This might help. What does the V stand for in VLAN? Virtual. VLANs are a method for emulating Real LANs in a switched network. The original poster seems disillusioned with VLANs. Well, I am too. :-) You can't do much with them that you can't do with a bunch of Real LANs connected by routers.
First we had hubs and bridges and routers. Then switches came out. They were cheaper and faster than routers, so everyone jumped on the bandwagon and started designing huge flat networks with mostly switches and maybe one router to get out to the rest of the world. Ah, but there was a problem! A L2 switch forwards broadcasts out all ports. And this was in the mid-1990s when PC CPUs were slow as molasses and got bogged down by broadcasts and multicasts. Dreadful protocols like SAP and RTMP and NetBIOS were rampant! Something had to be done. So, hummmm, should we go back to designing our networks with routers, which don't forward broadcasts? Nah, still too expensive. Better come up with a way to emulate LAN and IP subnet benefits on a switched networks. OK, let's invent VLANs! But how do the VLANs talk to each other? Oh dear, we better go back to routers. Nah, still too slow, though it will work in a pinch. I know! We could speed them up and call them L3 switches. One last rather serious comment. This is not a comment on the newbiness of the original poster, but I must say that I think it is common for newbies to get confused by VLANs. Cisco teaches VLANs without ever teaching basic networking 101. People can't understand VLANs unless they first understand a lot more about protocol behavior and traffic flow. VLANs are really an advanced topic and shouldn't be covered so early on in the Cisco test progression. Either that or CCNA should be beefed up to teach something useful, if you ask me, which they didn't. Priscilla The Long and Winding Road wrote: > > I've been following this thread, and have offered a comment or > two along the > way. Perhaps I should offer some thoughts here at the source. > > note that I have not read any of the exam study materials in > question, so I > don't know what is or is not being stated in the courseware. I > can offer > that just because it says so in the study materials doesn't > mean that's the > way it is. > > comments below.... > > > ""Stephen Hoover"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > I am studying for the CCNP Switching exam and it covers VLANs > and layer 3 > > switching moderately. It states that Cisco recommends a 1 to > 1 mapping of > > VLANs to subnets. It also states that VLANs can be used to > break up > > broadcast domains. > > this is a reasonable, simple approach, and thus one that > appeals to my > reasonably simple mind. > > > > > > When you create different subnets, you are already breaking > up broadcast > > domains, so does layer 3 switching require the use of VLANs > to actually do > > the switching? > > > this is where the confusion, no doubt introduced by the > marketing people, > set in. > > suppose you have a router with three ethernet interfaces, and > each of these > interfaces is plugged into a different hub ( no switch ) > > hosts on each of these hubs are in the same broadcast domain ( > same > collision domain too, but I digress ) hosts in each of these > domains cannot > reach hosts ( or servers ) in other domians, on different hubs, > without > routing. > > this would be true, even if you had all hosts on the same great > big hub with > 500 ports. You could have hosts on the same hub, but having > different L3 ( > IP ) addresses. communication between hosts on different > subnets, even if > they are on the same hub, require the intrercession of a router. > > vlans, made possible by various 802.1 specifications, are > really just a way > of expressing logical broadcast domains. > > layer 3 switching is really routing. an L3 switch has the > routing function > built into it, rather than using a separate piece of equipment. > > > > > > Say for instance I have 2 hosts on the same layer 3 switch, > but the two > > hosts are on 2 different IP subnets (No VLANs are defined). > Host A wants > to > > talk to host B. Can the switch not look up the routing info > and then know > to > > switch to that port? I am not seeing where the requirement > for the VLAN > > comes into play. > > despite what others have said, you can do this. it is wasteful, > in that a > host plugged into an L3 port would require 4 ip addresses > because you have a > subnet with two hosts ( the PC and the port, and the net number > and the > broadcast address ). whereas if you have a vlan, that vlan is a > virutal port > that represents the physical ports as a single subnet to the L3 > ( routing ) > function. > > > > > > If VLANs are required for layer 3 switching, is that pretty > much standard > > across the industry, or that a Cisco only thing? > > > forget this L3 switch versus router distinction. it is > confusing, and > misrepresentational. > > think instead in terms of how traffic moves through a network. > > think instead of a vlan as a virtual logical construct that > represents one > or more ports as a single broadcast domain to a router. it > doesn't matter > that the router is integrated into the switch hardware with an > ASIC and > code, or is an external device. > > HTH > > > > > > Thanks! > > Stephen Hoover > > Dallas, Texas > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=63195&t=63147 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
