12-20
Catalyst 3560 Switch Software Configuration Guide
78-16156-01
Chapter 12 Configuring VLANs
Configuring VLAN Trunks
Interaction with Other Features
Trunking interacts with other features in these ways:
• A trunk port cannot be a secure port.
• Trunk ports can be grouped into EtherChannel port groups, but all trunks in the group must have the
same configuration. When a group is first created, all ports follow the parameters set for the first
port to be added to the group. If you change the configuration of one of these parameters, the switch
propagates the setting you entered to all ports in the group:
–
allowed-VLAN list
–
STP port priority for each VLAN
–
STP Port Fast setting
–
trunk status: if one port in a port group ceases to be a trunk, all ports cease to be trunks.
• We recommend that you configure no more than 24 trunk ports in PVST mode and no more than 40
trunk ports in MST mode.
• If you try to enable 802.1X on a trunk port, an error message appears, and 802.1X is not enabled. If
you try to change the mode of an 802.1X-enabled port to trunk, the port mode is not changed.
• A port in dynamic mode can negotiate with its neighbor to become a trunk port. If you try to enable
802.1X on a dynamic port, an error message appears, and 802.1X is not enabled. If you try to change
the mode of an 802.1X-enabled port to dynamic, the port mode is not changed.
Configuring a Trunk Port
Beginning in privileged EXEC mode, follow these steps to configure a port as an ISL or 802.1Q
trunk port:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter the interface configuration mode and the port to be configured for
trunking.
Step 3
switchport trunk encapsulation {isl |
dot1q | negotiate}
Configure the port to support ISL or 802.1Q encapsulation or to negotiate
(the default) with the neighboring interface for encapsulation type.
You must configure each end of the link with the same encapsulation
type.
Step 4
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. This is the default.
• 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.
Step 5
switchport access vlan vlan-id (Optional) Specify the default VLAN, which is used if the interface stops
trunking.
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