At 12:16 PM 3/13/2003 -0500, Howard C. Berkowitz wrote: >At 2:43 PM +0000 3/13/03, Peter van Oene wrote: >>At 10:44 PM 3/12/2003 +0000, Orlando, Jr. Palomar wrote: >>>Without consulting any documentation, a couple of reasons I could think of >>>is forwarding rate and the switch-fabric (or the size of the backplane, >>>usually in Gbps). A full-fledged Layer-3 switch running at "wire-speed" >>>would be much more efficient in routing (and switching) between VLANs >>>compared to a router. >> >>Many routers route at wire speed and can do this on/between tagged >>VLANs. This is just routing. >> >>>Another point of comparison is port density. You can only have such and such >>>number of ethernet, fastethernet, or maybe even gigabit ethernet ports on a >>>router before the cost becomes quite prohibitive. >>> >>>Oh sure, you can use the "router-on-a-stick" method. And though it is a good >>>Cisco IOS feature, it was meant to be an interim solution when transitioning >>>from a flat to a segmented network. >>> >>>Anyway, if you only have a relatively small network, say 2 VLANs, you can >>>opt for the "router-on-a-stick" method. Or better yet, use a router with >>>dual ethernets or fastethernets. However, if you're supporting 4,5, or more >> >networks, that's what L-3 and multi-layer switches are for. > >Peter, would you agree that when someone says "that's what layer3 and >multilayer switches are for," they are really talking about router >packaging (as oppposed to fundamentally different technology) that creates >platforms with certain port densities, functionality tradeoffs, and price >points?
I would certainly agree. There is definitely a family of enterprise devices that package relatively high density layer two aggregation (ie lots of GE/FE ports) with a routing functionality such that you end up with an integrated device that can route or bridge depending upon configuration. However, such a device is in theory no differently that a router connected directly to a bridge via an external vlan trunked interface. The fact that the box happens to integrate the connection between router and bridge is merely a matter of convenience. In the end, the device either routes or bridges the frames it receives, but takes no action that can be distinctly described as layer three switching. Pete >Again, I call attention to the comment of "routing in hardware" as >misleading. I can't think of a "routing ASIC," where I actually looked at >the chip or chipset design, that wasn't some flavor of Von Neumann >stored-program computer. Certain of the specific designs might be >microcode rather than RISC or CISC, but they are still basically von >Neumann. FPGAs might be a special case, but they can't do the more >complex functions. > >In other words, an ASIC is a computer, just a specialized, optimized >computer burned into silicon (or whatever).. Some newer ASICs even are >partially reprogrammable, typically with electrically alterable gate >arrays and the like. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=65341&t=65215 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
