A larger MTU (Maximum Transmission Unit) brings greater efficiency in transmitting because each packet carries more data; however, a packet too large may be fragmented and results in lower transmitting speed instead. Optimizing the MTU value on the router's WAN interface can improve performance and avoid issues. This article shows how to find the right MTU size.
1. Go to WAN >> Internet Access >> Detail Page, click "Detect" at Path MTU Discovery.
2. Enter an IP for the destination as a testing target and the MTU size for the router reduce each time. Then, click Detect to start MTU path discovery. If the test is successful, the recommend MTU size will be displayed.
3. Click Accept to apply it to MTU setting, and click OK to save.
For routers that don't support Path MTU Discovery, we can find the proper MTU by sending ping requests from a PC behind the router. This can be done by entering “ping <IP> -l <packet size> -f” in the command prompt. (Where “-l” means to specify the ping packet size, and “-f” means don't fragment.)
Take the following topology for example, if you cannot get ping responses, it means the packet is too large to be sent to the destination without fragmentation. Then try a smaller size until you get responses with an MTU size as large as possible.
When you get replies from the ping target, it means the current packet size is okay to traverse the path without being fragmented. In the example below, it means a packet with size 1464 byte plus ICMP header (8 bytes) and IP header (20 bytes) won't be fragmented along the path, and the MTU size for router's WAN can be 1464 + 8 + 20 = 1492 bytes.