Use BT Home Hub 5A with external modem (Zyxel VMG3925-B10B)

I own a BT Hub 5A gateway with Openwrt 19.07.5. My vdsl connection drops here and there (known lantiq modem-related issue) and although the frequency by which it drops is not that bad, I want to make the connection really stable (30 days + or so without drop).
I was inspired by another post which referred to a setup with the Zyxel VMG1312-B30A modem, so I did my little research about good broadcom chipsets and ended up buying a used Zyxel VMG3925-B10B modem. Yesterday I rebranded it sucessfully via serial port etc and updated it to the latest modded firmware version with jumbo frames capability. Looks quite a decent piece of equipment.

Now I am trying to migrate my setup from an "all-in-one" solution to a 2-device solution.
I understand that Zyxel has to be in bridge mode and BT Hub 5A to act in switch/router mode. The devices would be connected to each other by the LAN 1 port of Zyxel to the red WAN ethernet port of BT Hub. Is that correct?

I put Zyxel to bridge mode and I defined the VLAN to 101 (that is the VLAN for BT here in the UK) in its web interface and disabled wifi and firewall. However I cannot see where I can enter the ISP details in such setup, and WAN connection says it is "down". Not sure if anything up to here is right.
Is it the case that the ISP details need to be entered in the 2nd device (BT Hub)? Do I need to specify in Zyxel who the only client is (BT Hub's MAC address)?

With regards to the openwrt setup of the 2nd device (BT Hub), how should the WAN (or WAN6 or whatever relevant to the red WAN ethernet port) interface (or the DSL tab?) be amended so that it successfully listens from Zyxel?

Also any changes to the firewall of BT Hub or SQM that need to be changed? Currently SQM is used in pppoe-wan (wan, wan6, wan_6).

Thanks

Refer to sections 7.5 and 9.2 (for PPPoE on WAN port) of installation guide for HH5a
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266

9.13 lightly touches on SQM

Cheers @bill888 !! Believe it or not , I was looking at your guide at the time you were writing the reply!
All done now. It was much easier than I thought:

  1. addition of the 2nd VLAN so to be able use eth0.2 interface
  2. change of the WAN physical interface from dsl0.101 to eth0.2
  3. disabled the DSL port (dsl_control service)
  4. no need to touch sqm or firewall as my setup was already fine.

By the way, I did not find the "override MAC address" option in the web UI of the WAN interface. Has this been removed recently? Also is the related bug fixed?

Thanks

I got a hunch the "override MAC address" field doesn't appear when using PPPoE. ie. my guide is wrong. It is visible and still required for HH5a when using say DHCP or static IP instead of PPPoE.

All right. So not to worry as it does not affect this setup.

Maybe I go a bit off topic here, but after I finalised the setup I did a couple of speed tests (via dslreports) and I get about 12 Mb lower actual download speed (not sync speed) compared to the download speed (~60Mb) when the devices work individually as all-in-one gateways using their internal modems. Does this preformance drop relate to the famous Bt hub 5a wan-lan speed issue or is it something more or less expected?

Thanks

You appear to have a different problem.

Download speeds should be similar to when HH5a was wired direct to your DSL connection.

What download speed do you get with PC (with PPPoE) wired direct to Zyxel modem?

Please note that Zyxel is currently at bridge mode hence I have not set up any ISP details there, but when I do so, I get similar speeds to BT Hub 5A (~60Mb).
The upload speed is the same either via 2 devices or via 1.

Your original post is not clear. Are you seeing 12mbps download speed, or 60-12=48 mbps download speed?

Configure Zyxel in bridge mode. Configure a PC to use PPPoE protocol and wire it to Zyxel bridge modem and check download speed.

I suspect HH5a may be misconfigured. Factory reset it and configure with just PPPoE connection on eth0.2 and check speeds. Do not install any packages or any customised settings etc.

Perhaps try the documented fixes for slow WAN to LAN throughput described in section 7.29 of installation guide, if you are only seeing 48 mbps download speed.

I see 48 Mb actual (NOT synced) download speed

This is done. I see synced speed in the Zyxel xdsl menu of 67Mb.

This is what I will try now. Thanks

Update: after some further testing and after disabling SQM QoS, I got results same as if I was using 1 device (i.e. 61 Mb download/18Mb upload). However the horrible bufferbloat (score F!) came back, so I am not willing to keep sqm disabled. Not sure if there is something in this setup that creates a bottleneck in the sqm process.
I will probably wait until 20.x is out to see if the WAN-LAN fix improves the download speed.

While the BTHub5 is a great device, because of its integrated VDSL modem and the rather good wireless, it still is on the slow side (with software flow-offloading enabed just over ~110-115 MBit/s, without it more around 84 MBit/s; obviously significantly less with SQM enabled). But now that you have a dedicated modem, there's not really any need to keep it - you are now free to choose a faster router as well (e.g. ipq40xx or mt7621, even contemporary ath79 would qualify (just barely, but still)).

Very valid point. I was thinking about it during this evening. Probably BTHH5A has served well its duties so far and it is time to move on, as it becomes limited in terms of capabilities.
I was thinking about Netgear R7800 (still a bit old but far more powerful) and it is possibly a good opportunity for me now to move towards this solution.

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