Isp router mtu settings

[l2eactionz]

Hello asked this before on the actual isp forum (was soooo helpful) lol.

 

Question.

My isp modem router that gets shipped out with broadband has a built in mtu that is not changeable after running command prompt and using the ping feature. I could work out that my isp uses an mtu value on the router of 1488.

On the xr500 this is set to the default 1492 for my pppoe.

I'm wondering why the isp is capping to 1488 and not allowing for the correct or default 1492. Do they know something about there own network they are not sharing with me?

Are they trying to optimise a low end broadband with mtu to optimise through put?

After reading everywhere that mtu doesn't matter and shouldn't be changed why does my isp cap there equipment to 1488?

Theories accepted lol.

And should I set my xr500 to 1488 juuuust incase there is some difference the customers aren't being told about?

Thank you

[Netduma Fraser]

They would have experimented with this and determined what works best on their network, usually a bigger MTU is more efficient, if it's a smaller ISP than the big players for example then their hardware/infrastructure may not be as up to date and therefore slower, in which case a smaller MTU would make sense. I would recommend you set the XR500 to use the same MTU as the ISP router so you don't risk fragmentation.

[Killhippie]

On 10/16/2022 at 1:47 PM, Netduma Fraser said:

They would have experimented with this and determined what works best on their network, usually a bigger MTU is more efficient, if it's a smaller ISP than the big players for example then their hardware/infrastructure may not be as up to date and therefore slower, in which case a smaller MTU would make sense. I would recommend you set the XR500 to use the same MTU as the ISP router so you don't risk fragmentation.

Smaller ISP's like AAISP and IDNet actually use more modern tech than the big boys, for instance AAISP uses a firebrick on every line to fault monitor and IDNet  allow HTTPS over DoH and TLS and use DNSSEC as standard for DNS . The router using PPPoE with a MTU 1488 with a separate router at 1492 is about right as it allows for the extra 8 octets to 1500 which is the standard MTU in the UK, for instance Vigor go up to 1520 to allow Jumbo frames. The XR500 would have auto set the MTU to 1492 after doing its line test as the optimum MTU for a PPPoE connection, it may be that 1488 was specific to the modem built into in the OP's modem router but not necessarily correct for the line itself with different hardware.