How to decode nested l2tp traffic?

[Joan <aseques () gmail com>]

I am trying to extract the data transmitted into a l2tp tunnel, I am
running thsark/tcpdump in the tunnel terminator. What I am using so far is
this (4291 is the tunnel number):
  tcpdump -n -i eth3.800 "udp port 1701 && udp[8:2] & 0x80ff == 0x0002 &&
udp[10:2] == 4291"

I took the filter line from here
http://networkingbodges.blogspot.com.es/2012/11/tshark-one-liners.html

The problem is that I would like to inspect the traffic inside the tunnel,
but I could'nt find a reference on this.

Any clues?

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