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| 2.5 |  | VLAN Trunking and Dynamic Trunking Protocol (DTP) |
| 2.5.2 |  | Configuring a VLAN trunk |

To create or configure a VLAN trunk on a Cisco IOS-based switch, you must configure the port as a trunk and to specify the trunk encapsulation. To do this, issue the switchport mode and switchport trunk commands.

> Switch(config-if)#switchport trunk encapsulation [dot1q | isl]
> Switch(config-if)#switchport mode trunk

dot1q Interface uses only 802.1q trunking encapsulation when trunking.

isl Interface uses only ISL trunking encapsulation when trunking.

When configuring switches that allow multiple trunking encapsulations, dot1q or isl, the switchport trunk encapsulation command must be used before the switchport mode trunk command. By default, the switchport mode trunk command will trunk all VLANs.

In 802.1Q trunking, all VLAN packets are tagged on the trunk link to indicate the VLAN to which they belong. Frames belonging to the Native VLAN are sent untagged on the trunk link. The Native VLAN contains ports not assigned to other VLANs that by default belong to VLAN 1. VLAN 1 is the Native VLAN by default, but VLANs other than VLAN 1 may be designated as the Native VLAN. However, the Native VLAN must be the same on trunked switches in 802.1Q trunking. If a VLAN other than VLAN 1 is to be the Native VLAN, it needs to be identified on the trunk ports. In the interface configuration mode of the trunk port(s), the IOS-based command to designate the Native VLAN is switchport trunk native.

> Switch(config-if)#switchport trunk native vlan vlan-id

The switchport trunk native vlan command is only necessary when changing the native VLAN from something other than VLAN 1 or changing it back to VLAN 1.

Remember that the Catalyst 3550 switch supports both 802.1Q and ISL while the Catalyst 2950 supports only 802.1Q.