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I understand that you can use IPSec to tunnel data securely. According to the Wikipedia page and a few other sources it can also tunnel IP packets and then route them through an interface. That would create a VPN where one subnet would be able to access another subnet in a very secure way.

However what I dont understand is why some people add L2TP to the stack. I get the idea that L2TP is secured by IPSEC, but if IPSEC already has a tunnel implementation wouldn't it just cause more overhead?

What is the attraction to L2TP/IPSEC when the same result can be reached with plain IPSec?

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  • IPSec -> Layer 3 auth and encryption
  • L2TP -> Layer 2 tunneling

From what I understand, IPSec wouldn't be carrying any Layer 2 information. That's where L2TP comes in.

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So if you wanted to have a VPN on the same subnet you would need to have L2TP to keep things like ARP working, i think i get it now. – Silverfire Oct 8 '11 at 3:13
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Windows especially the older versions have L2TP over IPSEC support builtin.

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this doesn't answer his question. Read it again. – Graeme Donaldson Oct 7 '11 at 7:30
Id say L2TP over IPSEC by default in a operating system is a great attraction. For us it was the only reason to ever consider adding something on top of IPSEC. RAW IPSEC on Windows is or at least was a pain to configure in my experience. – Miademora Oct 7 '11 at 7:41
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