MTU Issues with double NAT

[cedws]

I have an OpenWrt 24.10 device which I have freshly set up subnet 192.168.3.1/24 on. The WAN port is connected to another subnet 192.168.1.1/24 which I don't control, hence the need for double NAT.

My devices on the 192.168.3.1/24 subnet can route to the internet fine, but large downloads seem to stall and generally browsing the web feels slow and breaks intermittently, even though fast.com shows good download/upload speeds.

Connectivity is fine on the "WAN" network.

I suspect the problem is related to MTU but the things I've tried so far haven't worked. I've changed the Firewall settings and enabled MSS clamping on the lan->wan zone, and also tried decreasing the max MTU on the WAN port to 1400.

The MTU on the "WAN" network appears to be 1492, upstream connection probably PPPoE. (tested with ping, +28 to account for ICMP packet overhead).

What else can I try?

Thanks in advance

[ncompact]

In the meantime, welcome and Merry Christmas

If you remove the router managing the 192.168.3.x/24 network, do you encounter the same problem with a PC connected to the 192.168.1.x/24 router?

Can you show us the current configuration of the 192.168.3.1/24 router?

I assume you've only touched the minimum by changing the br-lan IP address. Did you change anything else?

@cedws

but large downloads seem to stall and generally browsing the web feels slow and breaks intermittently

can you show us what you mean by this?

ps: Assuming you're talking about the "edgerouter x" you read this:

Edgerouter-X with SQM on 850/45 network

So i just installed OpenWRT on my ER-X in an attempt to help alleviate the SQM issue on stock ER-X OS since my network is way over the 250mbps limit. (850 down, 45 up) I ran ifstatus wan | grep device and it reports Eth0 is the WAN port and when setting up SQM i applied 680000 for ingress and 34000 to egress and applied cake/ layers of cake. This fixed my issue with bufferbloat, but i am only getting 125/28 speed which is worse than what i was getting running ER-X default. I tried just using …

[brada4]

@cedws

large downloads seem to stall and generally browsing the web feels slow

@cedws

Connectivity is fine on the "WAN" network

Which one is it then? Please elaborate on how MTU is an issue?

[cedws]

Merry christmas

@ncompact

If you remove the router managing the 192.168.3.x/24 network, do you encounter the same problem with a PC connected to the 192.168.1.x/24 router?

No, this happens only on my subnet.

@ncompact

I assume you've only touched the minimum by changing the br-lan IP address. Did you change anything else?

Just that plus enabling MSS clamping and fiddling with MTU on the wan interface.

@ncompact

can you show us what you mean by this?

I've been using the Hetzner 10GB file as a test. I can start downloading it, speed starts off good (30MB+/s sec), after 500MB or so the download stalls and fails. Happens with various other sites too, I tried downloading a Ubuntu ISO too as a test and same happens with that. As for browsing the web, sites with a lot of images take ages to load.

@ncompact

ps: Assuming you're talking about the "edgerouter x" you read this:

Yep EdgeRouter-X. I didn't know the performance was expected to be below gigabit but it's probably fine for now. Can this issue be explained by bufferbloat?

Thanks.

[cedws]

@brada4

Which one is it then? Please elaborate on how MTU is an issue?

In the former statement I'm referring to the 192.168.3.1/24 network, not the "WAN" network.

[brada4]

Nothing indicates a MTU issue, if such existed you have to have that both sides of your router.
If you somehow know like upstream uses vlan and pppoe you could retract 12 from your WAN MTU to better fit in.

Please connect to your OpenWrt device using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button (red circle; this works best in the 'Markdown' composer view in the blue oval):

Remember to redact passwords, VPN keys, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall

[cedws]

After coming back to this a day later I can no longer reproduce the issue. No config was changed in OpenWrt, all that happened in the last day was a power cycle of the device. I have no clue what it could have been but at least it's resolved.

Thanks guys.