Problem connecting LEDE-router to cable modem (no link, no IP from modem)

Hi friends,
I am having a little problem here, and so far I have not yet an idea how to approach it.

Decided to buy a 2nd hand router (TL-WDR3500) to run LEDE as WLAN AP. So far so good, cheap hardware, flashing was easy, settings (as far as I need them) as well. LEDE router was connected to the outside world via my "old" router: ethernet connection from WAN port on LEDE (WAN-eth1) to a LAN port of the existing router (which then connects to the cable modem).

When I plug that LAN cable from my LEDE router on WAN (eth1) directly into my cable modem (SB6141), the router does not obtain an IP address, and the "link" LED on the modem goes red after blinking red for a while. I think this is a IPv6 issue but since IPv6 is really new to me, I have no idea how to troubleshoot this problem. Any advise is highly appreciated!

An interesting fact, that may or may not be related to my attempt of connecting my LEDE AP directly to my modem: After trying this for the first time today, I noticed after some while that my modem (at that time a Cisco DPC3008) went dead. It would establish a link via LAN to any device. I have tried several computers and routers as peers, as well as different ethernet cables, and of course hard-reset. The "link" LED would not light up, and a connection would not be established. So, I trashed this modem and I am lucky that my neighbor borrowed me his spare modem (Motorola SB6141). I know, sometimes it's just coincidence but I wonder if something may have caused the ethernet port on the modem to die. Power-over-Ethernet, as far as I can imagine not with these cheap routers...

Whatever, looking ahead, I would appreciate if someone could point me into the right direction or to an existing tutorial. Many thanks!

If you haven't tried yet reboot the modem (maybe the router too) after connecting the new router to it.

I have to do that with my cable ISP when connecting a different router to the modem. Else no lease can be obtained.

As far as I know this behavior is intentional for my ISP.

As an alternative, you can shut down the modem/the router, keep them off for a couple of minutes and then start them again.

Not obtaining any IP should not be related to IPv6. If you ISP runs a proper dual stack, IPv6 failing should just lead to IPv6 failing and not preventing you from getting an IPv4.

Thanks @hbr for your quick reply. It's kind of embarrassing to say but the rebooting the modem did the trick. It surprised me because I thought I had done so before.
Whatever, it works, and I am extremely happy about running LEDE on my router.

Many thanks to devs and supporters! Sorry for this really stupid question here.

As far as I remember the issue is that the DOCSIS modem will bridge data packets and the CMTS will only service registered MAC addresses. There is a short time window (typically triggered by a modem restart) in which the upstream elements are willing to learn a new MAC. The trick is to make sure that the DOCSIS system is in that state and make sure the new device is actually sending ethernet frames towards the CMTS. The result is that depending on the time your modem and router require to reboot/restart there can be an elaborate dance to make sure you hit the right time window (on a chart DOCSIS link I had repeated success with restarting both modem and router at the same time, but that is not guaranteed).
An alternative that often works is to clone the MAC address of the last device that eorked successfully on the cable link onto your router's WAN interface. Personally I would prefer to get the correct MAC into the ISPs CMTS/DHCP system even if that puts some constraints on the reboot order.... It is a bit sad that cable ISPs typically do not communicate this peculiarity better...

Best Regards

That's a good explanation. Thanks for the extra info.

Now it works just fine, also when power on modem and router simultaneously - this is important for me because I have a timer that cuts power to both pieces of equipment at night, and turns them on in the morning again. But OK, as you mentioned, now the CMTS already registered the MAC of my router.

LEDE is awesome! I wonder why it took me 13 years from willing to install openwrt (dd-wrt) to actually do it.