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My setup:

L3 Switch(tagged port) --> Unmanaged Switch --> L2 Switch (tagged port)

  1. I had created VLAN and IP addresses for 2, 3, and 4 in the L3 and L2 switches.
  2. Ports connecting the L3 switch to the unmanaged switch and unmanaged switch to the L2 switch are tagged.
  3. Connected three machines in 2, 3, and 4 VLANs to ports in the L2 switch.

My question is will the PC in the VLAN 2 network reach the L3 switch? Will the packets dropped or flow in this network setup to their respective VLANs?

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  • Depends on how the unmanaged switch treats tagged frames, so without knowing the vendor/model/code version, you can really only find out by testing.
    – YLearn
    Commented Apr 24, 2015 at 14:40
  • whether the traffic will flow..?
    – balamurugan
    Commented Apr 24, 2015 at 15:29
  • 2
    Yes. Different "unmanaged" switches operate differently. One may pass the traffic just fine, another may dropped tagged frames, or possibly it could fail altogether because it doesn't know what to do when it receives tagged frames. Most "unmanaged" switches are designed to be inexpensive and as such don't get much development resources allocated to the project....exactly how they behave outside of "simple L2 switching" (and sometimes even then) will vary from device to device.
    – YLearn
    Commented Apr 24, 2015 at 17:59
  • And sometimes the switch will simply crash. (ran into that with a cisco 2950. sent tagged frames to an access (untagged) port: *poof*)
    – Ricky
    Commented Apr 24, 2015 at 19:14
  • 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 provide and accept your own answer.
    – Ron Maupin ♦
    Commented Aug 11, 2017 at 16:26

2 Answers 2

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If I understand correctly, what you're asking is if the unmanaged switch can pass vlan tagged traffic to enable devices on the L2/L3 switches to communicate with each other.

This question has been asked on Serverfault, with the answer being: maybe.

Some switches will happily pass along the traffic, and some wont. It depends on the switching method used, and how the switch handles frames larger than 1518 Bytes.

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802.1q VLAN tagging (http://en.wikipedia.org/wiki/IEEE_802.1Q) is pretty much standard now. I don't think that Cisco even has switches supporting ISL encapsulation any longer. As a result, unmanaged switches should transmit tagged frames without any issue - it looks just like a regular ethernet frame. Since you're using an unmanaged switch to basically pass traffic between two managed switches, you may need to disable trunk negotiation between the two switches.

On a Cisco switch, switchport mode trunk, switchport nonegotiate, and if you want to manage this, switchport trunk vlan allowed #,#,#,# to specify exactly which VLANs you want to flow on this link.

Setting both L3 managed and L2 managed switch ports to trunk unconditionally will allow traffic to flow transparently to the managed switches.

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  • Dear Smith, thanks for your response.. I am not clear with your answer, could you please describe it further... Thank you
    – balamurugan
    Commented Apr 24, 2015 at 15:07
  • 2
    A) what does ISL have to do with anything? B) it's an unmanaged switch, so there's nothing to, you know, manage. It will either accept the frames, or drop them. The real problem is not with the ethernet protocol, but with the additional 4 bytes the tag adds. If the switch sees it as "oversized", it'll be dropped.
    – Ricky
    Commented Apr 24, 2015 at 19:18
  • My understanding is that 802.1q tagging did not alter ethernet frame sizes whereas ISL did. One of the reasons that 802.1q won out over ISL as the standard. ISL was the old ( old, old ) option for VLAN tagging and trunk encapsulation. Find an old enough switch, old enough IOS and you have to set your trunk encapsulation to 802.1q because ISL is the default.
    – Stuart Smith
    Commented Apr 24, 2015 at 20:58
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    @StuartSmith, your understanding is wrong. You are adding information (the 802.1q tag) to the Ethernet header, so how is this not going to alter the size? Dot1q doesn't add a trailer, so adds less overhead, but it still increases the size of the frame. The reason Dot1q won out is that it is standards based instead of Cisco proprietary and it supports more VLANs. Cisco has a history of working with the IEEE in standardizing many of their proprietary features, dot1q is just one of them (others include 802.3af/at).
    – YLearn
    Commented Aug 22, 2015 at 16:44

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