I have T-Mobile Home Internet and RT-AX88U, I have to update the WAN MTU to the correct size

[Logi]

Can I change the WAN MTU using the standard Asus FW or do I need Merlin FW?

Does anybody knows what is the correct size, I have seen 1420, 1436 and 1480?

Is there is simple method to perform the calculation myself, it seems to be a unique value to each installation

If I execute this on a macOS Terminal: ping -g 1300 -G 1500 -D 1.1.1.1

1474 bytes from 1.1.1.1: icmp_seq=166 ttl=52 time=59.682 ms
1475 bytes from 1.1.1.1: icmp_seq=167 ttl=52 time=55.723 ms
1476 bytes from 1.1.1.1: icmp_seq=168 ttl=52 time=56.773 ms
1477 bytes from 1.1.1.1: icmp_seq=169 ttl=52 time=42.378 ms
1478 bytes from 1.1.1.1: icmp_seq=170 ttl=52 time=129.950 ms
1479 bytes from 1.1.1.1: icmp_seq=171 ttl=52 time=106.052 ms
1480 bytes from 1.1.1.1: icmp_seq=172 ttl=52 time=44.120 ms
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 173
ping: sendto: Message too long
Request timeout for icmp_seq 174
ping: sendto: Message too long
Request timeout for icmp_seq 175

Does this means my MTU should be 1480 or I have to add 24 ?

Thanks

 

[bbunge]

Can I change the WAN MTU using the standard Asus FW or do I need Merlin FW?

Does anybody knows what is the correct size, I have seen 1420, 1436 and 1480?

Is there is simple method to perform the calculation myself, it seems to be a unique value to each installation

If I execute this on a macOS Terminal: ping -g 1300 -G 1500 -D 1.1.1.1

1474 bytes from 1.1.1.1: icmp_seq=166 ttl=52 time=59.682 ms
1475 bytes from 1.1.1.1: icmp_seq=167 ttl=52 time=55.723 ms
1476 bytes from 1.1.1.1: icmp_seq=168 ttl=52 time=56.773 ms
1477 bytes from 1.1.1.1: icmp_seq=169 ttl=52 time=42.378 ms
1478 bytes from 1.1.1.1: icmp_seq=170 ttl=52 time=129.950 ms
1479 bytes from 1.1.1.1: icmp_seq=171 ttl=52 time=106.052 ms
1480 bytes from 1.1.1.1: icmp_seq=172 ttl=52 time=44.120 ms
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 173
ping: sendto: Message too long
Request timeout for icmp_seq 174
ping: sendto: Message too long
Request timeout for icmp_seq 175

Does this means my MTU should be 1480 or I have to add 24 ?

Thanks

Changing the default of 1500 bytes is not necessary in most applications. PPPoE is one exception and your ISP will give you the value. Your results of the ping likely reflect an upstream fragmentation. If you are using a cellular modem, it is likely reducing the packet size and handling the fragmentation so fast your bandwidth will not suffer.

 

[Logi]

Changing the default of 1500 bytes is not necessary in most applications. PPPoE is one exception and your ISP will give you the value. Your results of the ping likely reflect an upstream fragmentation. If you are using a cellular modem, it is likely reducing the packet size and handling the fragmentation so fast your bandwidth will not suffer.

Thank you, I followed this guide:

With a Windows VM, everything works fine until 1472, at 1473 it start dropping packages, therefore 1472 + 28 = 1500, seems to be the optimal value

I also used the Android app Ping & Net the option Path MTU and the output value is also 1500

It seems there is no need to change the MTU even with the Cellular Modem from T-Mobile Home Internet Sagemcom 5688W.

 

[drinkingbird]

Thank you, I followed this guide:

With a Windows VM, everything works fine until 1472, at 1473 it start dropping packages, therefore 1472 + 28 = 1500, seems to be the optimal value

I also used the Android app Ping & Net the option Path MTU and the output value is also 1500

It seems there is no need to change the MTU even with the Cellular Modem from T-Mobile Home Internet Sagemcom 5688W.

Yes, you're good at 1500, which is the default for the Asus (and all ethernet devices) so you're good to go.