I am currently working in a developing country as a system analyst for a government department. My area of expertise is software projects, but I've come across a few issues with the network setup in my office. (Unfortunately, being a developing country, there's not a lot of professional help available for this sort of thing.)

Most recently, I am trying to diagnose a problem with slowness on the network. Our office is connected to the internet via an ADSL wireless modem/router (called Router). The modem is connected via ethernet to a switch (called Switch). The modem also acts as a wireless access point (called Wireless1), though because it is in a room at the end of the floor, it's range is limited. There are ethernet ports installed around the office. The cables of these all lead back to the same switch. In closer vicinity to the bulk of the client computers, there is another wireless router that acts as an access point for those clients (called Wireless2). That router is connected via ethernet to a wall port, and therefore to Switch. There is also a Windows server which acts as a DNS server (called DNSBox) which is located in the same room and is connected directly to Switch.

               DNSBox                           |         |--------------------
      --------------------|  Switch |---Other clients----
                                                |         |--------------------
               Wireless2 ------------------|

One final thing to mention about the network setup. All clients are configured with manual IP addresses. Their router/gateway is set to the IP address of Router, and their DNS server is set to the IP address of DNSBox (with a secondary IP set to an external IP - that of our ISP's DNS server).

Here are the symptoms we are experiencing:

  • Clients connected to Wireless2 AP experience slow and unstable connections to the internet. (Slow here is defined as speeds of ~1KB/s, though ping response times seem to be as normal.)
  • Clients connected via ethernet to Switch also experience the same slowness.
  • Clients connected to Wireless1 AP (i.e. connecting via wireless directly to the ADSL modem) experience normal connections to the internet.
  • Clients connected via ethernet to Router (i.e. connecting via ethernet directly to the ADSL modem) also experience normal connections to the internet.

I also tried to gauge the connection performance between two machines on the network via ethernet:

  • A file transfer between two clients who were both directly connected to Switch was the fastest;
  • A file transfer between one client directly connected to Switch, and one client directly connected to Router (which is directly connected to Switch) performed much slower;
  • A file transfer between two clients directly connected to Router also performed slowly.

Things I have attempted to diagnose the problem:

  • Restarted Switch -- no change.
  • We tried unplugging ethernet jacks from Switch 4 at a time and testing the internet connection. The thought here was that perhaps a client on the network has contracted a virus, and is possibly spamming the network with traffic? (Not very technical, I know.) Unfortunately we couldn't get any significant increases in performance using this method. There were a couple of times when it seemed to be better, but then the connection speed quickly dropped back to slow/dead pace. I didn't want to unplug all jacks from Switch because I was concerned that users might be affected or that I would re-plug in the jacks incorrectly (should I even be worried about that? a port is a port on a switch, right?)
  • I tried swapping the ethernet cable used to connect Router to Switch -- no change in performance.
  • I tried swapping the port used on Switch for Router -- no change in performance.

Anyone got any ideas on what this could be? Should I be mentioning specific brand names/models of the hardware used? Virii outbreaks are common in this country/office -- what could I be doing to figure out if a virus is at fault? If it is a virus, it doesn't seem to be generating a lot of traffic to/from the internet, because a) I can still get a good speed if I am directly connected to Router / Wireless1 and b) our ISP data usage has not risen suspiciously.

Thanks for your help!

Update #1

Here are the specs of some of the hardware:

  • Switch is an Edimax ES3132RL 32-Port 10/100 Rackmount Switch
  • Router is a D-Link DSL-G604T

Update #2

I just tried unplugging everything except a laptop and Router from Switch. Speeds are still slow. I guess that means that Router / Switch are not being flooded?

It seems more and more likely that the cause is something to do with the interaction between Router and Switch. However, I still can't find any useful resources on setting the LAN speed for either (and I'm not well-versed in these advanced networking configurations).

6 Answers 6


All but 1 of the problems you've described and all but 1 of the tests you've conducted appear to have the switch/router link in common (if I read and understood your post correctly). I know you've tried a different cable and switch port but have you looked at the configuration of the switch port and router port for that link? Check the speed and duplex setting on both ends and make sure they match.

  • How would I go about checking the configuration of the switch port? And same for the router port? Is there anything I can clarify for you in my original post?
    – tokes
    Feb 8, 2011 at 23:44
  • I'm assuming the router has a CLI or web interface that you can log in to and check the port configuration. The switch may or may not be a "managed" switch and may not have any administrative interface but you'll need to check the vendor documentation to find out. What make/model are the router and switch?
    – joeqwerty
    Feb 9, 2011 at 0:13
  • I've updated the original post with the make/model info. On Router at least, I can't seem to find any settings for port speed (half/full duplex).
    – tokes
    Feb 9, 2011 at 3:22
  • After looking at the user manual from the Edimax web site, it doesn't look like the switch has any management interface. Also, looking at the router user manual it doesn't look like there's any way to configure the speed/duplex settings on the LAN ports or to view the speed/duplex settings of the LAN ports.
    – joeqwerty
    Feb 9, 2011 at 15:28

Most probable cause is a duplex mismatch. 100 Mbit ethernet has a fault where occasionally one end will be running at full duplex while the other at at half. Check the lights on the switch and router for the duplex setting - both should be full. The characteristic symptom is the network seems to work well with only one computer attached for one way heavy data flow, but when using two-way heavy data flow or with multiple computers attached it falls apart. (Gigabit ethernet is much less prone to this fault due to improved duplex signaling). Solve it by forcing 100 Mbit & full duplex on both the router and switch ports connected to each other.

Cheap routers can have their CPU overloaded by performing NAT across too many connections, generally caused by bittorrent. Unfortunately it's hard to detect, and depends on the specific model of router.

  • @glenn-butcher Hi! I've updated the original post with the model names of both Router and Switch. There doesn't seem to be any settings available on Router for choosing the port speed. Have not yet figured out if it is possible to 'manage' the Switch. Neither have status lights that indicate half/full duplex. One thought I have a spare router at my disposal which has gigabit ethernet - perhaps I should give that a go?
    – tokes
    Feb 9, 2011 at 3:17
  • Scratch that thought - the other router I have is not an ADSL modem.
    – tokes
    Feb 9, 2011 at 3:37

For Wireless2's speed issues, is Wireless2 set up to act solely as an access point? Double NAT can be a problem in this case: if everything under your gateway (Router) has a NAT address, and if your second wireless is creating its own private network (even worse: if it's attempting to do so in the same subnet as your primary network) you can get some nasty behavior.

Could also be that your switch is overworked. How many clients have you got on it at any given time, and what make/model switch is it?

  • Off subject, but nice avatar. To be the man you have to beat the man. Whooo!
    – joeqwerty
    Feb 8, 2011 at 22:10
  • The switch is an Edimax ES3132RL 32Port 10/100 Rackmount Switch. I just had a look at the activity panel - only 13 ports are active at the moment. Would that classify it as overworked? Wireless2 is as far as I can tell set up to just be an AP. NAT is not enabled. DHCP is not enabled.
    – tokes
    Feb 8, 2011 at 23:38

I guess that either the link between the switch and the router is being flooded OR your MTU is being set incorrectly.

You can check for MTU setting by following the instructions here: http://groups.google.com/group/blogger-help/web/how-to-check-the-mtu-setting-on-your-computer?pli=1

Flooding of the connection would probably be obvious (a solid network traffic light rather than blinking). This wouldn't slow down copies from one machine to another on the switch, because its a switch and not a hub.

Also the firewall in the router can get overloaded if it is asked to do a lot of NAT port mappings. Eg. when a torrent client tries to establish lots of connections to other clients.

  • But would the MTU really be a factor if: multiple computers are experiencing the problem, and I can get a good internet connection if I am directly connected to Router? I think exploring the flooding scenario is probably worth it. Any ideas on how I would check for the presence of a client on the network running a bittorrent program? The activity lights on Router and Switch seem to be pretty constantly flashing.
    – tokes
    Feb 8, 2011 at 23:55

First, if the cabling is confusing you, label every cable with a tag made from masking tape. That will let you be confident about unplugging things.

You said:


  1. A file transfer between two clients who were both directly connected to Switch was the fastest;

  2. A file transfer between one client directly connected to Switch, and one client directly connected to Router (which is directly connected to Switch) performed much slower

  3. A file transfer between two clients directly connected to Router also performed slowly.


(1) indicates the switch is ok

(2) indicates that there is a duplex or speed mismatch on the link between "router" and "switch"

(3) is interesting on two notes,

a) the "router" may have a problem with duplex/speed mismatch, you can troubleshoot this, if you cannot set duplex/speed at the router, by plugging in a laptop with hard set duplex/speed combinations until you see what the port is giving you.

b) if the "router" acts congested, it could be compromised with a trojan. yes, there have been incidents of routers being hacked, especailly consumer grade routers.

There were some suggestions but no resolution here:


You still might find something there that triggers a eureka moment.

The general debug procedure should be to work backwards from the router. Get a good connection going at the router. Get a good connection going at the switch. And so forth.


I have solved the problems by replacing Router with another ADSL modem I had access to: Netcomm nb6plus4w. As soon as that was done, the entire network was able to access the Internet at a good speed.

Unfortunately, I haven't yet had time to check the link speed on the old router by connecting it to a fixed speed client, as my priority was to fix the problem. I might do that later and report back with my findings, but for now I would consider this done.

I'm not sure who to award the correct answer to -- I don't know for certain if it was a trojan/virus on the old router, incompatible link speeds, an overloaded firewall, or something else. Regardless, I can't vote any answers up yet as I don't have enough rep.

  • Good work getting the problem resolved. If you get around to it, I'd be interested in hearing what you find out about the old modem/router.
    – joeqwerty
    Feb 11, 2011 at 2:49

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