Comcast XFINITY → [Connectivity] SB6120 LAN stuck at 10Mbps!

I've just upgraded to 50/10 plan and ran the speed test. the UP/DOWN came back as 9.6/9.6;
After some troubleshooting, I've found that Ethernet link from the SB6120 and my router only negotiates at 10/full;
Since then I tried:
1) replace the cable
2) connect 2 laptops directly to SB6120
3) connect SB6120 to a gigabit triple speed switch
4) connect my router to triple speed switch

every result points to SB6120 LAN (LINE) port "stuck" at 10Mbps speed;

I called Comcast asking to check if there might be a chance of fat-fingering config and hardcoding LAN port to 10Mbps (I used to have 6Mbps plan!) - but no one seems to know of such setting. I called Motorola, they too have no clue.

I wonder if there is anyone out there who knows how to fix it. I really hope this is NOT a hardware issue.

thank you guys!

Maybe the problem is due to an auto-negotiation issue with that modem.

See: »SB6120 Connection Speed - LAN side

I suppose I can dig out my 10/100 switch and see if SB6120 negotiates with that correctly, but one would think that 4 different devices tried to negotiate with SB6120 and none got more than 10Mbps link - there is a good chance that 5th one will not either.
*4 devices are:
1- router (Cisco 1811)
2- dell laptop 10/100 NIC
3- lenovo laptop 10/100/1000 NIC
4- Trednet 10/100/1000 switch

by the way - I did try "Clear all settings" through web gui - it didn't help.

who knows - maybe my 10/100 Linksys switch will work... I will try and report back.

thank you for your reply!

Well then, like I meant, the auto-negotiation issue is with that modem.

Ok, The Linksys switch has 100 Mbps LED and FD LED;
Once I plug in the cable modem, the following is observed:
- the 100 and FD LEDs light up twice (about 1 sec apart) and then it goes dark for about 5 seconds
- the sequence above repeats one more time, after which the FD light comes up and 100 light stays off;

it seems the link is being tried at 100 and for some reason does not succeed.

I know that you said you tried replacing the cable, but there have been some previous posts on issues with cables: »[Speed] Free Speed Upgrade

Is the replacement cable a cat6 ? If not, give one a try for the halibut. 5e should suffice but some hardware is finicky.

Hello telcodad and EG,
yes, I've tried different cables. the original was cat.5 (not .e!) and I ended up with cat.6 now; I used the same cable to connect my laptop to the switch and it comes up (fast!) at 1G speed.

what do you think causes the delay I described while SB6120 is trying to negotiate the speed? It takes good 10 seconds (or so) for the link light to come up, while the rest of the equipment gives me the link light almost instantly?

if this helps any, here is my modem info:

Model Name: SB6120
Vendor Name: Motorola
Firmware Name: SB_KOMODO-1.0.6.10-SCM00-NOSH
Boot Version: PSPU-Boot 1.0.0.4m1
Hardware Version: 3.0
Firmware Build Time: Oct 29 2012 18:07:13

is there a way to check the LAN port setting through the web gui interface that anyone is aware of?

No, and I'm fairly certain that this isn't even configurable from the DOCSIS configuration file.

I don't think there is a way using the GUI.

The only way that I know of to tell what the LAN port speed mode is is via the Link LED indicator (orange = 10Base-T or 100Base-T, blue = 1000Base-T): »www.motorolasolutions.co ··· 01-a.pdf

That seems to be the case of bad hardware then, pity. Thank you!

Yeap, I've seen this guide. The light is always orange, which means 10 OR 100, but the other side of the link (either a switch / a router or a laptop) show me the 10Mbps link. Various speed test web sites confirm this. 9.6/9.6 Up/Down