Cisco 4500M Command Reference Manual page 508

switchport trunk
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For native VLAN tagging, the following guidelines apply:
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Examples
This example shows how to cause a port interface configured as a switched interface to encapsulate in
802.1Q trunking format regardless of its default trunking format in trunking mode:
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)#
This example shows how to enable 802.1Q tagging on a port:
Switch(config-if)# switchport trunk native vlan tag
Switch(config-if)#
Related Commands
show interfaces switchport
Catalyst4500 Series SwitchCiscoIOS Command Reference—Release 12.2(18)EW
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Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (MST) that defines the
spanning tree topology for all VLANs. When you connect a Cisco switch to a non-Cisco switch
through an 802.1Q trunk, the MST of the non-Cisco switch and the native VLAN spanning tree of
the Cisco switch combine to form a single spanning tree topology known as the CST.
Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on VLANs other than
the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and
flood them on all ports in the corresponding VLAN. Cisco switches connected to the non-Cisco
802.1Q network receive these flooded BPDUs. Because Cisco switches receive the flooded BPDUs
the switches can maintain a per-VLAN spanning tree topology across a network of non-Cisco
802.1Q switches. The non-Cisco 802.1Q network separating the Cisco switches is treated as a single
broadcast segment between all switches connected to the non-Cisco 802.1Q network through
802.1Q trunks.
Ensure that the native VLAN is the same on all of the 802.1Q trunks connecting the Cisco switches
to the non-Cisco 802.1Q network.
If you are connecting multiple Cisco switches to a non-Cisco 802.1Q network, all of the connections
must be through 802.1Q trunks. You cannot connect Cisco switches to a non-Cisco 802.1Q network
through ISL trunks or through access ports. This action causes the switch to place the ISL trunk port
or access port into the spanning tree "port inconsistent" state and no traffic will pass through the
port.
The no switchport trunk native vlan tag command disables the native VLAN tagging operation on
a port. This overrides the global tagging configuration.
The switchport trunk native vlan tag command can be used to reenable tagging on a disabled port.
The no option is saved to NVRAM so that the user does not have to manually select the ports to
disable the tagging operation each time the switch reboots.
When the switchport trunk native vlan tag command is enabled and active, all packets on the
native VLAN are tagged, and incoming untagged data packets are dropped. Untagged control
packets are accepted.
Chapter2 Cisco IOS Commands for the Catalyst 4500 Series Switches
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