Question

A Cisco switch port is configured as `switchport mode trunk` and `switchport trunk native vlan 99`. If an untagged frame arrives on this port, which specific VLAN does it belong to?

Accepted Answer

An untagged frame arriving on a Cisco switch port configured with `switchport mode trunk` and `switchport trunk native vlan 99` will belong to VLAN 99. The command `switchport mode trunk` configures the port to carry traffic for multiple Virtual Local Area Networks (VLANs) by adding an 802.1Q tag to frames, which identifies their VLAN membership. However, the concept of a native VLAN exists to handle frames that do not have such a tag. The command `switchport trunk native vlan 99` explicitly designates VLAN 99 as the native VLAN for this specific trunk port. When an Ethernet frame arrives on a trunk port without an 802.1Q VLAN tag (i.e., it is an untagged frame), the switch automatically associates that frame with the native VLAN configured on that port. Therefore, any untagged frame received on this port is implicitly assigned to VLAN 99 for processing within the switch.

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A Cisco switch port is configured as `switchport mode trunk` and `switchport trunk native vlan 99`. If an untagged frame arrives on this port, which specific VLAN does it belong to?

An untagged frame arriving on a Cisco switch port configured with `switchport mode trunk` and `switchport trunk native vlan 99` will belong to VLAN 99. The command `switchport mode trunk` configures the port to carry traffic for multiple Virtual Local Area Networks....