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WiFi DHCP / Connectivity Issues

By Genphlux ·
Okay I've been battling this Wi-Fi problem here at work and I thought I'd type it up and see if any of you could help me out.

Where I work, we have a warehouse area with an automation system & conveyor belt, as well as an administrative office area. I have recently set up a wireless network so that some of the directors and managers can take their laptops out into the warehouse and use them freely.

Configuration: Standard corporate Ethernet network. Win2k3 DHCP server is the most important information here. All Cisco switches, standard patch panels, etc. Wireless consists of two brand new Linksys WAP54G 802.11G APs. Clients are Dell & HP laptops using integrated 802.11G wireless adapters, and connecting via Windows wireless configuration. I have also tried Dell and Linksys for configuration software client-side.

Problem: No matter how I set up the network or configure the APs, the clients can rarely pull a DHCP address. And when they actually do pull one, it only stays for 2 mins or so, and they lose it again. I have tried with our WEP settings, with different WEP settings, and with the standard out-of-the-box wide-open configuration. I set static IPs for the laptops just as a troubleshooting measure, and at first it appeared to have worked OK. However, while they are connected, they have limited network access. If they try to use Citrix, it drops connection constantly. They are unable to hit network printers or shares. But they can browse the web and our intranet, intermittently.

Although I first suspected interference from our automation equipment, signal strength as well as SNR are both well within reasonable and acceptable limits. I don't think the issue is on the radio side.

I have spent countless hours on the phone with Linksys, but they are ultimately no help. Their actual official response was "Well it appears that DHCP probably isn't going to work through the AP, but it might.". WTF??

I spent some time with a packet sniffer (Ethereal) and watched the traffic. You can see that the client requests the IP address from the DHCP server like 4 times in a row before it ever gets a response back from the server. So obviously I start thinking network latency, but ping response times are just fine. The server also always replies, only once, with an IP, as if it were successfully assigned. But the client machine never officially gets the IP and uses it. Also, from the server side, looking at the DHCP logs, as far as the server is concerned it successfully assigned the IP. The logs show a successful assignment to that MAC address. Our DHCP pool has PLENTY of addresses left. We are nowhere near running out.

I have put a band-aid solution up, just to get them going, but I don't understand why it worked, and I don't need to keep it this way.

I brought in from home a Netgear 8 port SOHO router/switch. I seperated the network and used a completely different IP addressing scheme than what we use here. I patched the APs directly into the Netgear and allowed them to pull addresses from it rather than our Win2k3 DHCP server, and it works great now. It hasn't dropped once.

Also keep in mind that I have the APs set up for static IPs for the AP itself. I can remote into the web console no matter where the AP is plugged into the network, its just that the AP doesn't function as it should. It doesn't forward DHCP traffic normally.

The DHCP server works fine with any machine that is plugged into it, as long as its wired. Like I said, we have plenty of addresses in the pool and you can release/renew all day long with no problem until you try it wirelessly.

I'm sure I've left details out, as I have spent a considerable amount of time on this issue, but I need to get it working with our DHCP server and get my Netgear off the network. Any ideas?

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Welcome to Techrepublic...

by CarlitosWay In reply to WiFi DHCP / Connectivity ...

and wow you have quite the problem.

I got a question, how is the signal from AP when you try to assosciate with it while it is connected to the DHCP server. Can you logon to the setup screen with out a problem?

You might want to also check to make sure that the firmware of the AP is also updated to the latest version.

If you need the link it is:

Also, make sure that you have each AP on a different channel to cut down on interference, if they are in close proximity to each other. Try to overlap the signal coming from each AP but not to much, to avoid packet collisions. If they are far apart I wouldn't worry to much.

Let me know. I'll try to think of anything else that might be a problem.

Good Luck,


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Lease Time

by iandavid In reply to WiFi DHCP / Connectivity ...

I am interested in why the DHCP lease ends so quickly, even if they lose network connectivity, they should keep the address for the lifetime of the lease?

Forgive my lack of knowledge here, as I have never configured a linksys AP, but have you tried letting the ap's have a DHCP lease, if you have what happened?

What about the speed and duplex settings between AP and Cisco switch.

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by bgculler In reply to WiFi DHCP / Connectivity ...

About the only thing I can think of is potential VLAN issues. Are all the devices on the same vlan? Didn't know if you trunked the ports on the switch that the AP is connected but you also have to add a command telling the AP what the native VLAN is. Since you didn't mention anything about VLAN's I have to assume it's all the same vlan so none of what I mentioned above matters.
The fact that statically assinged addresses doesn't work either tells me it's more than a problem with DHCP. Something is killing that wireless connectivity, have you tried bringing this back to your office to see if it reacts the same?

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Broadcast domain and bandwidth

Until recently, I was the IT Director for a medium size government where we operated an outdoor wireless network with dozens of APs. We also faced substantial DHCP problems. Here's what we decided the problem was:

An 802.11b device has a theoretical maximum of 11 Mbps (Half duplex), meaning useful throughput of around 5 Mbps. Wireless is a shared media (rather than switched that we almost all have on our wired networks), so this bandwidth is shared between all wireless clients. If you are using an AP (rather than a router), all broadcast packets on your network will be sent out on the radio. A busy switched 100 Mbps network can easily tolerate 2 or 3 Mbps of broadcast traffic, but that amount of traffic swamps the shared radio channel, forcing lots of retransmissions both to and from you wireless devices. Since DHCP is fairly timing sensitive, this causes frequent DHCP failures. Once a DHCP address is obtained, excessive retransmissions of payload traffic may cause the wireless client to decide that it has roamed, causing it to release its hard-won DHCP address and ask for a new one.

So what can you do about this problem?
Instead of using an AP, use a combo AP/router (like the LinkSys WRT54G, ~$60). This does two things: 1) the router keeps the broadcast traffic off your radio channel; and 2) the AP/router can also offer DHCP services right on the AP, where the wireless client will get an immediate response. This magically cleared up all of our DHCP problems, and dramataically improved overall network performance as well. A little clever configuration of the AP/router can also add another security layer to your architecture at the perimeter.

Good luck!

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by CarlitosWay In reply to Broadcast domain and band ...

you bought up a pretty good situation, greg. Never really thought about a switch creating a issue. Hey,I learned something new today :)

Kudos to you!.


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Similar Problems....

by tm3k4s7 In reply to Interesting....

I had similar problems on our home system. I ended up on an open system. Before I came to the decision to go to this setup, I found that the firewalls (on our laptops and the main system) were creating issues as well, once I got past that, I still had dropped IP's and decided to go with wep/open system instead of the wap/shared key. This might be a less secure setup but a) it's a home system and b) no more latency problems. Haven't had a glitch since. I have a Linksys wrt54gs. Just a thought.

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by nuander In reply to WiFi DHCP / Connectivity ...

Thanks all for the info here which helped me sort my issues. I found that my WAP54G would not relay DHCP while using the WPA shared key security. This even after following advise to install the 2.08 firmware. I also found that setting the AP to use DHCP or Static IP for its own address had no affect.

I am now using two WAP54Gs with one set as a repeater. This set up works fine with DHCP provided I use the WEP key security (and not WPA). I also found that setting the SSID to a different string on each AP improved the wireless performance. The client computer is less confused about which AP its getting its packets from.

One more thing. I found that if I plug an ethernet cable into the repeater AP, that this also works. So it is acting like a repeater AND a bridge.


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Glad you found the problem......

by CarlitosWay In reply to WAP54G and DHCP

Linksys give you any reason's on why the AP wouldn't relay any packages to DHCP using WPA?


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Same here, even in 2007

by gjs@overdam In reply to WiFi DHCP / Connectivity ...

We experience the same problem, now we implemented a smiliar set up in a small office environment in our premises:

4 Linksys APs: (Wireless-N Broadband Router) WRT300N (latest fw 2.00.17) and a Fedora Core 6 server that does the DHCP leases.
Pretty much exactly the same behaviour, although some APs users have none or less issues, and some very much.

Added to that we have VOIP boxes connected to some, and they even seem to interfere with the AP.

I was reading a good suggestion in this thread, but we still are trying to tackle this issue.

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