You need to change the MTU value of Cyberoam whenever Cyberoam is connected with any device which uses PPPoE such as ADSL. This change is required due to PPPoE architecture.
Maximum Ethernet payload size is 1500 octets but in PPPoE transaction, PPPoE header size (6 octets) and PPP protocol ID size (2 octets) is added to the Ethernet payload which increases MTU value to 1508 octets. This increased MTU is not acceptable; hence it is necessary to reduce MTU value.
Reduce MTU value of Cyberoam to 1492 so as to make the maximum payload size to 1500 octets (TCP/IP payload: 1492 + PPPoE header size (6 octets) + PPP protocol ID size (2 octets)).