chloe] Do you know whether there is any network issue if we don't change MTU as same as our provider? eg: our upstream provider router is using mtu 1600 but our end is 1500.
If your MTU is lower than your upstream, you'll end up sending a few more packets than you need to. (for 1500 vs 1600, about 1.066 times to many) If your MTU is higher than your upstream, then you'll fragment your packets. When you try to send a single packet, it will get split into two packets. (if you have 1700 vs 1600, the 1700 will be split into a 1600 and a 100 byte packet.) That doubles the amount of packet overhead you have to transfer, and doubles the number of packets that have to be received. I'm not saying this authoritatively, but that's how I understand things to work.
