1 – Client or host connections between the client’s gateway,modem or router : Typically the mtu on the connection between your pc and the modem is 1500. Or in other word the default MTU on windows/Mac or any other pc is 1500. So the connection between the modem and BNG/BRAS should support MTU of 1500 or more. This includes the protocol overheads.
2 – For bitstream access, or PPPoL2TP/PPPoEoL2TP MTU should be, client MTU – extra overhead on the link (between the dslam-LAC-LNS).
PPPoL2TP header = 20 IP + 8 UDP + 12 L2TP (8 L2TP + 4 PPP) = 40 byte (L2TP header may lower). http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a0080094c4f.shtml
Thus 1540 byte MTU is required between the LNS and LAC. If all the way betweeen user device and BNG is 1500 byte, the extra over caused by the L2TP will lower the MTU on the client site to 1500-40 = 1460.
You can try to increase MTU between LNS-LAC to higer value but the link between modem and LAC is still 1500 which is not enough for ppp packets (ppp will add extra 8 byte to the packets). Assuming that LNS-LAC connection have higher MTU, there is still 8 byte ppp header overhead on the modem site. Thus this give MTU of 1500-8=1492 byte to the user.
In out network we increase the MTU on path between LNS and LAC to 1700. and lower tried to adjust tcp mtu(which will rewrite mtu of the output packets on the virtual-template) on the bng’s virtual-link to 1452. 1500 – ip header 20 – tcp header 20 – pop header 8.