Given multiple networks all with the same VLAN tag, is VLAN re-tagging a wise thing to do in order to keep them separated on the same physical network?

If so, how can I achieve this without getting too complex/costly?

Since I am limited to one physical network, I'd like to deal with them as follows:

WAN-1-VLAN-X becomes *VLAN-A*
WAN-2-VLAN-X becomes *VLAN-B*

WAN-1-VLAN-X and WAN-2-VLAN-X are two physically different networks I have no control over, while VLAN-A & VLAN-B are in the same physical network which I do have control over.

  • I don't really understand the question. VLANs are layer-2 constructs, and a VLAN will not cross a layer-3 device, e.g. router. VLAN tags are only applied on trunks; any traffic coming into or exiting an access interface will be untagged. You should edit your question to include the configuration. – Ron Maupin Mar 25 '18 at 4:15
  • Also, you should be specific in which brand and type of equipment you're using, taging it with both 'cisco' and 'ubiquiti' makes it even more confusing. – Teun Vink Mar 25 '18 at 20:26
  • 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 Apr 1 '18 at 23:13

Some Cisco switches support "vlan mapping". You'd do this (for your example):

Switch(config)#int <wan-1>
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport vlan mapping X A

Switch(config)#int <wan-2>
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport vlan mapping X B

When you merge two networks using the same VLAN tags you may want to keep them separate. There are several approaches, depending on the hardware capabilities:

  1. using Q-in-Q tagging you can use inner and outer tags where the inner tags have no interaction with the outer tags - requires expensive/current switches)
  2. on a trunk interface, you can translate the untagged VLAN from one switch to the other - this might preclude the use of MVRP/GVRP or MSTP)
  3. use a port loop on a switch that has one VLAN ID untagged on the one port and the other ID untagged on the other port - precludes STP and works on some switches while it doesn't on others, depending on the inner workings of the VLAN-MAC table

Items 2. and 3. are more or less ugly hacks and may easily a become a source of trouble once you have to replace your switches, so the only real solution besides Q-in-Q tagging is renumbering your VLANs...

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