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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter9 Creating and Maintai nin g VLA Ns Understanding VLAN Trunks
Configuring an Ethernet Interface as a Trunk Port
Because trunk ports send and receive VTP advertisements, you mu st en sur e that at le a st one trun k po rt
is configured on the switch and that this trunk port is connected to the trunk port of a second switch.
Otherwise, the switch cannot receive any VTP advertisements.
This section includes these procedures for configurin g an Ethe rnet inter face as a tru nk port o n the swit ch:
•Configuring a Trunk Port, page 9-25
•Defining the Allowed VLANs on a Trunk, page 9-27
•Changing the Pruning-Eligible List, page 9-28
•Configuring the Native VLAN for Untagged Traffic, page 9-29
Note By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport
mode dynamic desirable. If the neighboring interface supports trunking and is configured to allow
trunking, the link is a Layer 2 trunk or, if the interface is in Layer 3 mode, it becomes a Layer 2 trunk
when you enter the switchport interface configuration command. By default, trunks negotiate
encapsulation. If the neighboring interface supports ISL an d 8 02.1Q enc a psula tion an d b oth
interfaces are set to negotiate the encapsulation type, the trunk uses ISL encapsulation.

Configuring a Trunk Port

Beginning in privileged EXEC mode, follow these steps to conf igur e a port as an ISL o r 802. 1Q
trunk port:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter the interface configuration mode and the port to be configured for
trunking.
Step3 switchport trunk encapsulation {isl |
dot1q | negotiate}Configure the port to support ISL or 802.1Q encapsulation or to negotiat e
(the default) with the neighboring interface for en ca psula ti on t ype.
You must configure each end of the link with the same encapsulation type.
Step4 switchport mode {dynamic {auto |
desirable} | trunk}Configure the interface as a Layer 2 trunk (required only if the interface
is a Layer 2 access port or to specify the trunking mode).
•dynamic auto—Set the interface to a trunk link if the neighboring
interface is set to trunk or desirable mode.
•dynamic desirable—Set the interface to a trunk link if the
neighboring interface is set to trunk, desirable, or auto mode.
•trunk—Set the interface in permanent trunking mode and negotiate
to convert the link to a trunk link even if the neighboring interface is
not a trunk interface.
Step5 switchport access vlan vlan-id (Optional) Specify the default VLAN, which is used if the interface sto ps
trunking.
Step6 switchport trunk native vlan vlan-id For 802.1Q trunks, specify the native VLAN.
Step7 end Return to privileged EXEC mode.