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In the scenario where the following standard trunk-port configuration is not working on a Cisco switch:
switchport mode trunk
switchport trunk encapsulation dot1q
Check to make sure the global command "dot1q tag native" is not invoked. Here is Cisco's description for the command:
When enabled, native VLAN packets going out all IEEE 802.1Q trunk ports are tagged.
When disabled, native VLAN packets going out all IEEE 802.1Q trunk ports are not tagged.
You can use this command with the IEEE 802.1Q tunneling feature. This feature operates on an edge
switch of a service-provider network and expands VLAN space by using a VLAN-in-VLAN hierarchy
and tagging the tagged packets. You must use IEEE 802.1Q trunk ports for sending packets to the
service-provider network. However, packets going through the core of the service-provider network
might also be carried on IEEE 802.1Q trunks. If the native VLANs of an IEEE 802.1Q trunks match the
native VLAN of a tunneling port on the same switch, traffic on the native VLAN is not tagged on the
sending trunk port. This command ensures that native VLAN packets on all IEEE 802.1Q trunk ports are
Any traffic passed over the native VLAN should be untagged to ensure proper communication.