Is it possible to encapsulate or double tag frames in a VLAN?

[michaelbs]

Hi all,

we are currently using two linux routers that are connecting to the Internet via PPPoE over DSL connections. Each router and its corresponding DSL modem reside in a dedicated VLAN and are connected over a trunk:

Now the ISP requires the PPPoE frames to be tagged with VLAN ID 7 in order to establish the connection. This means that both routers need to tag its PPPoE frames with VLAN ID 7. In order to achive this I could put the routers PPPoE interfaces in access ports with VLAN ID 7 so the switches perform the tagging. The modems are then put on trunk ports so they receive the frames tagged with VLAN ID 7 as required:

However, as both routers frames need to be tagged with VLAN ID 7 in order to successfully estabilsh the connection no seperation with different VLANs is possible anymore. Would the following be possible?

1. Attach router and modem to access ports with VLAN ID 22 (first picture)

2. Tag the routers frames with VLAN ID 7 already in the router's OS

3. Send the already VLAN-7-tagged frames over the access port to additionally tag them with VLAN ID 22

4. Send the frames over the trunk with VLAN ID 22

5. Remove the VLAN ID 22 tag from the frames as they arrive on the modems access port in VLAN 22

6. Let the modem receive the frames with VLAN ID 7 tag still available

As far as I know this is not possible since access ports usually discard tagged frames, so the VLAN-7-tagged frames from the router would be dropped while entering the access port of the router's switch. But maybe I'm wrong and there is an alternative that allows to double tag the frames or achive the seperation another way?

Any help is highly appreciated.

Thanks

Michael

[Richard Burts · Accepted solution]

Michael

I am not sure whether it is the solution that you are looking for but the technique sometimes called Q in Q does create Ethernet frames with two tags. You can find some discussion of it in this

https://en.wikipedia.org/wiki/IEEE_802.1ad

HTH

Rick

[Joseph W. Doherty · Accepted solution]

As Rick notes, you're probably want Q-in-Q support.

Not all switches support it.

See if yours support the interface command:

switchport mode dot1q-tunnel

[michaelbs]

Thanks Richard and Joesph,

that would indeed be the technology I'm looking for. It however seems not support on the Cisco 2960 and in VMware (other than on the images the routers are actually virtual machines).

I'm having a look into PPP on linux - maybe it's possible to explicitly specify the modem so that it is guaranteed that a given router dials the connection over a particular modem.

Thanks,
Michael