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17-17
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-10
Chapter 17 Configuring VLANs
Configuring VLAN Trunks
• Make sure the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the
native VLAN on one end of the trunk is different from the native VLAN on the other end,
spanning-tree loops might result.
• Disabling spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on
every VLAN in the network can potentially cause spanning-tree loops. We recommend that you
leave spanning tree enabled on the native VLAN of an 802.1Q trunk or disable spanning tree on
every VLAN in the network. Make sure your network is loop-free before disabling spanning tree.
Default Layer 2 Ethernet Interface VLAN Configuration
Table 17-5 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 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:
• Interaction with Other Features, page 17-18
• Defining the Allowed VLANs on a Trunk, page 17-19
• Changing the Pruning-Eligible List, page 17-20
• Configuring the Native VLAN for Untagged Traffic, page 17-21
Note The default mode for Layer 2 interfaces is switchport mode dynamic desirable. If the neighboring
interface supports trunking and is configured to allow trunking, the link is a Layer 2 trunk.
Table 17-5 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode switchport mode dynamic desirable
Allowed VLAN range VLANs 1 to 4094 when the EI is installed and 1 to
1005 when the SI is installed
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN (for access ports) VLAN 1
Native VLAN (for 802.1Q trunks) VLAN 1