Cable modem won't connect to router with gigabit-capable WAN port

[#1]

I've encountered a perplexing issue recently on my home setup recently, and I'm at a loss as to what could be causing it.

I have a BT Openreach ECI cable modem supplied by PlusNet, connected to a NetGear WNDR3700v4 router. The NetGear provides my wireless network, as well as a wired gigabit network using desktop D-Link gigabit switches and CAT5e cabling.

Recently after a weekend away I had to powercycle both the cable modem and the router to restore my connection. After doing so, I've found that the cable modem will no longer communicate with the router on its WAN port. The modem is working normally, as is the router - but they won't connect.

I have tried a number of working cables, and the issue is consistent. If I connect an intermediate non-gigabit-capable device in unmanaged switch mode (e.g. an old DIR615 router running OpenWRT) as a bridge between the two, everything is fine. If I try to use a gigabit-capable device as the bridge, no dice. Depending on the device being used, the autonegotiation seems to be an issue as well (the first unmanaged switch I tried auto-neg'd down to 10Mbit).

In practical terms I'm not having a significant issue with my connection - I'm still seeing upload and download rates in line with what I'd expect. But I would like to understand why my cable modem now requires an additional device in order to communicate with my router, and ideally find out how to remove this requirement.

[#2]

From the diagnostic tests you have performed, it looks like the ethernet port on the ECI modem has developed a minor fault.

How old is the ECI fibre modem?  

is it earliest Fub/i or later Fub/r model ?

The underside label should tell you when it was manufactured too.

It could be the capacitors going bad inside the modem, and/or bad power adapter. Or power surge has damaged the ethernet port/chip.

Only way to confirm where the fault is, and hopefully fix the issue, is by substitution of the ECI modem.

[#3]

According to the label on the modem, it's a little under 4 years since manufacture and has been in use for just under 3 years. It's the later Fub/R model.

I'll be contacting the support team about it, but as it's not stopping me from accessing the service it's not massively high-priority.

As much as I'd like it sorted I'd also like to understand what's happened, because I can't see any obvious reason for it - even if it were a case that some of the contacts in the ethernet port were damaged somehow, I'd expect it to auto-negotiate down to 100Mbit, rather than not connect at all...

[#4]

Could be faulty electronics.  Electrical noise.  Out of tolerance.  Substitution is only way to confirm it I'm afraid.

fyi, Openreach stopped supporting their fibre modems just over a year ago.  There is no way of obtaining a replacement from Openreach or Plusnet.

I would guess Plusnet will probably just offer you a new Hub One modem router for cost of postage if you agree to recontract.

Unfortunately, the Hub One cannot be put into 'bridge' mode, to mimic the function of the Openreach fibre modem, if you wish to continue to use your Netgear as the main router.

The last ECI modems were made in early 2016.  You may be able to find one on eBay if you need one as a backup unit in case your ECI unexpectedly dies.  Alternatives to consider are 'BT Business Hub 5', Zyxel VMG1312, and TPlink TD-W9970 which all offer 'bridge' mode, but I've not personally tried them myself.

[#5]

Heh, I suppose being able to just get another of the same units would've been too simple I did think that seeing the stated warranty end date on the modem as being sometime in 2019 was an unexpectedly long lifespan.

Thanks for the recommendations on the models, and for the heads-up re: lack of bridge mode in the Hub One - I've yet to find an ISP-branded router that doesn't have either flaky firmware or a really rubbish DHCP stack, so I prefer to have the modem in bridge mode if at all possible. (I'm also aware my router is a bit long in the tooth and probably due to be replaced since Netgear are no longer releasing updates for it, although OpenWRT might extend its usable lifespan somewhat...)

[#6]

Alternatives to consider are 'BT Business Hub 5', Zyxel VMG1312, and TPlink TD-W9970 which all offer 'bridge' mode, but I've not personally tried them myself

If it helps, I use the td-W9970 in bridge mode on my FTTC connection. It has a pretty good modem according to kitz http://www.kitz.co.uk/routers/tplink_TD-W9970_review.htm , is a doddle to setup in bridge mode and can be bought for <£30. Only downside is lack of stats.