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Author Topic: Anyone forcing MTU1500 on Zen FTTP (OPNSense)  (Read 88 times)


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Anyone forcing MTU1500 on Zen FTTP (OPNSense)
« on: August 06, 2022, 11:55:53 AM »

Hi all,

Is anyone implementing MTU of 1500 on Zen FTTP using OPNSense? Having done some reading, I thought it was as simple as setting Interfaces > WAN > MTU to 1508 (it then shows calculated PPP MTU as 1500). If I do that, internet access is 'Broken'. Pages load really slowly, if at all, gets stuck on finding best server etc.

Do I need to set MSS clamping manually in the WAN interface?

I use IPv6 (as per Zens instructions for DHCPv6) - is that the issue?

If anyone has a similar set-up and has it working, any help would be appreciated!


Alex Atkin UK

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Re: Anyone forcing MTU1500 on Zen FTTP (OPNSense)
« Reply #1 on: August 06, 2022, 05:12:57 PM »

All I can say is on pfSense all you do is set the PPP connection to 1500 MTU and the internal logic handles the rest.  It automatically adjusts the parent interface to 1508.
Broadband: Zen Full Fibre 900 + Three 5G Routers: pfSense (Celeron N5105) + CPE Pro 2 H122-373 WiFi: Zyxel NWA210AX
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