View Full Version : Cable modem loses internet route
Hi,
This has been annoying me for the past few months. I have a pretty elaborate setup, with a FreeBSD machine configured as a
router to share my NTL internet connection out between multiple PC's, as well as act as a firewall.
The problem is this- everything will go dead, the upload/download lights on the modem will stop blinking, I cannot reach
the status page on the modem, traceroute, renew IP, nothing. I have eliminated it being the FreeBSD machine by plugging the
modem straight into a Windows machine and the same thing happens, no IP, no access to the status page (even when I manually
configure a 192.168.100.x IP), nothing. HOWEVER if i restart the cable modem, everything works again.
I'm pretty convinced I've found a firmware bug in the Ambit modem that causes the traffic routing to die. When I examine
the modem log after a reboot, there's never any mention of any network fault. The problem seems to happen more often when 2
of the machines on the network are running eMule.
Anyone got any ideas ? I don't fancy trying to explain this to the script-readers at NTL tech support!
Emule? You sure you aren't overloading your modem bandwidth? If you don't run the mule does the connection still drop?
Anyone got any ideas ? I don't fancy trying to explain this to the script-readers at NTL tech support!
:welcome:
This "script-reader" knows exactly what you are talking about ;)
I know of a bug with the ambit 200s that mean you can only get to the internal config page straight after a rebbot, but i am not aware of a bug that causes all dataflow to stop.
Do you lose the SYNC and RDY lights on the modem when the connection drops, or just the activity from the D/S and U/S lights?
quadplay
30-05-2005, 21:10
I have eliminated it being the FreeBSD machine by plugging the
modem straight into a Windows machine and the same thing happens, no IP, no access to the status page (even when I manually
configure a 192.168.100.x IP), nothing. HOWEVER if i restart the cable modem, everything works again.
And this "script-reader" says that's normal. The modem will only communicate with one MAC address between reboots (or two, if you're on the appropriate QoS).
Emule? You sure you aren't overloading your modem bandwidth? If you don't run the mule does the connection still drop?
Quite certain. I never run eMule at full pelt as I like to actually use the connection while stuff is downloading. It has happened without eMule running also, just takes longer to happen.
This "script-reader" knows exactly what you are talking about
Sorry. I realise there are exceptions but most of NTL's support (specifically the Indian support centre) just seem to read from scripts which is infuriating when a knowlegable experienced person like myself is trying to deal with them.
I know of a bug with the ambit 200s that mean you can only get to the internal config page straight after a rebbot, but i am not aware of a bug that causes all dataflow to stop.
I have also seen this bug you mention. The config page seems to work for some time here, then just suddenly stops. I put it down to either a) the route inside the modem expiring or b) the HTTP server in the modem crashing :)
Do you lose the SYNC and RDY lights on the modem when the connection drops, or just the activity from the D/S and U/S lights?
Well.. just the U/S light.. The D/S light flickers occasionally but only due to what i presume is ARP traffic on the cable side. There is no response to any traffic from the ethernet side.
And this "script-reader" says that's normal. The modem will only communicate with one MAC address between reboots (or two, if you're on the appropriate QoS).
OK i best clarify that. I tried it on a Windows machine with the same MAC address as the "external" NIC in my FreeBSD gateway to test. No response at all. I'm aware of the cablemodem learning one NIC MAC at a time and needing a reboot to relearn NIC MAC.
I have seen this happen in routers because of them running out of memory keeping track of their NAT connections, but not in an ntl CM - I suppose it could be running out of internal memory, but I can't think why.
I have seen this happen in routers because of them running out of memory keeping track of their NAT connections, but not in an ntl CM - I suppose it could be running out of internal memory, but I can't think why.
Time to get techy... the cable modem is in effect a router. It's whats called a transparent learning bridge, so in effect it's a router, that doesn't do NAT. I've head of Telewest's SciAtl Webstar modems being killed by excessive traffic and I'm just wondering whether the Ambit is the same. The Motorola Surfboard, is apparently bombproof
Another thing I have heard is that the Ambit is susceptible to an SNMP Flood attack on the external port, which can crash or lock it up. This has been verified by a friend of mine who has crashed his own NTL cable modem by doing SNMP Queries on it from his place of work.
My Terayon seems pretty reliable too, although I can't say the same for the new wireless router which often needs rebooting after a night or two of bittorent (which is odd as an older non-wireless modem from the same manufacturer never had any problems)
My Terayon seems pretty reliable too, although I can't say the same for the new wireless router which often needs rebooting after a night or two of bittorent (which is odd as an older non-wireless modem from the same manufacturer never had any problems)
Personally, I would like to have a Motorola Surfboard 4100 modem, as I know these are pretty much solid, having used one on Telewest Blueyonder for many years. I am tempted to buy one from eBay but NTL no longer allow the customer to use their own equipment. (and i'm not naughty enough to try forging the CPE MAC!)
Time to get techy... the cable modem is in effect a router. It's whats called a transparent learning bridge, so in effect it's a router, that doesn't do NAT.
It's a transparent bridge, it's not in any way shape or form a router (it would have an IP address on its interfaces if it did, but it doesn't). It's a purely layer 2 device with a separate management IP address. It would have to be a layer 3 device to be considered a router in the classic sense. It doesn't route packets, it passes them.
Another thing I have heard is that the Ambit is susceptible to an SNMP Flood attack on the external port, which can crash or lock it up. This has been verified by a friend of mine who has crashed his own NTL cable modem by doing SNMP Queries on it from his place of work.
No he hasn't. Unless his place of work is the same as mine he won't be able to reach the 'external port', by which I assume you mean the cable modem's private management IP address. These addresses are non-routable across the public internet and thus by definition are not susceptible to an SNMP flood attack, unless someone in ntl with access to that particular network is doing it.
Depending on where you live the Moto 4100 might or might not work - if you're in a EuroDOCSIS area it won't, it's only Ambits that work in those areas.
eMule kills my ADSL modem/router sooner or later, so I'd suspect that - it shouldn't of course, but it evidently puts high demands on networking equipment.
It's a transparent bridge, it's not in any way shape or form a router (it would have an IP address on its interfaces if it did, but it doesn't). It's a purely layer 2 device with a separate management IP address. It would have to be a layer 3 device to be considered a router in the classic sense. It doesn't route packets, it passes them.
Yeah, that is what i meant, i was using "router" in the loosest sense of the word (in that it sends packets from one interface to another)
No he hasn't. Unless his place of work is the same as mine he won't be able to reach the 'external port', by which I assume you mean the cable modem's private management IP address. These addresses are non-routable across the public internet and thus by definition are not susceptible to an SNMP flood attack, unless someone in ntl with access to that particular network is doing it.
Hmm... this was quite a while ago, back when DocsDiag used to work... so I'm not sure how he managed it. It is possible he had SSH'd to one of his own boxes (on the customer side of the modem) and ran SNMP queries from there.
Depending on where you live the Moto 4100 might or might not work - if you're in a EuroDOCSIS area it won't, it's only Ambits that work in those areas.
I live in Norwich.. so I don't know :) I know there is a EuroDOCSIS version of the 4100 (the 4100E?) and I guess that would work?
Just out of interest, since as you work at NTL, if I lived in a suitable area and bought an SB4100, how would I go about using it ? Wouldn't you have to register it's CPE MAC address on your system in place of the Ambit's before I could use it? Would repeating the procedure with autoreg.autoregister.net work?
eMule kills my ADSL modem/router sooner or later, so I'd suspect that - it shouldn't of course, but it evidently puts high demands on networking equipment.
I do actually have a Netgear wireless router, but I don't trust these things, and want more firewall control than standalone routers will give me, so i use FreeBSD 5.3 with dhclient and natd.
Just out of interest, since as you work at NTL, if I lived in a suitable area and bought an SB4100, how would I go about using it ? Wouldn't you have to register it's CPE MAC address on your system in place of the Ambit's before I could use it? Would repeating the procedure with autoreg.autoregister.net work?
.
well i know there isn't an answer for that on my script :P
actually, your correct
well i know there isn't an answer for that on my script :P
actually, your correct
I do apologise for the "script" comment but as I have said, 9 times out of 10 when you ring NTL Tech Support, you get someone in the Indian call centre who is reading from a script. I have asked to be transferred to the other UK tech support centre and they have refused.
What's the chances of convincing someone at NTL to register a new CPE MAC for me ? I'm betting nil :(
What's the chances of convincing someone at NTL to register a new CPE MAC for me ? I'm betting nil :(
You're placing a good bet there ;)
What about via the intelligent handsome kind NTL employees that roam this forum ? </suckup_mode> ;)
ian@huth
31-05-2005, 23:15
I do apologise for the "script" comment but as I have said, 9 times out of 10 when you ring NTL Tech Support, you get someone in the Indian call centre who is reading from a script. I have asked to be transferred to the other UK tech support centre and they have refused.
What's the chances of convincing someone at NTL to register a new CPE MAC for me ? I'm betting nil :(I don't really get this reading from a script business. In any troubleshooting there is a best way of approaching the issue. If all tech support agents use the best approach then they will all ask the same questions and ask the customer to try the same things. If every tech support agent did something different I would be a little worried that they weren't fully clued up with troubleshooting that particular issue. It doesn't matter what level of expertise the customer has the same approach should still be followed as even the most highly qualified customer has moments of forgetfulness.
Raistlin
31-05-2005, 23:15
What about via the intelligent handsome kind NTL employees that roam this forum ? </suckup_mode> ;)
:sick:
What about via the intelligent handsome kind NTL employees that roam this forum ? </suckup_mode> ;)
:LOL: you obviously haven't seen any of us in the flesh :p:
Raistlin
31-05-2005, 23:22
:LOL: you obviously haven't seen any of us in the flesh :p:
Now there's a thought :erm:
I'm sure you're all lovely really.....
OK, it happened again... emule wasnt even running. SYNC and RDY lights stayed on, but D/S and U/S went out even though i was trying traffic. Checked cable, NIC, everything, all fine... powercycled modem and it came back.
Should I try requesting a replacement?
I don't really get this reading from a script business. In any troubleshooting there is a best way of approaching the issue. If all tech support agents use the best approach then they will all ask the same questions and ask the customer to try the same things. If every tech support agent did something different I would be a little worried that they weren't fully clued up with troubleshooting that particular issue. It doesn't matter what level of expertise the customer has the same approach should still be followed as even the most highly qualified customer has moments of forgetfulness.
Ill agree with you on that, and i got to say when your talking a customer through fixing most problems it does feel scripted because you've done it so much but a script wont help you if the customers got a problem that isn't included in the script which catches the india agents out alot.
OK, it happened again... emule wasnt even running. SYNC and RDY lights stayed on, but D/S and U/S went out even though i was trying traffic. Checked cable, NIC, everything, all fine... powercycled modem and it came back.
Should I try requesting a replacement?
You can try. i think the company policy is they'll give you a replacment if you need to reboot it more than 4 times per day
Ill agree with you on that, and i got to say when your talking a customer through fixing most problems it does feel scripted because you've done it so much but a script wont help you if the customers got a problem that isn't included in the script which catches the india agents out alot.The giveaway is when they stop using logic, and will not accept reasonable answers like "I have already tried that" or "Why would the SYNC light on my modem flashing need my PC to be rebooted". A flow diagram to walk the customer through is no substitute for an experienced technician even if he does use a standard routine for helping people with faults.
I have never gotten to speak to the "experienced" techs at NTL - only the ones who are script readers.
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You can try. i think the company policy is they'll give you a replacment if you need to reboot it more than 4 times per dayI'm inclined to think they will say "You reset the modem and it works now sir ? Then it's OK sir, thank you for calling..."
Sadly I've been down that route before :(
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