Intermittent L3 Connection Loss - Bad DHCP?

pentala · 21-02-2011, 17:52

Hi,

I've been having some issues now for approx 6-8 months with what appear to be L3 connection losses. I've spoken to tech support a number of times but seem to be getting nowhere.

I'm on my 3rd router now - having used a mixture of linksys, netgear & dlink devices - I'm now running on a DLink Dlink-DIR615 rev d using the DD-WRT firmware - DD-WRT v24-sp2 (07/16/10) std - build 14815

Every few weeks or so it appears as if the IP address becomes stale. No L3 traffic is passing, no access to the internet. Can't even access the admin interface on the cable modem... Refreshing the WAN IP address on the router will bring the connection back to life. Normally I would say that this would be a HW/SW issue with the router but as I have used 3 different devices and they all exhibit the same problem...?

I've just spoken to Virgin Support again to be advised that in order to maintain a working connection I should be re-booting my router at least every 2 days because the memory will fill up.....!

They have also advised that they cannot support a configuration where the PC is on a wired connection to the router, only wireless.....i've tried explaining that a laptop on a wireless connection also exhibits the same connection but they aren't interested.

Speaking with the support guys they suggest that the only checks they will do are at L2 to the modem, they are not interested at any layers above that. Indeed if I call them when the error occurs, the first thing they do is reboot the router - which has the same effect as renewing the lease i.e. it starts working....

Can anyone please help me as to how I can progress this further? Tech support will not troubleshoot above L2 and they won't escalate the call to the network support guys......

HELP!

Andy.

pip08456 · 21-02-2011, 18:38

I think you'll have to wait for Ignitionnet to come on to answer this one.

What I find surprising though is support saying they don't support wired connections as all tests the ask you to perform should be done wired not wireless.

Have you tried with a wired connection direct to the modem to rule out a router problem or config?

qasdfdsaq · 21-02-2011, 20:32

I have a funny feeling this is the same as what I get after a disconnect (which used to happen quite often on my modem, but it's now once a week or so).

Without really knowing why, it seems the modem waits for an explicit DHCP renew after a short loss of connectivity or reinitialization even if the old lease is still valid. As you say, you can't access the modem status pages or the internet until the router's WAN interface gets reset.

It feels a bit like the "DHCP required" option on Cisco wireless controllers which, after certain kinds of reconnect, will not allow any data to be passed until the client does a DHCP renew even if the old lease is still valid. The client doesn't realise, thinks it's old lease is still valid, and simply sees 100% packet loss.

Anyway, it looks like you might have the same problems as me - short, quick disconnect/reconnects without the modem rebooting, after which the modem/CMTS expects a DHCP request from the router but the router not realizing it needs to do one. I have a script as a workaround that monitors the WAN connection and issues DHCP renew requests if the modem drops, but doesn't actually stop the disconnections. VM have so far failed to address the actual cause of the disconnections, but at least now the connection comes back up without me having to reboot the router myself.

Chrysalis · 22-02-2011, 07:26

I initially had the same on ddwrt, it was when I was using the mac clone feature, I was cloning my pc mac on ddwrt so I would keep my old ip, problem was every dhcp renewal killed my net as I had to keep manually renewing it so I disabled the feature and its been fine since. I do still see occasional dhcp warnings in modem logs but I no longer get outages associated with it.

pentala · 26-02-2011, 20:34

A VM engineer came out this morning but couldn't find any problems. Strangely enough the router lost connection again last night - i've now got a watchdog configured on DDWRT that will re-boot the router if it looses internet connectivity - last time was 00:00 this morning... The engineer was going to look out for further problems but I haven't seen any fixes yet...

Ignitionnet · 26-02-2011, 21:18

Can you set up a TCP dump with a filter on the DHCP server you took your lease from?

Shouldn't get very large and should tell you what you need to know.

alto- · 26-02-2011, 23:01

I have had exactly the same issue since having an Ambit 256 (had a sci atlantic before, no issues). If the modem's connection drops for any reason, the router must explicitly request a new DHCP lease or else the modem will not talk to the router at L3. It's been present across two routers and four firmware types (stock x2, thibor, tomato, ddwrt x2).

As qasdfdsaq said, it seems like a configuration requirement for a "DHCP required" option is enabled. If that's the case, I don't think there's anything end users can do to actually fix it, only work around.

Ignitionnet · 28-02-2011, 07:25

OK I read the thread properly this time.

Yes - it's exactly that. Until the cable modem sees a DHCP transaction it doesn't talk to CPE and won't add it to its clients table. Odd way of doing things but I do get why it is done.