[bjensen2]

Update…
After a couple weeks of running with random MAC addresses I had mixed results. Usually the router would work for a day or two and then the problems would begin. Power cycling the devices (to get a fresh MAC address) seemed to reset the cycle. But this Thursday I seemed to run out of options. After three power cycles I was still experiencing masses of dropped packets (I’m not sure that’s a technically accurate description of the issue, but it will suffice). This made me think that the duplicate-MAC theory can’t be correct, because the odds of getting three duplicates in a row are astronomically low. Also, it was pointed out to me that Comcast may well have structured their network in such a way that duplicate MAC addresses aren’t actually a problem.

Thus I went back to cloning the MAC address off my working router. And lo and behold, that didn’t work either. So at last I removed the PCWRT router from the system and put my gigabit router in its place. That instantly resolved all issues. I’ve had an unbroken VPN connection ever since, streaming works, etc. I didn’t even have to reboot the cable modem.

From reading this forum it seems there are many users who are employing these routers without the kind of issues I encountered. I don’t know, perhaps they are just lucky. But my home setup is pretty plain-vanilla when it comes to devices, using a common cable modem that millions of other people are or soon will be using as DOCSIS 3.1 is rolled out across the USA. In any case, I can’t spend any more time on this, and will have to figure out another way to restrict devices on my home network.

[bjensen2]

Arris supplies the majority of modems used on cable internet service in the USA. There are millions and millions of them deployed. A systematic problem with Arris modems seems highly unlikely. What’s described in that post sounds like something Spectrum has imposed on their subscribers. An ISP can modify the behavior of the modem pretty much at will. But I’m not on Spectrum, and I’ve had no issue with several devices connecting to the T25, including my PC and a couple of different routers. This problem only arises with the PCWRT. I have a more plausible idea about what I’m seeing.

The issue I’m encountering will manifest if there are multiple devices with the same MAC address on the network. One thing that makes me suspect this might be the case with the PCWRT is that the burned-in address doesn’t match the sticker on the bottom of the router (it lists XX:XX:XX:XX:XX:0C). Another thing is that the first WRT router, which was manufactured at least three months prior, if not earlier, had a very similar MAC address. The difference between the old and new MAC addresses is only 36 in decimal. Presumably more than 36 routers were manufactured during those months. So the MAC addresses of these routers are not sequentially incremented, and if they were randomly distributed across the 16 million possibilities available, the odds of me receiving two routers with random addresses <= 36 bits apart are phenomenally long. So my hunch is that many PCWRT routers ship with the same MAC addresses – hundreds or maybe even thousands. In fact, the reuse of MAC addresses is pervasive enough that I’ve now had two of them that were not unique on the particular segment of Comcast’s network to which I am connecting. That is, some other frustrated Comcast/PCWRT customer was experiencing the same problems that I was. There’s a way to test this theory… and I did.

I tried the “random MAC address” option in the internet settings. This seemed to work just as well as the cloned address (and I actually do think the cloned address took effect, it is just a bug in the UI that does not display it on the status page). The reason that a random address would work is pretty simple – the randomly selected address is one of the 16 million possibilities that doesn’t match any used on any PCWRT routers manufactured to date, or at least not any that have been put into service on my network segment.

I don’t really want to go forward with a cloned MAC address as that could later lead to a problem if the gigabit router is put back into service. I think the randomized MAC address is a reasonable fix, although I’ve never needed to do this with any other piece of networking equipment.

If you do not believe that duplicated MAC addresses are the source of my issue, please explain why the randomized MAC address option (which, by the way, does seem to display correctly on the status page) would resolve the problem.

[bjensen2]

So I’ve been running with a cloned MAC address for about four days, and… so far it seems to have resolved the issue. The gigabit router and the old Netgear wireless access point have of course been removed from the network and powered down. The PCWRT router is handling the WAN connections and all the home devices.

I am not sure, however, that the cloned MAC address has really “taken”. My PCWRT router shipped with an public-facing MAC address of XX:XX:XX:XX:XX:0F and a private-facing MAC address of XX:XX:XX:XX:XX:0E. In Settings -> Internet -> Override MAC Address I entered the MAC address off the Gigabit router YY:YY:YY:YY:YY:C1. However… on the Status page, under Internet MAC Address in the top left hand corner, the old XX:XX:XX:XX:XX:0F is still displaying. I would have expected this to match the YY:YY:YY:YY:YY:C1 address that I set in Settings->Internet. Anyhow, populating the “Override MAC Address” and saving that setting is what appears to have fixed the router.

I have the following questions:

1. Why would overriding the MAC address fix the issue? Is there something “bad” about the XX:XX:XX:XX:XX:0F address, or something “special” about the YY:YY:YY:YY:YY:C1 address? The gigabit router was just purchased off the shelf and the YY:YY:YY:YY:YY:C1 address was what it shipped with;

2. Would it work just as well to set any other random address, as long as it was not XX:XX:XX:XX:XX:0F?

Thanks!

[bjensen2]

It’s an Arris T25, running on Comcast gigabit internet service.

[bjensen2]

I received the replacement router, thank you. The only difference I can see is that it’s running a newer firmware, version 2.03 I think. I set it up in place of the temporary setup I’ve been using with the gigabit switch, that is, I just patched it directly to the cable modem as I’d originally tried back in November.

For an hour or so things seemed fine. I was able to reconfigure my household printer and add devices to the new wireless network. But soon enough my work VPN connection began dropping every few minutes, and other devices began to experience losses of internet connectivity, operations timing out, etc. I cannot tell whether it is any better than the original router, but I don’t think it is. So, I put the switch back in the mix as I describe above in my Dec 20 post and once again, as soon as I connect to the network hosted by the old Linksys router all the problems vanish.

So – what next? Should I try disabling DHCP? I presume that if I do that I can reverse the changes by executing
/etc/init.d/dnsmasq enable && /etc/init.d/dnsmasq start
? Or is there something else I can try?

[bjensen2]

Sent an e-mail.

If I am willing to lose filtering functionality for the purpose of testing – is it possible to configure the device as a wireless bridge?

Would it be possible to see anything in the logs? I am comfortable with using the command line.

Thanks!

[bjensen2]

I had to return the gigabit switch. I found that the PCWRT router was still dropping connections every 30-60 minutes even with the additional device between it and the modem anyway. I can’t really wait to get this settled as I need to be able to work from home, so I bought a gigabit router and restructured my home network a bit.

At present I have the gigabit router handling NAT off the cable modem. The gigabit router has only two devices connected to it. The first is the old Linksys device which I reconfigured as a wireless access point and it serves a wireless network that I use only for my work PC. The second is the PCWRT router providing a separate wireless network that is used by all non-work devices. I can hold a VPN connection all day this way, and other members of the household are more or less OK, although they’re still telling me that they get “kicked off” periodically.

So – a couple of questions:

1. If I obtain a replacement router from you, how will it differ from the one that I already have?
2. Is there a way to configure the PCWRT router as a wireless access point, that is, as a bridge only that does not perform routing, just extends the LAN created by the gigabit router?

Thanks!

[bjensen2]

I tried the troubleshooting steps you outlined.
1. Turning off Access Control made no difference.
2. I tried two other cat 5E patch cables and this made no difference.
3. I dug up an old Linksys router and turned off DCHP on the LAN. I inserted this device in between the cable modem and the PCWRT router so that it functioned as a switch. This does seem to resolve the issue, although the speeds I saw topped out at about 96mpbs (not surprising – it’s a 100 base T switch).
4. I borrowed a gigabit switch for testing and replaced the Linksys switch with the gigabit switch. The speeds are now as high as 260mbps when my PC is on the 5ghz wireless network and near the router.

Do these results indicate an interframe gap issue or something of that nature? Is there any way to fix this problem short of buying a redundant switch to sit permanently between the PCWRT router and the cable modem?

[Author]

Posts