[c-nsp] What does L2PT actually do on the wire?

[Howard Jones]

We have a metro-ethernet link between two sites, where our CE are a pair 
of 3750G switches. We run q-in-q for a couple of internal things, with 
all the L2PT options enabled (cdp, vtp, stp, lacp etc etc). We've found 
that even with all that enabled, CDP (for example) doesn't get to the 
far end, and it turns out that our MPLS provider's default product 
blocks a series of L2 protocols. We see encapsulation counters for L2PT 
going up, but 0 for decapsulation.

My question though is: what is L2PT actually doing? My original 
assumption was that it was encapsulating these packets in another frame 
header, so they would no longer look like STP or CDP or whatever to 
intervening devices. That doesn't appear to be the case though, or they 
wouldn't get blocked.

Thanks in advance for any illumination...

Howie