use the option on. This option
enables the trunk and requires that you also specify a tagging mechanism for the
trunk. For IOS devices, the command switchport mode trunk is equivalent to
the set trunk mod/port on command. When specifying the option on, you must
also choose a tagging mechanism (see Step 1b).
Many Cisco switches employ an automatic trunking mechanism known as the
Dynamic Trunking Protocol (DTP), which enables a trunk to be dynamically
established between two switches. All integrated switches can use DTP to form a
trunk link. The options of dynamic auto, dynamic desirable, and trunk config-
ure a trunk link using DTP. If one side of the link is configured to trunk and
sends DTP signals, the other side of the link dynamically begins to trunk if the
options match correctly.
If you want to enable trunking and not send any DTP signaling, use the option
nonegotiate for switches that support that function. If you want to disable
trunking completely, use the no switchport mode trunk command.