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I'm currently studying about L2TP and there's something I don't quite understand, even after studying a tutorial and some parts of the RFC. Specifically, when connected via L2TP, how does the client know where to forward data frames?

For example, in regular ethernet, the client uses ARP to find out the MAC address it needs to send to, writes that as the destination, and sends it. Does ARP work over L2TP and the destination is the MAC address of the device on the other side of the tunnel? Or does it use the MAC address of the router as the destination of its frame? Or am I misunderstanding this entirely?

Thanks in advance for any answers!

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Or does it use the MAC address of the router as the destination of its frame?

Yes, that's exactly right. The VPN router typically utilizes proxy ARP to answer ARP queries on behalf of the actual destination host. When the packets are received by the router, they're forwarded on to the actual destination host.

From Wikipedia:

Proxy ARP is a technique by which a device on a given network answers the ARP queries for a network address that is not on that network. The ARP Proxy is aware of the location of the traffic's destination, and offers its own MAC address as (ostensibly final) destination. The "captured" traffic is then typically routed by the Proxy to the intended destination via another interface or via a tunnel.

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  • You are absolutely correct, but you may also want to mention that Proxy ARP is a security hole. It is now disabled by default on network equipment, and many businesses require it to remain disabled. – Ron Maupin Apr 9 '16 at 1:24
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Does ARP work over L2TP and the destination is the MAC address of the device on the other side of the tunnel? Or does it use the MAC address of the router as the destination of its frame?

It depends on l2tp setup.

If it use BCP (Bridge Control Protocol) PPP extension, then l2tp works like Ethernet (forward L2 frames), so your first assumption is right.

If it plain l2tp (which forward L3 packets only) it forwarded by routes or by combination of routes and arp manipulation (Arp Proxy or some other), then your second assumption is right.

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