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16-16
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 16 Configuring VLANs
Configuring VLAN Trunks
Default Layer 2 Ethernet Interface VLAN Configuration
Table 16-5 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 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:
• Interaction with Other Features, page 16-16
• Defining the Allowed VLANs on a Trunk, page 16-18
• Changing the Pruning-Eligible List, page 16-19
• Configuring the Native VLAN for Untagged Traffic, page 16-19
Note The default mode for interfaces is switchport mode dynamic desirable interface configuration mode.
If the neighboring interface supports trunking and is configured to allow trunking, the link is a Layer 2
trunk.
Interaction with Other Features
Trunking interacts with other features in these ways:
• 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 that setting 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.)
• 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.
Ta b l e 16-5 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode
Allowed VLAN range VLANs 1 to 4094.
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN VLAN 1
Native VLAN (for IEEE 802.1Q trunks) VLAN 1