Configuring An Ethernet Interface As A Trunk Port; Interaction With Other Features - Cisco Catalyst 3750 Software Configuration Manual

Configuring VLAN Trunks

Configuring an Ethernet Interface as a Trunk Port

Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one
trunk port 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 configuring an Ethernet interface as a trunk port on the switch:
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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 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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Catalyst 3750 Metro Switch Software Configuration Guide
10-20
Interaction with Other Features, page 10-20
Defining the Allowed VLANs on a Trunk, page 10-22
Changing the Pruning-Eligible List, page 10-23
Configuring the Native VLAN for Untagged Traffic, page 10-24
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
– 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.
Chapter 10 Configuring VLANs
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