What MTU should I set on my router?

Although larger MTU (Maximum Transmission Unit) may bring higher efficiency, but if the router's WAN MTU is too large, the packet will be fragmented when traversing, thus it's important to configure a proper MTU size to have better performance and avoid Internet access issue. Vigor Router supports Path MTU Discovery to help network administrator find the proper MTU.

Use Path MTU Discovery

1. Go to WAN >> Internet Access >> Detail Page, click Detect at Path MTU Discovery

   

2. Enter an IP as the destination of path MTU discovery and the MTU reduce size for router to try. Then, click Detect to start MTU path discovery. If the discovery is successful, the recommend MTU size will be shown.

   

3. Click Accept to apply it to MTU setting, and click OK to save.

   

Use Ping command

For older Vigor Routers (Vigor2830/2850/2920/2710) that don't support Path MTU Discovery, we can find the proper MTU manually by the enter command “ping <IP> -l <packet size> -f” on the PC behind Vigor Router, where “-l” means to specify the ping packet size, and “-f” means don't fragment.

Take the following topology for example:
PC 192.168.1.100 --> GW 192.168.1.96 --> GW 192.168.239.1 --> Server 8.8.8.8

PC can ping GW IP with ping size 1432 when DF is set.
But PC is unable to ping 8.8.8.8 with ping size 1432 when DF is set.
It means there is one of node's MTU smaller than 1432+28=1460. (28 bytes are 8 bytes ICMP header and 20 bytes IP header)
Hence, user should try to ping with a smaller size to find the proper MTU for nodes from PC to 8.8.8.8. 

   

When you get replies from the server, 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 we can use 1492 as router's WAN MTU.

   

 

 

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