L2 vpn

Hi,
we have made a lab setup to knw about l2/l3 Vpn.The setup is like

traffic generater(ce)--PE---P-------P---PE---Traffic gen(ce)

we are sniffing the data between the P devices.

And we have found that L3 Vpn is looks in generic fromat as below

Mac layer--->mpls (outer label)-->mpls(inner label)---> ip layer

when we test with L2 VPN(ethernet vlan-ccc) the we found format as
below

(network) MAC layer--->mpls outer label----mpls inner label--->00 00 00
00---> customer MAC layer---IP layer.

My query is wht is these 4byes of zero's .... is it control word.

1)is it standard 4byes ????? for all types of encapsulation.????.
2)wht will be the value fo these control word for diff encapsulation
like ppp-ccc,hdlc-ccc,for FR-ccc

3) and one more question will this conrol word will be there in TCC
also.
if yes wht will be the value and size of the control word.???

soms

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L2 vpn