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Posts: 515
Joined: Tue Feb 10, 2009 4:08 pm
Location: UK

L2TP and proxy-arp

Mon Sep 12, 2011 12:48 pm

Hi all,

Bit of a problem here...

Got a RB acting as a LNS in a data centre with a /24. I would like to allocate one address from this pool to an L2TP client.

Both the LNS and the client are running ROS 5.6. Both have only one ethernet port.


ether1 IP

I want to give the L2TP tunnel the IP address

I can get the connection working perfectly when using addresses from a different network block (say using and adding routes as appropriate), but not when I want to allocate one of the public /24s to the client. The tunnel comes up and shows the correct address, but is not pingable/routable from anywhere other than the LNS.

In any other situation, I would configure proxy-arp on the LNSs ether1, but as soon as I do in this case, the LNS becomes unresponsive to all connections until the proxy-arp setting is removed.

Any ideas?
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Joined: Tue Feb 24, 2009 11:23 pm

Re: L2TP and proxy-arp

Mon Sep 12, 2011 3:43 pm

Well... it is a rather dirty fix, but you could add the address to your LNS'es ether1, give your L2TP client a private (192...) address,
and then NAT the whole official IP through using two netmap rules in /ip firewall nat

I also noticed, that activating proxy-arp while an interface is running often causes routerboards to stop working, until you either reboot, or disable and enable the corresponding ethernet interface - however i am not sure why this is happening.
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Re: L2TP and proxy-arp

Mon Jun 25, 2012 2:49 pm

Hi Nick - a little off your topic, but I would like to set up an RB1100 as a LNS server. I can set up the L2TP server easy enough, and have set up lots of PPPoE servers for Ethernet based networks. The bit I am missing is how to get the PPPoE server running over the incoming L2TP tunnels. If you have a spare minute for a "30 words or less" answer, it would be greatly appreciated. Not looking for a config - just some guidance on how to make the PPPoE and L2TP work together.

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