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Will 100mbps modem/router limit Gigabit switch connections?

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August 6, 2007 8:24:49 PM

Hi all,

Just wondering if anyone could help with a small concern I'm having regarding an intended network setup?

Basically I've just had ADSL activated and due to a somewhat noisy line (even at the BT master socket using an ADSL Nation XTE-2005 faceplate) I would prefer to have the ADSL modem/router (a Sky supplied Netgear DG834GT) plugged in to this rather than running it off an extension.

My 2 primary desktop PCs (labelled PC #2 & PC #3 - both with dual onboard gigabit LAN ports) are on the 1st & 2nd floor but due to the slightly complex layout of rooms in the house I'd prefer to run only a single cable upstairs.

I'm also intent on avoiding using wireless (been using this for laptops for many years and have never been 100% satisfied) and powerline networking for these machines (old house, wiring not ideal) and so have decided upon the below layout as being easy enough to implement.

My question then concerns whether the computer-computer connection between PC #2 & PC #3 will be at the gigabit speed, even though the router is limited to 100Mbps?


August 7, 2007 7:48:29 AM

I have a similar configuration here at my place. If you are transferring data from 1 PC to another, you only need to be concerned with what is in between the 2 points. So transferring from #2 to #3, you'll get full gigabit speed. However, going from PC 1 to PC 2 or 3, you will be bottlenecked at 100mbit because your router is 100mbit.

I wanted all gigabit myself, and the way I got around it was I bought an 8 port gigabit switch for about $20 a few months ago when they were significantly discounted. Then I plug the new switch into the router, and then everything else plugs into the new switch. Problem solved. :) 

-Cyberjock
August 7, 2007 3:34:59 PM

Thanks for the response - I've heard the same on other forums so I'll consider the matter resolved.
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November 6, 2009 3:13:39 AM

I have a question somewhere along the same line. I have a fantom drives gforcenas box with 2 WD SATA 500GB RE2s in it. It is attached to a trendnet 5-port gigabit network switch along with the rest of my networked devices, computers, etc. all connected at 1000 with the exception of my 100Mbit Gigaset/Siemens ADSL Modem/Router (frontiernet internet). As far as I can tell, the nas box pulls it's ip address off the slow modem router so that it is in the same subnet (192.168.254.XXX) as the rest of the devices on the switch or modem (I have nothing attached to the modem except the switch, phone, and power cord). But I think even though the nas box shouldn't need to go through the modem, something is limiting it to ~ 9MBytes/sec (less than 100Mbit/sec). Any ideas why I'm only getting 100Mbit speed over what should be 1000Mbit network, except for the modem?
November 6, 2009 3:53:52 AM

What's happening is since you're NAS is getting it's IP from the routers DHCP client, it's also supplying the NAS with a default gateway, being the internal IP of the router.

So, when the NAS wants to communicate with someone on the same LAN, it sends it to the router, and the router forwards the data to its next destination...thus limiting it to 100Mb/s.

Of course, this depends on whether or not im picturing your topology right. Maybe a picture/diagram would help.
March 12, 2014 5:09:02 PM

I have a similar scenario as OEXT. All computers have GB NICs, talk among themselves at GB speeds UNTIL I connect the Internet access point (it is 10/100 not changeable right now). Now all traffic between the computers crawls at 10/100 speeds. Disconnect the Internet access point and speeds more than quadruples. I even tried adding a physically separate network (see drdumont.webs.com/example), but traffic never moves over the faster network.
Any suggestions will be much appreciated.
March 12, 2014 5:42:08 PM

This is a very old thread to dig up. The answer by cyberjock is correct and the one by ryanmission is completely wrong.

When you use a switch and the IP are in the same subnet the IP are not used only mac addresses are used select the path. The traffic always goes directly between the end machine and not via the default gateway since that is only used when you need to leave the subnet.

First I would use a tool that is not impacted by complex stuff like DNS and file systems to test your speed. Something like the old tool iperf works well. First test without the router attached to your switch. Then run the test again and while the test is running add the router. If it slows down the router is somehow contaminating the switch mac tables. I strongly suspect you will not see any drop in speed. Also confirm with the ARP -a command that the IP address of the machines are tied to the proper mac addresses. A misconfigured router could be in effect poisoning you mac address.

Pretty much what you describe can't happen it is against fundamental principles on how switches work so if it is there must be something major that is configured wrong.
March 13, 2014 9:02:54 AM

bill001g:
My system? Something configured wrong? Of course not! I could never make a mistake!
Why, I ... <harrumph>
Welll, I er, uh, that is, since I knew I thought I had the problem whipped once, I decided to listen to bill001g, and use my 50 years of learned the hard way diagnostic techniques, so I started with the power cord and moved
forward. Yours and everyone elsewhere's comments were just not working, and that, Captain, was illogical. SOMEONE had to be right!
Sure enough, on one of the machines, someone (must have been the bad fairy) had misconfigured the add
on NIC in one of the machines. I really don't know how in Hades I did it, unless I got confused as to which
machine I was connected (two machines and a KVM for quick testing).
So I set things back up with the separate 10/100/1000 network and switch, and sure enough, files scream at
top speeds between machines using the 10/100/1000 network, at about the same rate as my inittal test
with two machines only connected to each other over that new system. (To see how fast the machines' innards would permit disk transfers).
OK. both networks set properly, priorities set right, connections tight, fresh cup of coffee.
Final test - started up file transfers over the Internet to remote machines accessible only through the Internet,
and then initiated a transfer between machines.
Sure enough, according to the indicator lights and a couple of speed widgets, inter-machine traffic is screaming at
GigaByte speeds (seemingly limited only by the computer's internals), and Internet traffic zips along as fast as before. I've got a 35/35 FiOS pipe but their old access point. I have my doubts about its throughput, but they want $100 to update it - that's another tale.
Problem appears to be solved. My file transfers no longer bog down the house's network, kids' game playing, VOIP traffic, movie streaming.
Thanks again, TomsHardware, but most of all >>> bill001g <<< for the facefull of cold water. I probably wouldn't have done the Start From Square One reality check for awhile.
-- Doc
!