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Is it possible to connect a VLAN switch into a dumb switch ?

The router will know how to reach network connected by a VLAN through the dumb switch please ? No edge effect ?

By example, a schema of the project :

enter image description here

Thanks !

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  • Normally it would not be a problem.
    – user36472
    Nov 12, 2018 at 15:53
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    Possible? Yes. Recommended? No. A "hub" that isn't vlan aware could do anything with tagged traffic. The biggest problem: the frames are now 4 bytes bigger, which can cause them to be dropped as oversized. (1518 vs 1514) In the absolute worst case, your "hub" crashes when presented tagged traffic. (a Cisco 2900XL, for example)
    – Ricky
    Nov 12, 2018 at 19:05
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    Can you reconfigure things so the unmanaged switch is not in that path? Would be a better network design if so.
    – Criggie
    Nov 12, 2018 at 19:43
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Dec 21, 2018 at 2:49

3 Answers 3

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An unmanaged switch will only have one VLAN. Some unmanaged switches will drop tagged frames as damaged, others will strip the tag, and some will simply pass the frames unchanged. Unless you try it, you have no way to know how your switch acts. For the switches that strip the tags, all the frames will be placed into the native (untagged) VLAN as they pass through the switch.

If your VLAN switch is a layer-3 switch, then you are better off using it as the LAN router and creating a routed link to the router, which can act as the WAN router. You will need to either use static routes on the WAN router, or you will need to run a routing protocol between the WAN router and the layer-3 switch as the LAN router.

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  • Your VLAN doesn't work like mine. Then again, mine won't allow two VLANs to communicate between switches on the same wire.
    – Joshua
    Nov 12, 2018 at 17:27
  • @Joshua if that's true, do you really even have vlans? 802.1q, which allows for multiple vlans in the same switch port via tagging, has been a thing for a loooong time now, and it's hard to find a managed switch that won't do this.
    – Joel Coehoorn
    Nov 12, 2018 at 17:28
  • @JoelCoehoorn: If I can reassign machines to broadcast domains remotely it's a VLAN right?
    – Joshua
    Nov 12, 2018 at 17:30
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Yes, you can do what you're suggesting without any problem.

In practice, the dumb switch treats everything as if it's on vlan1. Technically though, vlan tags are removed when leaving access ports and applied when entering. Vlan tags are only kept in the frame between switches when crossing a trunk port. Since you're not creating a trunk between to the dumb switch (or router), then vlan tagging is not an issue.

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  • The router will have the VLAN 10 declared, any problem with your last sentence ?
    – Puma
    Nov 12, 2018 at 16:03
  • If you're tagging at the router with a sub-interface, then generally you want to have that go into a trunk port on the switch. Since the dumb switch is incapable of forming a trunk, then it might not work as you expect. If you can get the router and the smart switch to both trunk to the dumb switch, then whatever packets that cross the dumb switch will still contain the vlan tags and you'll be good to go. However this is an unusual design and it's better to just put the trunk straight to a proper layer3 switch.
    – aletoledo
    Nov 12, 2018 at 16:15
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    "whatever packets that cross the dumb switch will still contain the vlan tags" That is not a given. The behavior of an unmanaged switch when faced with VLAN tags is undefined, and some unmanaged switches will drop the frames as damaged and some will strip the tags.
    – Ron Maupin ♦
    Nov 12, 2018 at 16:44
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Did this today.

One side was a Cisco 3560 switch, the other was a Netgear GS305. I was shocked to see the Netgear could use vlan 2.

On Cisco side

switchport trunk encapsulation dot1q switchport mode trunk switchport trunk native vlan 2

Netgear Side

No configuration because its unmanaged.

Verify trunk on Cisco with "show interfaces trunk". Your new trunk should be established

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    Of course you can use the uplink port's native VLAN. The point is that tagged frames likely leak all over the dumb switch and any node can join them.
    – Zac67 ♦
    Jul 4, 2020 at 20:32
  • The point is that the behavior of an unmanaged switch when faced with tagged frames is undefined (no standard). Your test of untagged frames (a native VLAN is the VLAN that does not tag frames) is meaningless, and even if you have success on that switch with tagged frames, other unmanaged switches do it differently.
    – Ron Maupin ♦
    Jul 5, 2020 at 1:04
  • I am a complete noob about vlan.. how can I test whether a dumb unmanaged switch is dropping tagged frames or not?
    – yeahman
    Aug 21, 2020 at 18:11

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