Office LAN - gigabit problems

[puremetal]

Hi all, I work in a small office for a small business (5 or 6 employees), and we've been here for a long time. We have a bunch of computers and systems (routers, NAS, etc) that run on 100mbps ethernet, and some newer stuff (including main NAS and main switch) that run at gigabit speeds.

My question is this: Does having a mix of 100/1000 speed devices on the same network mean the whole LAN will only work at the lowest (100mbps) speed? I ask this because that's pretty much what's happening. This laptop here has a gigabit NIC, it goes straight to a gigabit switch, which has a gigabit NAS attached via another gigabit router. In my mind, that should mean I can access the NAS at 1000mbps, but I can't (I can access it but only at 100mbps). Is this because there are other slower devices on the same switch/router?

That's question #1. Assuming that all to be the case, how would I go about getting things to work at faster speeds, bearing in mind we're on a shoestring budget and can't replace a bunch of the slower stuff. If I keep the two speeds fairly seperate - eg have ALL 100mbps devices plugged into one gigabit switch, and all gigabit devices plugged into the other (and Cat6 between the two) - would this work? I have enough Cat6 cable to run rings round the office lol

Any help would be massively appreciated, as I'd love to get on with improving our speeds ASAP! :P

[jdMorgan]

Each subnet will run only at the highest speed supported by all attached devices. So partition your network in such a way that gigabit devices such as your NAS (and laptop) are not on the same subnet as any slower devices.

This may be easy or it might be impossible, depending on your physical wiring and/or switch locations, unless you can afford to add a few switches at strategic locations to 'isolate' slower subnets from your gigabit 'core network.'

Jim

[puremetal]

Thanks very much for the reply Jim :)
I'm afraid I now need to ask the dumb, but obvious, question: how do I seperate my network into different subnets? Physical wiring isn't a problem as its a small office and there are wires everywhere anyway, and I'm sure we can scrape together enough to get a few extra switches if needed.

So, is it:
a) Physically seperate the two speeds of the network by a switch (ie. have all the 100mbps devices running off a single switch, like a 100mbps spur off the gigabit network), as per this handy plan I made <snip>

or b) is it anything to do with setting Subnet Masks on the switch(es)?

Thanks again for your time

[edited by: engine at 8:08 am (utc) on May 30, 2009]
[edit reason] No urls, thanks [/edit]

[jdMorgan]

Yes, option "a" with the "spur." It's not related to subnet IP address masks, just physically partitioning the network into high- and low-speed segments using switches.

In order to avoid major headaches, label both ends of each cable with where the *other end* goes and what the speed is. For example, the cable on your desk that you plug into your laptop should say, "To Gigabit switch #1 under Bob's desk. Attach 1GB/s devices ONLY."

The other end of that cable, at the GigaBit switch should say, "To puremetal's desk, 1GB/s." And the switch needs a label too, designating it as "1GB Switch #1." Basically, label everything to save time and money today and later.

Warn all co-workers about the effects of plugging low-speed devices into the high-speed network segment. Anyone who might need to plug in both low- and high-speed devices needs two cables at their desk -- one for each network segment, and this will likely be easier to provide right now than later.

If someone plugs a low-speed device into the high-speed segment, just go to switch #1 and unplug each cable one at a time until the speed goes back up. Then read the label on the un-plugged cable, and it'll tell you what desk to go to to find the problem.

Aren't transitional networks fun? ;)

Jim

[mcavic]

Each subnet will run only at the highest speed supported by all attached devices.

I don't buy that at all. Why, then, would anyone make a switch that has both 100mbit and gigabit ports? The whole point of a switch is to allow device A and device B to talk at one speed, and device C and device D to talk at another speed.

What if you plugged a 10mbit device (like a modem or router) into a 100mbit switch? It certainly won't slow the network to 10mbit.

[jdMorgan]

It won't as long as there are no 1 GB/s devices sharing that same physical port on the switch. It is the clock speed used per-port (i.e. on each individual switch port) that is important here.

See the diagram puremetal posted above. The main network is 1GB/s, and one of the ports on the 1GB/s switch goes to a second switch, and everything plugged into that second switch is 100 MB/s. The devices below the second switch communicate at 100 MB/s, but the link from that second switch to the first switch will happily run at 1GB/s. So, it is the second switch that partitions the 1GB/s and 100MB/s segments.

Jim

[lammert]

You can connect mixed Gb, 100Mb and 10Mb devices one one single gigabit switch and still get gigabit connection speeds between any two attached devices which support gigabit speeds. I have such a configuration running with a 3Com Gigabit switch without problems. Transfer speeds between my desktop and server are around 30 megabytes per second independent on other devices attached to the switch (100mbit/sec ADSL router, 10mbit/sec printer server)

Partitioning an office network over multiple gigabit switches should not be necessary under normal circumstances.

There may however be other problems in your setup which practically limit the speed to 100 mbit/sec.

• It may be a cheap switch which supports multiple speeds, but not multiple speeds simultaneously.
  
• There are "switching" hubs and "switched" hubs. Switching hubs are supposed to to the forwarding of network packets to destination ports automatically by remembering the MAC addresses of the attached devices. Switched hubs allow manual partitioning of subnets via a serial or IP console connection. Both types of hubs are often called "switch".
  
• One or more of your network cables / patch cables may be of poor quality (less than Cat 5e/6 or a badly crimped connector to a cable end)
  
• Settings in the windows registry of your computers (especially the ForwardBufferMemory and NumForwardPackets settings) which practically limit the speed to about 100 mbit/sec.

The best way to test it is to disconnect all devices from your switch, except the NAS and your Gigabit laptop. If you get speeds around 30 megabyte/sec sustained (the maximum for most 32bit PCI network cards), your Windows settings and cables are fine.

Now attach more devices and see when the speeds falls. With every new device, an intelligent switch will only shortly slow down the speed when a slow device is attached. Once it has recorded the MAC address of the newly attached device, it will only slow down traffic to or from that MAC address, but allow high speed access between all other MAC addresses. If this is not the case with your switch, it is either a "switched" hub which requires manual configuration, or effectively a hub without routing intelligence.

[tangor]

Partitioning an office network over multiple gigabit switches should not be necessary under normal circumstances.

Shouldn't be, but always works. Interaction between devices of different speeds is problematic. Isolating those problems generally gives good results. (Running a subset of devices myself).