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Michael
Hi Miss Anderson, thanks a bunch for giving us this interview. Gillian Hi ya cutie, you can call me Gillian. And I'm happy to be here, I hope I can answer your switching questions. Michael Let's get started then. Here at RouterGod Online Magazine we're only interested in Cisco switches and routers, is that a problem? Gillian Certainly not! You know, when I was a struggling actress, I had to put up with a lot of cheap crap. I lived in a crummy apartment and had this horrible HP ProCurve switch. The really nice thing about becoming successful is you are able to afford nice things like houses, cars and Cisco switches. I'll never forget the day that I was hired to play Agent Scully on the X-Files, the first thing I did was go out and buy a brand new Catalyst 2924 XL. What a wonderful, wonderful switch... Michael Very good, switches have become very popular, can you tell us why? Gillian Sure cutie, for one thing the cost of switches has come way down. They can directly replace hubs too. You can literally replace a hub with a switch in seconds, no configuration required. Michael Whoa, whoa, wait a minute. Switching is a huge subject, there are issues about spanning tree and trunking protocols and VTP domains and etherchannel...what about that? What about that? Gillian Oh boy! You just got worked up there, didn't you? Relax cutie, you are talking advanced CCNP type concepts there, I'm just a CCNA but let me explain what I mean. Cisco's philosophy about switches is "do no harm". All ports are by default in the same vlan, vlan1 to be exact. So you can plug in the switch and plug in the cables and now every port is in a separate collision domain. Michael Many of our viewers are still fixated on your picture at the top of the page, can you explain about collisions domains, for their benefit. Gillian Not a problem Michael, I get that all the time. Switches work at layer 2 of the OSI model. They listen on each port and write down the source MAC address of every Ethernet frame they hear. This is put in what Cisco calls a CAM table, it's a listing of every port and what machines can be found on each port. These ports are not connected to one another. The switch can connect them when it wants to but otherwise they are not connected. Michael Nobody ever put it like that, so the switch is just a series of ports that are not normally connected, under what circumstances does the switch connect all the ports? Gillian Connect all the ports? You would have to be more specific, is there any multicasting going on? Are there different vlans? Michael No, you know what I mean, why would the switch decide to connect 2 ports together? Gillian Now that's a different question, you said "all ports" at first. Let me start from the beginning. We're only talking one vlan right now. Remember the switch pretty much knows what machines are on what ports, it's been listening and creating these entries in the CAM table. So it receives a frame coming in on port number 1. It looks at the frame and reads the MAC address of the destination and checks it's CAM table to see if it has already heard that machine on another port, if it has, it will forward that frame out the port that the destination machine resides on. No other ports will hear this transmission. On a single switch in a single vlan you could have a machine on port 3 sending a file to a machine on port 21, a machine on port 17 sending email to a machine on port 9, a machine on port 11 logging in to a server located on port 16, all at the same time. If you tried to do that with a hub, you would have collisions as only one data stream could travel the Ethernet at one time. Michael Excellent. In your example the switch has tripled the available bandwidth. What if the switch looks in the CAM table but there is no entry listing the MAC address of the destination machine? Does the switch just drop the packet? Gillian Listen up, cutie! You just said "packet"! A packet is a layer 3 data unit. We use frames thank you very much. And no, the switch will not drop the frame. If the switch does not have an entry in it's CAM table, it will copy the frame and send it out all ports at once. When it gets a response it will add that machine to the CAM table and all future frames will be switched to that port. Michael OK, now I know. What about broadcasts, what does the switch do with them? You know, ARP broadcast and stuff like that? Gillian Broadcasts are always flooded out all ports that are members of the same vlan. Michael There! you said it again, vlan! What's a vlan? Gillian My goodness! I love your enthusiasm! A vlan or "virtual lan" is simply a group of ports that you have put in the same broadcast domain. Take a 24 port switch for example, by default all ports can hear broadcasts from all other ports. The switch is maintaining a CAM table of all 24 ports so that it can switch frames between ports. This is a single lan, or vlan if you like, now take a hacksaw and saw the switch in half. Now you have 2 vlans. Computers in one vlan cannot talk or hear broadcasts from the other vlan. Michael So instead of actually sawing a switch into pieces, you can administratively create different vlans and assign ports to these vlans? Am I getting that right? Gillian Exactly, the only switches that come with hacksaws are SMCs and the lower priced Addtrons. On a Catalyst you create these separate vlans from the command line. Each port can only be a member of one vlan though, there are exceptions like SPANning a port and trunking but for now, the rule is one vlan per port. Michael Ok, here's a stupid question, how does this really work in the real world? How do subnets figure in? This is kind of confusing. Gillian You are such a cutie! You've got it figured out and you don't even know it! Every vlan is a separate subnet! You create a vlan called Accounting, all the computers in that vlan have IP addresses that place them in the same subnet. All machine on the same subnet (vlan) hear broadcasts from the other machines on the same vlan. Michael How does the switch route traffic from one vlan to another? Gillian They don't. Michael If the switches don't route the packets from one subnet to another, how do the packets get routed? Gillian (laughing hysterically) Hummmm...Let's see...how do packets get routed? Humm, that's a tough one! They should invent something that routes packets.....they could call it....wait.....a ROUTER! (more girlish giggling) Michael AAHHHhhh! So the switch always needs a router connected to it if it's going to route packets between vlans! Gillian You got it, Buckaroo!!!! Check back often as Gillian
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