@Xanadu_2 wrote:
... ... I noticed that there was no recommended MTU setting.
What figure would you suggest for ADSL.
Your ADSL broadband is capable of an MTU of 1500.
As a general rule, set the maximum MTU values that your modem and router configurations will allow, because when your router is establishing the PPP session with the upstream DSL cabinet (or telephone exchange) it will negotiate the maximum achievable MTU that your router can then advertise to the devices on your local network. Ideally all of your devices should be using "path MTU discovery", so the router discovers the maximum MTU of the incoming line, and then your local networked devices auto discover the MTU of your LAN/WiFi from the MTU that the router has negotiated. The MTU setting in your modem and/or router just provides you with the ability to override to a lower value the maximum MTU that you may want the router to negotiate when you have a specific technical requirement to do so, but generally is unnecessary for most people to set MTU to anything other than the maximum that the modem or router will allow.
With a combined DSL modem and router, it should be possible to have an MTU of 1500.
If you have a router with a separate external modem wired together and configured using PPPoE protocol, then the PPPoE will require the use of eight bytes of the MTU for signalling. If either of the modem or router is only capable of having the maximum configurable MTU set to 1500, then you will likely and up with an negotiated MTU of 1492 (i.e. 1500 - 8 ). If both router and modem support "baby jumbo frames" then it should be possible to set both modem and router MTU to 1508 (i.e. 1500 + 8 ) to overcome the 8 bytes used by PPPoE, and resulting with your local network achieving and MTU of 1500 (i.e. 1508 - 8 ).
If you are trying to get every last bit of speed out of your limited ADSL line, I used to find that fixing an MTU of 1430 or 1458 used to give marginally faster speedtest results and slightly better latency than 1492 or 1500. Unfortunately many of the online tools for automating the process of MTU optimisation seem to have been retired from use, as MTU tweaking was a worthwhile hack back in the days of Windows XP when ADSL was the most used broadband technology, but the world has moved on and those tools are no longer maintained by their developers.
If you want maximum compatibility then aim for 1500, if your equipment can only do 1492 then that'll do, but if you are a gamer and need every millisecond advantage then experiment with lower values.
For testing the actual MTU that your router negotiated with the cabinet/exchange, rather than use terminal ping commands to work out the maximum unfragmented MTU, there is a handy website that automates the test -
http://www.letmecheck.it/mtu-test.php
Just be aware that this test remotely pings your WAN address and expects the appropriate ICMP response - which I understand recent Plusnet supplied routers are apparently incapable of doing 🙄 !, but otherwise should work perfectly on almost any third party routers 😀.
