802.1q VLAN trunk over bridged DSL (ATM)

Unanswered Question
Oct 16th, 2009

Recently, i got some hints on the forum to get my 2 ATM PVC's to bond together via bridging on a 7204VXR. This is working now and i can ping from left to right and ARP seems to work ok.

Now i attached 2950's on both sides and configured a VLAN trunk on the interfaces facing the DSL network. Unfortunately, there's something going wrong in replying to ARP.

When trying to send something from A to B, there's an ARP request going to B. B replies with it's MAC, but this reply is not received by A. I included a packet capture and a diagram so you guys can see what i'm talking about. 7200 config is pretty straightforward :

bridge irb


interface atm1/0.162055 point-to-point

bridge-group 10

pvc 16/2055

encapsulation aal5snap

interface atm1/0.162056 point-to-point

bridge-group 10

pvc 16/2056

encapsulation aal5snap


bridge 10 protocol ieee

Any help would be very appreciated.


Remco Bressers

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andrew.prince@m... Fri, 10/16/2009 - 06:44

The last time I checked you cannot bridge a vlan tagged frame.

Try configuring a flat VLAN on both switches and test again or make the vlan with the machines you are trying to ping to the native vlan on the trunk interfaces for testing.

signet Fri, 10/16/2009 - 06:51

If i sniff the VLAN trunk itself, it does add the VLAN-id in the trace, so it seems that the VLAN header does make it through.

Paolo Bevilacqua Fri, 10/16/2009 - 07:05

Are you doing a lab or real application ?

The proper way to do that is to front-end the router with an ATM switch, like an LS1010 that is extremely cheap on the used market.

Giuseppe Larosa Fri, 10/16/2009 - 08:09

Hello Remco,

in one of the links I've posted in your other thread explains that this setup works for a vlan only.

to be able to carry multiple Vlans you would need a PVC and a bridge-group for each.

Hope to help



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