recomended MTU

john7 · Dec 04, 2020, 11:44:57

Hi I am having to change router and the new one is a Netgear with a default MTU of 1500 I take it this should be changed to what I have now of 1492?

But I have just seen my 130 modem is using 1520 so am a bit lost. If the Netgear MTU is for internal network traffic 1500 may well be best but I thought PPPoE used 1492 which may well be determined by the 130. I can't try changes with the current router as the highest it will allow is 1492. So my query now has widened, should the modem have the same as the router as well as the size?

nowster · Dec 08, 2020, 10:38:22

The use of 1492 (or 1458) is to best fit in ATM framing. Whether it's useful on VDSL (FTTC) is another matter.

john7 · Jan 15, 2021, 10:29:52

Thanks, its looking rather academic as it looks like IDNet caps MTU at 1460!

L2020 · Feb 15, 2021, 09:42:05

Quote from: john7 on Jan 15, 2021, 10:29:52
Thanks, its looking rather academic as it looks like IDNet caps MTU at 1460!

As per my other post this clamping seems to have been removed now and MTU is back in our control

I'm currently running at 1500 MTU on FTTP no issues at all. Day to day I hadn't noticed the change and not sure when it happened, but as I work in IT and occasionally need to troubleshoot potential MTU issues (or at least rule them out) it is helpful to be able to test with 1500 MTU.

john7 · Feb 16, 2021, 21:49:29

I was told "1458 is the recommended MTU setting for DSL circuits provided via the BT Openreach network in the UK. Ethernet LAN MTU values of 1500 or greater will cause packet fragmentation". I don't know if right or not?

L2020 · Feb 17, 2021, 08:46:52

Quote from: john7 on Feb 16, 2021, 21:49:29
I was told "1458 is the recommended MTU setting for DSL circuits provided via the BT Openreach network in the UK. Ethernet LAN MTU values of 1500 or greater will cause packet fragmentation". I don't know if right or not?

That might have been the case sometime ago due to issues with a certain route or website or how packets were encapsulated over ATM with ADSL or the use of tunnels by the ISP, but in recent years and with FTTP/VDSL those issues are gone. The Internet mostly works fine on 1500 MTU, there may still be occasions where packets are fragmented or MTU is negotiated lower of course. If you wanted to avoid any fragmentation ever then the setting to use for MTU on IPv4 would be 576 bytes (but very inefficient on fast connections), which all routers must be able to handle without fragmenting. So even using 1458 never guaranteed packets were not fragmented somewhere sometimes.

Usually the maximum MTU we see is 1492, this is because in the UK ISPs on Openreach tend to use PPPoE that adds 8 bytes, and our own routers tend to work to 1500 MTU maximum for everything including PPPoE. So we can only send a maximum 1492 packet size to the Internet as the router needs to add 8 bytes for PPPoE to steer the packets to our own ISP first, the ISP strip off that 8 bytes when the traffic goes on the Internet so 1492 it becomes again.

Mini Jumbo frames can be used on our own routers to tell the router that 1508 MTU is okay, now we can send 1500 + 8 bytes PPPoE = 1508, and when the PPPoE is stripped off at the ISP its now 1500 MTU to the Internet. Not all routers support this setting, but if it does this is supported by the Openreach and all works well.

If you are always using a VPN for example there may be good reasons to use a lower MTU by default, but generally most people will be defaulting to 1492 on equipment from their ISP.

Just a quick edit with a tech document for anyone interested, it seems the industry has been actively working towards supporting mini jumbo frames (1508) over PPPoE (to allow for MTU of 1500 after PPPoE) for broadband connections as it seems it helps their networks if things can travel around unfragmented and optimised.

https://tools.ietf.org/html/rfc4638

nowster · Feb 17, 2021, 10:23:29

The caveat for that is that there may be isolated pockets within the OpenReach network, especially in rural areas, where they haven't yet upgraded the backhaul.

L2020 · Feb 17, 2021, 11:57:22

Quote from: nowster on Feb 17, 2021, 10:23:29
The caveat for that is that there may be isolated pockets within the OpenReach network, especially in rural areas, where they haven't yet upgraded the backhaul.

Perhaps, only Openreach could answer that one. I found some good info here about it all https://kieran.ie/mtu-baby-jumbo-frames-and-fttc/

The fact ISPs seem to be deliberately supporting baby jumbo frames to allow 1508 over PPPoE in order to get 1500 maximum on the Internet seems to suggest it is the accepted practice now.

I think it is always better our end to support the maximum MTU possible on the Internet to prevent fragmentation on incoming packets. If the source sends us smaller that is fine, but if we are capped lower and the source is sending larger packets, it's our end forcing the fragmentation if MTU discovery doesn't work. As I understand it anyway.

nowster · Feb 18, 2021, 09:59:04

There are still places where ADSL2 is the best you can get, and that means ATM framing, even if you use PPPoE.

L2020 · Feb 18, 2021, 11:12:08

Quote from: nowster on Feb 18, 2021, 09:59:04
There are still places where ADSL2 is the best you can get, and that means ATM framing, even if you use PPPoE.

Indeed there are, I have relatives stuck on ADSL. Reminds me of the days of having fun messing around with various tweaks and tests with MTU at 1500 or 1454, however I always found the results were inconclusive despite making sense why the lower MTU would be better with ATM framing, so stayed with the higher MTU for compatibility, as back then the lower MTU stopped me using a company VPN.

nowster · Feb 18, 2021, 11:21:01

Quote from: L2020 on Feb 18, 2021, 11:12:08
Indeed there are, I have relatives stuck on ADSL. Reminds me of the days of having fun messing around with various tweaks and tests with MTU at 1500 or 1454, however I always found the results were inconclusive despite making sense why the lower MTU would be better with ATM framing, so stayed with the higher MTU for compatibility, as back then the lower MTU stopped me using a company VPN.

I can't use a customer VPN when the laptop is tethered to EE 4G, and I've had to adjust my personal OpenVPN config to use "fragment 1300" and "mssfix" to work when tethered.

john7 · Feb 23, 2021, 21:32:47

So unfortuantly I look to have been told porkies by support!

L2020 · Feb 24, 2021, 10:55:09

Quote from: john7 on Feb 23, 2021, 21:32:47
So unfortuantly I look to have been told porkies by support!

I think the information you had been given was okay, just didn't the answer the question for you.

ADSL at MTU of 1458 is considered optimal, but doesn't break if greater. When they say "LAN MTU values of 1500 or greater will cause packet fragmentation" should really be 'LAN MTU values greater than 1500 will cause packet fragmentation'.

The real issue was they were intercepting our connections to servers and changing the value to MTU 1460 as the supported maximum, even if the server we were connecting to said "Hey I'm a great server and optimised to the hilt and happy with 1500 MTU lets go to the max", we saw it as "Hey you can't send me more than 1460 as I'm a bit flaky". If you Google "idnet 1460" you can see several mentions over the years of people being puzzled by this.

Sorted now though.

Tech Dude · Mar 18, 2021, 21:57:27

Netgear guidance on MTU size: https://kb.netgear.com/19863/Ping-Test-to-determine-Optimal-MTU-Size-on-Router
TP-Link guidance on MTU size: https://www.tp-link.com/us/support/faq/190/