Beginners guide to network switches

[Tim Streater]

Yes, that last bit is nice. I'm using an old 5-port switch (Netgear) that I retired some years ago, but since getting rid of an old printer I'm not sure I even need that any more.

[Graham.]

Bloody good stress-test though.

[Graham.]

Feel free, but they are pretty foolproof these days (That sounds more polite than idiot proof).

15 years ago you would have uplink ports, and crossover cables to contend with, but now that's all taken care of.

Best one was the Sweex router I bought when I first got broadband, I nearly took it back as faulty, then I discovered that all four switch ports worked fine with a NIC when a crossover cable was used, and a straight cable was needed to connect to a second switch. I've still got it somewhere.

[newshound]

[newshound]

Once in a while (months?) you might find things stop working, just switch off and then on again.

[Andrew Gabriel]

I think all switches detect that and isolate those segments (without the need for Spanning Tree which is used in larger networks to prevent loops).

It was much more of an issue with hubs, which generally didn't have the intelligence to detect it, but you can't buy hubs for a decade or more now, even if you want one.

[Graham.]

So do I. I had a customer with a pharmacy, maybe a dozen workstations, EPOS etc, all in a Windows workgroup, all nice and simple, except, for reasons that escape me, someone had installed this expensive managed switch.

I had been asked to install an additional workstation, and the required network cabling had been installed, but all the spare ports on the switch had been disabled and thee was no one available who could administer the switch, so, I had to find a consumer grade D-Link that we normally use for our customers, to split one of the working ports two ways.

[Andy Burns]

You're lucky the BOFH hadn't enabled port security on the switch to limit it to a single MAC address per port :-)

[John Rumm]

That pretty much concurs with my expectation. However I have had odd situations where a modern switch has been floored by odd cabling circumstances.

One was a managed switch that did support spanning tree - but it was turned off by default, and that did get the hump with a loop back. The other one was quite recently where a switch just refused to reliably pass traffic, while at the same time looking very "busy" judging by the LEDs. In the end I traced that[1] to one wall port connection. Patching that one port was basically enough to send it off into an odd state, where machines could still DHCP through the switch, but not actually pass useful traffic.

[1] In the end I had to unpatch everything on the switch, just leaving the router and my laptop. Left that pinging the router, and then watched carefully as I re-patched each wall port - eventually found one that caused it to apparently die. Testing that one cable run showed a mix of shorted and broken pairs. Which on closer inspection looked like a mouse had been having a nosh in one of the floor ports!

I cut off the nibbled socket, and cut back the cable a few inches. Reterminated it, and its been fine since.

[Andy Burns]

Maybe newer el-cheapo switches do, but older D-links don't and is the reason I cheerfully remove them whenever I find one lurking under a desk ...

[T i m]

I really can't think of sometime any more 'plug and play' than an Ethernet switch. It's about the same as a multiway trailing socket. ;-)

Cheers, T i m

[The Natural Philosopher]

The DGS-1005D 5-Port 10/100/1000BASE-T Gigabit Ethernet Switch was designed for easy installation and high performance in an environment where traffic on the network and the number of users increase continuously. ? Five 10/100/1000BASE-T Gigabit Ethernet ports ? Cable Diagnostics function at Switch boot up ? Supports Auto-Negotiation for 10/100/1000Mbps and duplex mode ? Supports Auto-MDI/MDIX for each port ? Supports Full/Half duplex transfer mode for 10 and 100Mbps ? Supports Full-duplex transfer mode for 1000Mbps ? Full wire speed reception and transmission ? Store-and-Forward switching method ? Supports 4K absolute MAC addresses ? Supports 128KBytes RAM for data buffering ? IEEE 802.3x flow control for full duplex ? IEEE 802.1p priority queues ? Back pressure flow control for half duplex ? Jumbo Frame Support at 1000Mbps (9000Bytes ) ? D-Link?s Green Technology

Where in that list, do you see ANY mention of DHCP??

[News]

In message , T i m writes

Good! I will report back, when it arrives.

Strange that Andrew mentioned hubs, up there ^. I do have a 3com hub, which I tried. It worked, but killed the speed.

[Andrew Gabriel]

Probably only 10Mbits/s. ICBR, but I think hubs had vanished before 100Mbit ethernet became popular (possibly because they can't convert speeds to perform interworking).

[Andy Burns]

3COM did make a few 10/100 "fast hubs" which IIRC were essentially a 10Mb hub and a 100Mb hub in the same case, with a bridge between them, the speed of a given port determined which hub it got connected to.

But yes, 100Mb switches became sensibly priced at around the same time, so 10/100 hubs were short lived, a few years ago they were a bit sought after as 100Mb "tap" interfaces, but now you can use mirror ports on managed switches or buy dedicated 1Gb mirror devices.

[AnthonyL]

Well that's pretty well how I ended up with the Cisco. For some reason the customer I had supplied with hardware and software for years bought the managed switch for his small network. He couldn't get it to work with the Novell server so I got involved. Once I showed him a simple switch would do the job fine he got one from me and gave me the Cisco which for interest I quite easily configured for our near identical Novell network. But I've never found a use for a managed switch also preferring KISS so it's barely ever been used.

[AnthonyL]

The only reason I ever kept a hub was I had some old ethernet 10Base2 cabling to an old server and the hub had a BNC connector. I could then connect the hub to a switch to join the whole network.

Now - what do I do with loads of 10Base2 cable and connectors?

[The Natural Philosopher]

I've never seen a managed switch under a sort of thousand up machine network

Where you want certain segments to talk to each other, others not.

Typical use might be fort say 3 segments - two sets of clients one set of servers - where every client can talk to the servers but the clients are not to talk to each other.

[Tim Streater]

We have to watch the terminology. When I started with Ethernet (1983), you used yellow coax as the backbone, at 10Mbps. Your backbone could be up to 500m long and you could have extra 500m segments connected to it via repeaters. Total end-to-end length between any two points not to exceed 1500m.

A repeater just streamed the bits through it, reshaping the signal - that's all. No storage, and the first bit went out a bit-time or two after receipt by the repeater, so a packet would span two cable segments as it went down the wire. Repeaters only had two ports, IIRC.

If you wanted to go further than as described above, you needed to use a bridge, which did store-and-forward. It read the whole packet in, and then consulted its table of MAC addresses to see which port to send it out on. You could thus connect several networks together in a simple tree. If you made loops that way, then the bridges (which communicated with each other) used spanning tree to detect loops and decide how to disable enough ports to get you back to being a simple tree again.

I think these days what was then called a bridge is now called a switch. Not sure what a repeater is called now. Hub, possibly.

Then there was thinnet, still at 10Mbps but using thinner coax and possibly shorter segments, and simpler connectors.

Much better is twisted pair at 100 and 1000 Mbps, of course, simpler connectors, much more reliable, no more worrying about bend radius of your cabling, or the reliability of the taps on your backbone.

What's helped even more is the reduction in the size of the electronics, and cost. One connection of a PC to ethernet when I was first doing it required a whole card slot inside the PC, and for the connection to the backbone a box about the size of three paperback books stacked together, with a fat cable between the two.

[John Rumm]

I have one on my home network... its quite handy for dropping a not to subtle hint to the sprogs that its too late to be playing minecraft or streaming films etc ;-)

Indeed - quite handy for situations in even small offices where you have two unconnected businesses sharing resources or internet connections, but you want some isolation between them.