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According to this article Cisco recommends VLAN 99 for the "native VLAN".

But what is the point when operating under switchport mode trunk?

Doesn't switchport mode trunk filter all tagged frames such that the native VLAN (untagged PVID) would be meaningless?

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  • Cisco actually recommends not using a native VLAN at all, nor allowing the default VLAN (VLAN 1) on a trunk interface. Also, disable VTP and set allowed VLANs on a trunk to only those actually used.
    – Ron Maupin
    Aug 23, 2022 at 2:20

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Doesn't switchport mode trunk filter all tagged frames such that the native VLAN (untagged PVID) would be meaningless?

No. A trunk has optionally one untagged VLAN and many tagged VLANS.

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https://en.wikipedia.org/wiki/IEEE_802.1Q

The native vlan is the vlan that would be used if no vlan is tagged on that link.

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    The question is tagged Cisco and there your answer is not correct. Cisco's "native" VLAN is the single untagged one in a VLAN trunk. The untagged VLAN used when reconfiguring to access port would be the "access VLAN".
    – Zac67
    Aug 23, 2022 at 13:35
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The switchport mode command is actually controlling Cisco's DTP (dynamic trunking protocol) feature. By setting it to Trunk, you are setting DTP to an 'ON" state. Setting it to access sets DTP to an off state. The default value on Cisco switchs today is dynamic desirable.

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