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I have an HP ProCurve swicth configured as follows (simplified, with U for Untagged and T for Tagged):

VLAN_1:

U 1,3,5,7-10,12-16
T 2,4,6
No U 11

VLAN_2:

T 1,3,5

VLAN_3:

T 1,3,5

What is the purpose of a vlan containing only tagged ports?

What is the purpose of 2 identical vlan like VLAN_2 and VLAN_3?

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What is the purpose of a vlan containing only tagged ports?

Likely, those (trunk) ports interconnect VLANs spanning multiple switches. It's also possible that a trunk leads to a VLAN-aware host or a router.

What is the purpose of 2 identichal vlan like VLAN_2 and VLAN_3?

VLANs separate layer-2 traffic. VLANs can look similar but may be used for entirely different purposes. Generally, VLAN should be properly named to document that.

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  • Ah.. didn't think about vlan aware things.. and yes, multi-switch vlans.. thanks
    – user815129
    Feb 10 at 7:59
  • The most general use case of vlans is quite simple. Affect ports to different vlans and designate as trunks those used to interconnect switches and/or vlan-aware hosts. But some switch vendors complicate things (probably for more flexibility) the configuration task by introducing notions like ingress vlan egress vlan, tagged/untagged port ...etc. All these can confuse even a skilled network admin
    – Soulimane Mammar
    Feb 11 at 6:45
  • @SoulimaneMammar Mostly true, but tagging is generally required for trunk ports.
    – Zac67 ♦
    Feb 11 at 7:16
  • @Zac67 I know but it's more simpler to configure a port as trunk by a single command and then eventually forbid/allow some vlans on it (if there is a need) than a bunch of commands that are error prone. Network admin's life is already complicated, no need to add on that
    – Soulimane Mammar
    Feb 11 at 10:38
  • Yes, switchport mode trunk is easy, but Procurves/HPE Provision/Aruba don't do it that way. They require explicit tagged/untagged configuration (which I actually find more transparent) or GVRP/MVRP.
    – Zac67 ♦
    Feb 11 at 10:45

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