13-19
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter13 Configuring VLANs
Configuring VLAN Trunks
Default Layer 2 Ethernet Interface VLAN Configuration
Table13-6 shows the default Layer 2 Ethernet interface VLAN configuration.
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.
These sections contain this configuration information:
•Interaction with Other Features, page 13-19
•Defining the Allowed VLANs on a Trunk, page13-22
•Changing the Pruning-Eligible List, page13-23
•Configuring the Native VLAN for Untagged Traffic, page13-24
Note By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode
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:
•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.
–
STP port priority for each VLAN.
Table13-6 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode switchport mode dynamic auto
Trunk encapsulation switchport trunk encapsulation negotiate
Allowed VLAN range VLANs 1 to 4094
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN (for access ports) VLAN 1
Native VLAN (for IEEE 802.1Q trunks) VLAN 1