Moving on to the core network the core network is the center of the campus and it's your top level hub and spoke let's talk for a minute about routing versus switching which is layer 2 versus layer 3. routers provide more isolation between devices because they stop broadcasts routing is more complicated but also more sophisticated and make much more efficient use of the network particularly if there are redundancy elements such as loops as we think about layer two versus layer three and we will look at this in a separate section of where we do a networking refresher but if you're on a layer 2 network and i don't know if you've ever thought about this but when you're on a windows machine and you click on add a printer and you say oh well it's a network printer and pretty soon a list of printers will show up how does that happen how does that work well that works by your computer sends out a broadcast that says hey are there any printers out there and the printers will all respond now in small networks that works fine but if your network has twenty thousand devices and there's a thousand printers believe me the level of broadcasts in that network is going to be crazy because not only do you have broadcasts when you're looking for a printer or looking for a server but both macs and pcs and linux boxes broadcast on a regular basis for example a pc will broadcast on a regular basis it says hey i'm a pc and my netbios name is whatever the netbios name is and as you get more and more and more computers the level of broadcast in large networks is a huge problem so moving to a layer 3 segmented network to where you only have oh 250 or so computers in a broadcast domain on an individual subnet that will make things work much much better additionally segmenting your network has some security implications for example if you have servers that are on the same subnet as your users the users can take over that server and the way that happens if you remember how do we translate an ip address into an ethernet address so we can send a packet well that's called arp and so if for example somebody on a large broadcast sub domain arps for a server well the server will spawn and say hey that's me and the the client machine will put that arp entry into its arp cache and happily then send the traffic to the server there's nothing to prevent me as an interloper to come into your network plug in and if this is if we're all on the same ip subnet i can send an arp reply to the client machine the pc that's talking to the server and say hey the server's ip address or ethernet address is my address not the servers and the client machine will happily just overwrite the server's ethernet address with mine so that all traffic that's sent to the server is actually sent to me i can on the same hand i can send to the server an unsolicited arp reply that says hey the this client machine its ethernet address is me not the client machines i the ethernet address and now all traffic between a client and the server and from the server to the client comes to me and i can simply forward that i can record that traffic and forward it on and now i have a man in the middle attack and nobody knows that i'm in the middle moving back to this core network concept at your core network this is going to be the center of your network to where fiber optic cable runs from all buildings and if the core network is unreliable then your entire network is unreliable so reliability is the key in your core network so your core network ought to be the place where you invest in battery backup possibly a generator and you must have good ear handling as well one of the things i've noticed in many emerging regions is that grounding and bonding is sometimes an issue and you would notice this if you ever touch a rack or a piece of network gear and you get a shock that means grounding and bonding has not been done properly and you should call an electrician to have that done if you don't have good grounding and bonding this can cause all kinds of problems not only can hurt you with the electricity but it can cause all kinds of reliability problems here's a diagram of what we typically will want at your core network you're going to have a core router and again that might be a layer 3 switch configured as a router but you're going to route on this again the routers give isolation between subnets from that core route location you are going to run fiber optic cable to every remote building so this is a very typical design and we will use this typical design throughout the rest of this course.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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