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=63190&t=63147 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
