Question

Way to change MTU on router

  • 20 March 2021
  • 7 replies
  • 4429 views

Userlevel 3
Badge +3

Is there a way to change the MTU on these new nokia 5g silver routers?


7 replies

Userlevel 5
Badge +5

They have the Nokia locked down pretty tight.

Besides, unless you wanted to reduce it for some reason, it should be left as is.  It is set to 1420 for a reason, likely tied to the 464XLAT "tunnel" they use (has a sort of VPN impact), or other security/stability concerns they have upstream.

If you need to lower MTU to get a VPN working, then it should be done on the client side, ideally in that VPN's config.  Alternatively, could try forcing it through the OS or your own router if you are doing the double-NAT thing.  But the best place to do it is in the VPN config itself to avoid any potential fragmentation weirdness.

Userlevel 3
Badge +3

They have the Nokia locked down pretty tight.

Besides, unless you wanted to reduce it for some reason, it should be left as is.  It is set to 1420 for a reason, likely tied to the 464XLAT "tunnel" they use (has a sort of VPN impact), or other security/stability concerns they have upstream.

If you need to lower MTU to get a VPN working, then it should be done on the client side, ideally in that VPN's config.  Alternatively, could try forcing it through the OS or your own router if you are doing the double-NAT thing.  But the best place to do it is in the VPN config itself to avoid any potential fragmentation weirdness.

How can you tell the MTU is set at 1420 on the router?

Userlevel 5
Badge +5

Just makes sense for it to be... if not something in their router, it would likely be the next link upstream.  I have their Askey model, and there are no accessible MTU settings there either.

All my stuff is set to use path discovery, and their router is negotiating the tunnel.  It would be the first thing in the localized chain getting set that way by TMO... and my devices are using 1420.

You can test from your clients to see what MTU gets through without fragmentation. Or just go to something like speedguide.net's TCP Analyzer.

Userlevel 3
Badge +3

Just makes sense for it to be... if not something in their router, it would likely be the next link upstream.  I have their Askey model, and there are no accessible MTU settings there either.

All my stuff is set to use path discovery, and their router is negotiating the tunnel.  It would be the first thing in the localized chain getting set that way by TMO... and my devices are using 1420.

You can test from your clients to see what MTU gets through without fragmentation. Or just go to something like speedguide.net's TCP Analyzer.

The reason I ask because I was getting a pause when requests go out and lower send speeds, but when I tested my packets to google they were fragmented and timing out until I adjusted the MTU on my computer down to 1392 +28  = 1420  Now they are not fragmented and up speed seems more stable and my pings are much better.

This makes me think the router is set to 1500 and no way to adjust it other then per machine.

Userlevel 5
Badge +5

If PMTU Discovery is not active(or working properly), it may be defaulting to the ethernet standard of 1500.

My Windoze laptops and Asus router were   picking up 1420 through discovery, but my PS4 was set statically due to some issues with PSN years ago.  It started behaving much better after dropping it down.

Just be mindful that dependingbon what all is going on, may need to set MSS to 1380.   May need extra room in the headers. (IP header 20 bytes, TCP header 20 bytes is kinda the "norm")

specifically for pulse secure, T-Mobile tech support identifies 1340 as the correct mtu value if you are having problems connecting to the VPN.

I don’t think any of my clients are using anything less than 1500.  I could put another router in between to make them smaller, but obviously not ideal.

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