One NE80 interconnects with one Ne40 via HDLC protocol through pos interface, the MTU value of NE80 interface is 9600Byte, while that of ne40 interface is 1500Byte, at this time, it is communicated from ne80 to the 2000Byte packets on NE40. Because HDLC does not negotiate the MTU value of interface, so the packet sent by ne80 should be 2000Byte packet, the MTU of ne40 is only 1500Byte, it should not accept 2000Byte packet, so the packet should be discarded, and ne80 should not communicate with NE40.
Because NE80 limists the size of the host packets, it allows 1500Bytes packets to travel through at most. For the packets whose size is more than 1500Byte, NE80 will do fragmentation automatically, at this time, the packets sent by NE80 less than or equals to 1500Byte, ne40 can accept normally, ne80 can communicate with ne40.
Generally, in order to protect all the forwarding service normal, the MTU value at the interconnecting interface must be consistent.
NE80 does fragmentation process for the out packets.