A trunk port carries traffic for many VLANs over a single cable by tagging each frame with a 4-byte 802.1Q header. This chapter explains the tag format, the native VLAN concept, and the configuration on both sides of a trunk.
When a frame leaves a trunk port it gets a 4-byte tag inserted between the source MAC and the EtherType. The tag carries the VLAN ID (12 bits, allowing 1–4094) plus a 3-bit Class of Service field (PCP) and a Drop Eligible Indicator (DEI) bit.
When the frame reaches the next switch, the tag is read, the frame is delivered to that VLAN's broadcast domain, and the tag is stripped before the frame leaves any access port.
Switch(config)# interface Gi0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30,99
Switch(config-if)# switchport trunk native vlan 99
Switch(config-if)# end
Switch# show interfaces trunk1 more section in this chapter, plus a 3-question quiz.
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