Defining The Allowed Vlans On A Trunk - Cisco Catalyst 2950 Software Configuration Manual

Chapter 13 Configuring VLANs
This example shows how to configure the Fast Ethernet interface 0/4 as an 802.1Q trunk and shows
several ways to verify the configuration. The example assumes that the neighbor interface is configured
to support 802.1Q trunking.
Switch# configure terminal
Enter configuration commands, one per line.
Switch(config)# interface fastethernet0/4
Switch(config-if)# switchport mode dynamic desirable
Switch(config-if)# end
Switch# show running-config interface fastethernet0/4
Building configuration...
Current configuration : 112 bytes
!
interface FastEthernet0/4
switchport trunk encapsulation dot1q
no ip address
snmp trap link-status
end
Switch# show interfaces fastethernet0/4 switchport
Name: Fa0/4
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Protected: false

Defining the Allowed VLANs on a Trunk

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094
when the enhanced software image is installed, and 1 to 1005 when the standard software image is
installed, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing
traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the
switchport trunk allowed vlan remove vlan-list interface configuration command to remove specific
VLANs from the allowed list.
You cannot remove VLAN 1 or VLANs 1002 to 1005 from the allowed-VLAN list.
Note
A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN,
and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the
VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the
enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk
port, the trunk port does not become a member of the new VLAN.
78-11380-04
End with CNTL/Z.
Catalyst 2950 Desktop Switch Software Configuration Guide
Configuring VLAN Trunks
13-23