Stumped by DHCP issue... (XP)

[Ichinisan]

I work for a local cable ISP. A customer had been leasing a cable modem from us and exchanged it for one that has a built-in wireless router (about $5 extra each month).

Original model:

• Cisco / Scientific Atlanta
• "WebStar"
• DPC2100
• DOCSIS 2.0
• No internal NAT or anything like that.
• Single RJ-45 connection
• Single USB connection

Newer model:

• Motorola
• "Surfboard"
• SBG941
• DOCSIS 2.0
• 802.11g
• Internal router does NAT and assigns local IPs
• 4x RJ-45 LAN ports
  

After the swap, wireless computers work perfectly fine...but the wired Windows XP desktop cannot get an IP address and shows "Limited or no connectivity."

- I walked the customer through downloading the WinsockXpFix utility from another computer. We copied it to a USB drive, ran it on the XP machine, and restarted. DHCP still fails.

- Tried Safe Mode w/ Networking. DHCP still fails.

- Tried manual IP configuration...

IP:

192.168.0.50​

Gateway:

192.168.0.1​

Preferred DNS:

192.168.0.1​

...and attempted to ping 192.168.0.1, but ping request times-out. Switched IP and DNS settings back to obtain automatically (DHCP).

- Customer had an older Netgear router they weren't using (I don't think it has wireless capability). Connected the computer to that router w/ no Inet connection and the computer obtained a valid IP address (192.168.1.2)!

- Connected back to the SBG941. Changed SBG941 internal IP to 192.168.1.1 and set DHCP range to 192.168.1.2+ (similar to the Netgear router). Rebooted all equipment. Still can't get an IP.

- Customer says they have tried wiring another computer directly and the wired connection works for another computer (though I can't trust that WiFi was properly disabled while testing this).

- Customer says they took the computer to a professional, who said the computer connects to the Internet just fine.

So my basic question is this:

Why would the computer be able to obtain an IP address from one router, and not another? The router appears to work fine with other computers...but there is some kind of compatibility issue here.

 

[theevilsharpie]

Speed/duplex mismatch or a defective port/switch on your router.

 

[bobdole369]

updated your systems MAC table to allow this new MAC (the new CM?)

 

[Ichinisan]

Speed/duplex mismatch

It would be the first one I've heard of with this model. The thought had crossed my mind, but I'm not sure what settings to recommend. I believe I've seen the speed/duplex options in device properties...but I don't think the dialog screens are consistent with different network chipsets and drivers. On my workstation, I just went through all the options on the "Advanced" tab for the network controller and there was no option for speed/duplex (Marvell Yukon 88E8056).

... or a defective port/switch on your router.

Perhaps, but it sounds like they've already tried swapping with other computers. Our tech went out and I believe he tested the port with his laptop. Also, I believe two SBG941 modem/routers were tried. Maybe the network cable is bad and they used a different Cat5 cord to test the other devices...? :hmm:

updated your system's MAC table to allow this new MAC (the new CM)?

Yeah. The modems don't work at all if they aren't provisioned. The built-in wireless router is the only CPE MAC our system sees. It has a valid IP assigned. I even had the customer visit www.whatismyip.com from the working wireless laptop and verified that its connection was through our system. The customer can have 254 devices behind the modem/router, if they want to.

 

[drebo]

Could be an arp issue on the workstation (especially if he has a switch between the cable modem and the PC). Clear the arp cache.

 

[bobdole369]

The built-in wireless router is the only CPE MAC our system sees.

And is that CPE MAC all that you normally would see? On our system with a wifi router/cm that's it. We will see the CM, the CPE from the wan side of the router, and the MTA (if equipped). Thats all. OK so you ruled out the ISP end of things.

Bad patch cable? Works in one switch, not the other? Maybe the port on the switch somehow isn't MDI-X and its an xover? or hes plugging into the uplink port (god do those still exist?)

 

[Lemon law]

My first guess is missing network protocols. Things like internet protocols for tcp/ip ver 4 or 6, client for microsoft networks, file and printer sharing for microsoft networks, or other protocols the router may need to have in the XP computer not otherwise installed and activated. Or there may services the XP has, but that do not start up automatically. Again things that are easy to fix, but often hard to find without trial and error.

Of course it just a guess, but it gives our OP another tree to bark up. Barking up the wrong tree never leads to a fix.

 

[Ichinisan]

Could be an arp issue on the workstation (especially if he has a switch between the cable modem and the PC). Clear the arp cache.

Well, the only switch is the 4-port switch built into the modem/router. The device was reset multiple times.

I'm pretty sure one of the following actions we performed would rule-out the ARP cache:
- Restarted PC and all equipment multiple times.
- Tried Safe Mode w/ Networking
- Ran Winsock XP Fix

 

[Ichinisan]

And is that CPE MAC all that you normally would see? On our system with a wifi router/cm that's it. We will see the CM, the CPE from the wan side of the router, and the MTA (if equipped). Thats all. OK so you ruled out the ISP end of things.

From my end, I would see the MAC IDs of multiple devices when a customer uses a regular modem with a switch or hub to share the connection (our CPE limit allows 2 IPs). If a customer connects the modem to the wrong port on a router, I can usually tell.

When the modem is properly-connected to a router that does NAT, I would only see the router's MAC and it would be the only device behind the modem. The combination modem/router behaves this way. I only see the MAC of the built-in router and I do not see the devices behind it. However, I *can* access the internal configuration page of the built-in router device. From there, I can see LAN devices that have requested DHCP.

Bad patch cable? Works in one switch, not the other? Maybe the port on the switch somehow isn't MDI-X and its an xover? or hes plugging into the uplink port (god do those still exist?)

Ugh! I remember how confusing that can be for customers...especially when the "uplink" port is electrically-connected to a numbered socket -and you can't use both. It's not a factor in this case. The Motorola modem/router doesn't have an "uplink" port and I would assume it auto-senses cross-over. It's a fairly-new model, and a brand-new unit. Definitely newer than the Netgear router the customer tested.