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Chapter 13
Configuring VLANs
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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
OL-12247-04
Changing the Pruning-Eligible List, page 13-22
Configuring the Native VLAN for Untagged Traffic, page 13-22
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:
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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.
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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.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
Configuring VLAN Trunks
13-19
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