Hi Sree,
the rule for MPLS (hence for VPLS too) is to have the highest MTU you can configure on the core interfaces. Usually ISPs have nowaday 9000.
If this is not possible for VPLS you need to take into consideration the following for Max Frame Size:
Max Frame Size = Link Header + Tunnel label + VC label + Control Word +Transported L2 Header + Payload.
Link Header is the one of your DSL circuit (likely using ATM link header.. anyway it something you should know/ask your provider).
Tunnel label + VC label + Control Word = 12 Bytes (4 bytes per label + 4 bytes for control word which is optional as it sepends on underlying transport media and on vendor implementation... anyway you should consider it)
Transported L2 Header = it depends on what you transport. For Ethernet II encapsulation with dot1q consider 18 bytes (or 14 bytes if customer vlan id is not transported. This depends on implementations once again).
So in the worst scenario where you have CW and you transport customer vlan id you need at least 1530 Bytes + the DSL link header on the core facing interface.
On the customer facing interface I would leave the interface MTU you have (1500 if it is Ethernet - even though the actual value is higher as it considers the L2 link overhead already). Or else you need to change the MTU on every host.. not too handy.
Riccardo
