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On my old Allied Telesis router I had a single interface for internal traffic. I had multiple networks talking on this one interface. For example 192.168.2.X was vlan 1-2, and 192.168.3.X was vlan 1-3. This was all achieved without any vlan tagging or trunk ports. My switches are just acting as dumb switches without any vlans at all.

Can anyone tell if this possible on a juniper srx router or cisco? I've read through documentation but pretty much all of it recommends to do vlan tagging/trunking across switches.

thanks

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  • I think what you were doing was just ignoring the vlan tags, which basically negates the purpose of having them.
    – NickW
    Jan 24, 2014 at 11:45

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Can anyone tell if this possible on a juniper srx router or cisco? I've read through documentation but pretty much all of it recommends to do vlan tagging/trunking across switches.

Yes they can, not the smartest way of doing things but yes, yes they can indeed.

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Consider two swiches and a network with two VLANs. Each switch has two ports, and both VLANs flow over both ports on both switches.

The first switch ignores the VLAN tags. It just copies packets it receives on port 1 to port 2, and vice versa.

The second switch processes the VLAN tags. It receives a packet on a port, checks and removes the VLAN tag, and transmits that packet out each port in the VLAN, tagging it appropriately if the port is so configured. The switch is configured with both ports in both VLANs and with tagging on.

From the outside, how could you tell the difference between these two switches? If you think about it, only by doing something "wrong". For example, if you send a packet tagged for a VLAN that is not being used, the first switch will repeat it out the other port and the second won't. But compliant traffic cannot tell the difference.

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