L2VPNS - CE to PE

[cloughy1]

In my job we have to troubleshoot L2VPN connection issues. One of the issues starts with 'wrong / incorrect' port encapsulation set up on the PE devices, at one end or the other or both ends. Im not sure why this would cause an issue and need it clear in my head - The port encapsulations that are available are: Null, dot1q or q-in-q. in simple english, which service does each provide and how do they work, why would they not work? Do certain combinations work each end or do they both have to be the same encaps both ends?

Do they work differently depending on what the customer is sending? i.e. no specific vlans, 1 vlan, many vlans

Sometimes frames dont reach the far end because of incorrect encpasulation on the PE port (s). I just need to know why this might be?

CE equipment is generally Omni 6450 going into an ESS 7450 or SR PE device

Any help appreciated