Interaction With Other Features; Configuring A Trunk Port - Cisco Catalyst 3750-X Software Configuration Manual

Configuring VLAN Trunks
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By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode
Note
dynamic auto. 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 and IEEE 802.1Q encapsulation and both interfaces are set to
negotiate the encapsulation type, the trunk uses ISL encapsulation.

Interaction with Other Features

Trunking interacts with other features in these ways:
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Configuring a Trunk Port

Beginning in privileged EXEC mode, follow these steps to configure a port as a trunk port:
Command
Step 1
configure terminal
Step 2 interface interface-id
switchport trunk encapsulation {isl |
Step 3
dot1q | negotiate}
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
15-18
Changing the Pruning-Eligible List, page 15-20
Configuring the Native VLAN for Untagged Traffic, page 15-21
A trunk port cannot be a secure port.
A trunk port cannot be a tunnel 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.
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STP port priority for each VLAN.
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STP Port Fast setting.
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trunk status: if one port in a port group ceases to be a trunk, all ports cease to be trunks.
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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 IEEE 802.1x on a trunk port, an error message appears, and IEEE 802.1x is not
enabled. If you try to change the mode of an IEEE 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
IEEE 802.1x on a dynamic port, an error message appears, and IEEE 802.1x is not enabled. If you
try to change the mode of an IEEE 802.1x-enabled port to dynamic, the port mode is not changed.
Purpose
Enter global configuration mode.
Specify the port to be configured for trunking, and enter interface
configuration mode.
Configure the port to support ISL or IEEE 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.
Chapter 15 Configuring VLANs
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