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Switch - how does it actually do it?

S

shadow

Golden Member
Ok, I've learned that a switch only connects 2ports at any one time, it may seem like it connects more than one port at a time but it simply seems that way because switches switch so damn fast.

Can anyone shed some light onto this - whether a switch only connects 2 ports at a time, or whether is connects 2/4/6/8/..... ports at a time.

Thx 🙂
 
Hey guy,

I have a Linksys 5 port switch (instead of a hub) and a WIN 98 peer-to-peer network with a server and client. I use it to share a DSL connection and 3 printers as well as files. When I was at a national computer store (Microcenter), I bought a hub and then talked to a customer in the check out line who suggested I buy a switch. The beauty of the switch is it has more electronics in it and I can use it for 10/100 Meg NICs. It costs more but it provides better service. For some reason, my DSL is running faster with both PCs running on it than with only on PC. Go figure.

Ed
guardfish2000@yahoo.com
 
First, thx for the article bober 🙂


From the Article Bober:



<< During peak data transfer periods, cut-through switches must buffer the data. This brings the latency of the cut-through switch up to that of a store-and-forward switch, but without the error checking capability. >>



That is all I can surmise about this question &quot;cut-through switches must buffer the data.&quot; Which can be implied to support either the 2ports only or multiple ports. So I still don't know for sure. I've done alot of searching myself, but haven't come across any thing definitive.

thx.

 
A hub &quot;broadcasts&quot; all traffic over the entire network, and the computer that a packet's addressed to picks it up and all the others reject it. A switch directs packets only to the computer they're addressed to, leaving the other lines open for other traffic.
 
that's why large networks on only hubs, even if they're 100mb, creep to a standstill while a fully switched, or switch/hub network still maintains speed. quakecon was 900 computers fully switched...shweeeet
 
A switch is nothing more than a really fast mutli-port bridge.

Let's say you have a simple 12 port switch. Each of these 12 ports has its own collision domain meaning that a collision on port 1 will not be propogated to any of the other ports. A switch also normally supports full-duplex with totally eliminates collisions all together by allowing a port to transmit and receive simultanously.

As soon as a frame hits the ingress (in) port of a switch it looks up the destination port of the destination address, ships the frame across the switch fabric and onto the egress (out) port ready for delivery. It does so by recording the MAC addresses associated with each port (bridge table/CAM table in cisco world).

So the beauty of a switch is that every port can use the full bandwidth.

Hope this helps. In this day and age you should never use a hub unless it is for a few computers only.

spidey
 
NO, can't do. Need white board. 🙂

Basically it is a crossed fabric of circuit paths between ports. Intelligence in the form of a processor controls these paths and places frames onto them.

Search www.cisco.com for more info.

 
Ok, let me take a crack at this...

There are a few types of switches, let's cover &quot;store and forward&quot; what this means is the switch can listen on all ports at the same time, and send on all ports at the same time. Ok, here is the catch, in order to go from listening to sending the switch has to receive the &quot;packet&quot; process the &quot;packet&quot; find the correct port, then transmit the packet. The processing is the &quot;storing&quot; of &quot;store and forward&quot;.
So basically it does have to take turns on each port, but given that(i'll use a netgear switch for this example) a netgear switch can:

Forward rate (10 Mbps port) 14,800 packet per sec
Forward rate (100 Mbps port) 148,000 packet per sec
Latency (10 to 100 Mbps) 80 usec max

All this happens so fast it will seem like it all occurs at the same exact time. Along with the ability to handle full-duplex operation.

Your real question makes me wonder do you know the difference between a switch and a hub?

To put it simple a hub can only handle, half-duplex and it will echo all errors to all clients where a switch would filter these errors. These errors being echoed is what really kills a network. More clients = more errors, it happens. These errors can be anything from collisions to broadcast storms.

Hope this helps...
 
TERRAPART, I'd have to disagree.



<< So basically it does have to take turns on each port >>



You have accurately descibed very cheap switches where the frame is input, processed, then output.

Most &quot;real&quot; switches generate this ingress-to-egress path without any processor intervention at all after the address is learned. Once a destination address is recognized then superfast ASIC hardware places the frame bit by bit on a bus to the egress buffer. The bus I'm referring to is the backplane.

It is for this reason that you can have a 384 port switch support 192 simultaneous conversations at FULL WIRESPEED.

There is some latency of course but this depends on frame-size and is variable.

ps - I'm referring to store-and-forward switches (most of the world)

oh well, just trying to be a geek. I'll get off my soapbox now. And desparately look for a PS2

🙂
 
Please read my post before replying its only fair...

quote -> &quot;There are a few types of switches&quot;
quote -> &quot;let's cover &quot;store and forward&quot; <--basic style 10/100 switch i described, not a gigabit or higher style. Yes, you are correct the higher grade switches can provide direct piping. However, i doubt this level of hardware is really what the poster was interested in.
But then again i may be wrong. 🙂

BTW: The switch family spidey07 referred to is the 3com 4007 family(gigabit++).
&quot;These modules leverage 3Com's SAGE (Switching ASIC for Gigabit Ethernet) and award-winning FIRE (Flexible Intelligent Routing Engine) ASIC chipsets, which provide the hardware-based switching speed of Switch 4007.&quot; <-quote from 3com.
These 2 items are NOT part of the average switch. 🙂 This switch has a &quot;Backplane capacity: 120 Gbps (passive, full duplex)&quot;
And as i stated this type of switch is a gigabit style which handles:
&quot;Gigabit Ethernet ports: 54 &quot;
&quot;10/100 Mbps Ethernet ports: 216 &quot;
Definatly NOT the basic style i explained in my earlier post. 🙂 Gigabit anyone? 🙂 🙂 🙂
 
Agreed, terrapart.

I was delving into switching operation and architecture. I'm trying for my CCIE and it helps me to openly repeat what I've learned or even get into a little debate.

The switches I'm referring to are Cisco Catalyst 6500s. We've got about 25 here on our campus that I put in about a year ago.

9 slots (1 for supervisor/route processor)
48 port blades
16 port gig blades
or ATM
32/256 gigabit backplane

Cheers!
 


<< However, i doubt this level of hardware is really what the poster was interested in.
But then again i may be wrong. >>



Dead wrong 🙂

Hit me with the big guns - or start a new thread if you wish to do so.

I'll try and learn what switch fabric is for it too 🙂
oh yeah, and &quot;direct piping&quot; (how it works on a circuit board level)
 
Ahhhh So you want to learn the big stuff...

My mistake, your original question leaned toward entry level equipment. I do recommend you learn the entry level equipment long before you jump into the big stuff. The terms and technology used can be crazy, unless you have a good foundation.

I'll look for you spidey07 if i ever need switching advise/recommendations. 🙂



 
hmmm, 4 network classes (2 in progress) going through CCNA right now...

hmm I think I have a big portion of the entry level covered.
 
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