Router configuration with ISP FTTH modem/router

I have a BT home hub 5a device. Up to a week ago, it was configured as xSDL modem with LAN, WLAN and a lot of "customizations" (services installed). Now, I want to put it in cascade to ISP FTTH modem/router (I have to use phone port on it :frowning: ). So I already read here and I think that router with double NAT configuration is the best option in my case (correct me if I'm wrong). I just need to reach ISP FTTH modem/router from my LAN for maintenance.
I deleted the wan interfaces (IPv4 and IPv6) from LuCI, then I setup:
ISP modem/router:
router: with default gateway:
Moreover, I tried to add wan interface with static and DHCP configuration, but I do not have any Internet access on router LAN and WLAN. Finally, I disabled the firewall to be sure, but nothing changed.
Almost sure, I'm missing some stupid steps...

You need two interfaces, traditionally lan and wan, since it will be routing.

lan: IP Do not set gateway or DNS here.
wan: IP gateway DNS

Generally you can keep WAN a dhcp client (the default setting) to obtain IP and gateway settings automatically from the ISP modem / router.

it is better remove nat from openwrt router

router (interface wan) is in different network (

If you setup ISP router to be network

Then reset openwrt router to default.

It will work by default, just need to setup your wifi again

Obviously you are connecting WAN of openwrt router to ISP router LAN

Thank you at all for your support.
I was able to config openwrt properly. I made a mistake about VLAN (I assigned the wrong interface to my WAN, it was eth0.2).
Now I have Internet connection between modem/router of my ISP and openWRT BT HH 5a.

lan: IP no gateway/DNS
wan: IP 168.1.1 DHCP client

I have two remaining problems:

  1. Internet connection speed is very slow downstream of BT HH 5a: about 65/60 Mbit/s instead of 950/300 Mbit/s as downstream of the modem/router of my ISP. I tested it from a PC connected to openWRT with different website, (PC: ethtool enp8s0 | grep -i speed -> Speed: 1000Mb/s, openWRT: swconfig dev switch0 show -> speed:1000baseT full-duplex auto). I tried to disable SQM (wan) without any gain. I do not think that it is an hardware problem since a netgear gs605 v4 (stupid switch) allows to reach the same speed as with ISP modem/router.
  2. I would like to reach modem/router of my ISP and a device on its network from a PC on openWRT network (maintenance SSH, telnet, web). What is the best strategy? By using a static route? Do I need to disable NAT?

Thank you very much

The HH5A does not have anywhere near enough CPU power to NAT and route a gigabit service.

With the standard configuration you can reach 192.168.1.X IPs including the ISP router through the existing NAT. A route is already in place to send such requests to the WAN port.


In the master branch, the transition of lantiq xrx200 to the DSA driver, and further proposed patches promise higher throughput, but still less than 1 gigabit/s:

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Is there something that I can do? Can I configure BT HH 5A in such a way to improve the speed?
You are right, I am able to reach that network.

At the moment I am on the latest stable release 19.07.7. Is the patch going to be released with 21.02 version that has 5.4.x linux kernel?

Even with all optimizations currently imaginable in place, performance is still an order of magnitude off, compared to what you need. You need a router that has the hardware performance to cope, the BT HH5a is not that.

Thank you for the link. I also have a xiaomi ax3600 equipped with quad core cortex-a53 @ 1.4 GHz (qualcomm IPQ8071A ARMv8 64 bit) and dual core NPU @ 1.7 GHz, I hope to have openWRT for it soon so it would allow to reach speed closed to the maximum.

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