topic Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900 in Tech Help - Software/Hardware etc

Unifi UDR 7 - Potential MTU issues Full Fibre 900

J77j7bk — Tue, 11 Mar 2025 14:04:52 GMT

Hi all, only been with Plusnet for a year but it's been super stable and I had zero complaints so far! Until two weeks ago I was using a Google Nest WiFi Pro as my router from the Openreach OTN and whilst the mesh reach/penetration was poor, at least the speed throughput was ok.

I've just upgraded to a Ubiquiti Unifi Dream Router 7 and so far it's been nothing short of a nightmare!

I've been through Unifi support who said it might be something to do with MSS clamping or the Plusnet DNS servers so I thought I'd ask here.

Basically, in our house the following happens;

kids want to play Roblox - it won't load now (kids annoyed with me)

Wife want's to surf instagram - it will only partially load now (wife annoyed with me)

I want to VPN into work and join teams calls - no outbound teams audio now (boss annoyed with me)

In short, I'm stuck, can someone please help!?

Setup is as follows;

DNS;

Primary: 8.8.8.8

Secondary: 8.8.4.4

Jumbo frames are turned on

MSS Clamping: Custom - Max TCP 1380

Everything else seems to be working fine and speeds are right on as advertised but I can't get the above services to work, if I use a VPN on mobile devices for instagram, roblox etc it's fine.

Thanks in advance!

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

Protech — Tue, 11 Mar 2025 14:44:58 GMT

@J77j7bk wrote:

Jumbo frames are turned on

MSS Clamping: Custom - Max TCP 1380

First thing to try is to turn off jumbo frames and let the MSS Clamping return to its default value.

Reboot the Unifi after doing this and see if it makes a difference ?

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

J77j7bk — Tue, 11 Mar 2025 15:27:59 GMT

@Protech thanks for the suggestion, just gave it a test but unfortunately it didn't make any difference.

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

MisterW — Tue, 11 Mar 2025 15:44:52 GMT

@J77j7bk this https://www.reddit.com/r/Ubiquiti/comments/jfyujs/mss_clamping_on_udm_do_i_have_this_right/ sounds like it might be similar to your problem

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

outcast — Tue, 11 Mar 2025 16:02:46 GMT

It it were me, after experimenting with different MTU and MSS values, and having put them back to default,

the path discovery packet sizes could be messed up anywhere.

I would switch EVERYTHING off (including the ONT),

then switch back on each device in order from the incoming fibre, and wait a little to allow each device to complete booting

i.e. first the ONT, then router, then network switches, wireless access points, and lastly the laptops PCs etc.

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

iannewson — Tue, 11 Mar 2025 19:36:13 GMT

According to this a temporary fix is to enable smart queues in internet settings until they release a fix.

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

J77j7bk — Tue, 11 Mar 2025 19:38:45 GMT

@MisterW yep, pretty much the same, I'm going to set it to 1440 and then shut it all off and bring it back up again in order as per @outcast suggestion too!

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

J77j7bk — Tue, 11 Mar 2025 20:00:43 GMT

@iannewson that fixed it!!! Honestly, that's two weeks of my life I'll not get back but I am VERY relieved to have this sorted, even if it is a slight bodge. Expect it will get sorted in the next patch.

For anyone else having the same problem with either a UX7 or UDR7, do the following;

Log into affected setup in site manager then;

Settings (cog bottom left) > Internet > Primary (WAN) > Smart Queues (enable) > Set Downrate/Uprate > Apply Changes.

Thank you everyone else for taking time to suggest solutions, greatly appreciated!

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

outcast — Tue, 11 Mar 2025 20:02:41 GMT

@J77j7bk what speed test results does your Full Fibre 900 setup achieve with Smart Queues enabled ?

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

J77j7bk — Tue, 11 Mar 2025 20:05:53 GMT

@outcast just ran it and it was 903 Mbps / 104 Mbps which is pretty much the same as it was before.

Re: Unifi UDR 7 - Potential MTU issues Full Fibre 900

outcast — Tue, 11 Mar 2025 20:07:17 GMT

Thanks that is useful to know for the next person having Unifi problems.