Again good information, thanks... I think as my modem and router are separate I am stuck with PPPoE(oA) though!

FWIW I have just set up a Cisco 887VA on my GEA FTTC line with Zen and with no special configuration on my part I am successfully getting 1500 MTU, confirmed by https://www.speedguide.net/analyzer.php

Still not working on ADSL.

I don't trust Zen to not "upgrade" (downgrade) their FTTC stuff too so as soon as my cab is fibre enabled (~ 1 month) I'll be off.

Nice knowing you Zen, but this "upgrade" knackers IPv6 that doesn't play nicely with MSS clamping on most routers. IPv6 "works" but a lot of connections get reset when there is other traffic on the interface.

ISP recommendations welcome. Aquiss & A&A where best bets last time I looked?

On a Zen FTTC connection with a Cisco 887VA handling PPPoE but with a hardware firewall internal to it, Speedguide shows 1500 as the MTU.

On a Zen FTTP connection with just a firewall handling PPPoE I'm currently seeing 1492. I haven't done any tweaking on this yet.

Zen FTTC (80/20) customer here. I've got RFC4638 working correctly with a HG612 3B + EdgeRouter PoE. They definitely haven't dropped support for it, at least on fibre connections.

Excuse my lack of knowledge, but what's the significance of this for a regular Zen Fibre customer, such as myself? (if any)
Thanks.

In reply to a post by AstroflashTB:

Excuse my lack of knowledge, but what's the significance of this for a regular Zen Fibre customer, such as myself? (if any)

This blog post explains it well.

https://kieran.ie/mtu-baby-jumbo-frames-and-fttc/

I think baby jumbo frames are working for me right now on Zen FTTC.
I'm using an Edgerouter X with a HG612 modem.
I have IPv6 enabled on my Zen connection.
I just set the MTU of the PPPoE interface to 1500 and I seem to need to set the MTU of the port the carries the PPPoE to/from the modem to 1516 to be able to send the biggest IPv6 packets, but I haven't quite figured out for sure what is going on. 1508 would definitely be the correct figure for just IPv4 and I'm sure 1516 can't do any harm.

PPP adds 8 bytes so 1508 is the correct MTU for IPV4.
IPV6 would be less than that.

The MTU is the same for both IPv6 and IPv4. One just has a bigger header.

You don't change the IP MTU. That stays at 1500 for both IPv6 and IPv4.

The Ethernet MTU will need to be either 1508, or 1522 depending if the router is including the Ethernet header in its MTU size. Setting it too high shouldn't cause issues.

Edited by Geordish (Mon 08-Nov-21 16:59:28)