As I understand it, the real problem with centralized forwarding architectures (and electrical forwarding architectures in general) is memory access speed. The total throughput of the router cannot exceed the memory access speed. Even if there was a way to process packets (lookup/decrement/move across bus, etc) at OC-192 rates in a single instruction at infinite speed (1EHz, say), the packet must be written into and out of memory. Given 1ns (possible today?) read/write times, and assuming 64 byte cell-based packet read/writes for efficiency, you can write 64*8/1ns = 512,000,000,000 bits/sec. Divide by 2 (read/write) = 256Gbps Which is equivalent to 25 OC-192s at line rate. Or, 8 OC-768s at line rate. But since my initial assumptions are currently impossible, then the results are also not possible using centralized forwarding. For example, by doubling the memory speed, we lose 12 OC-192s! I would expect even those vendors that use centralized forwarding engines to go to distributed ones in order to achieve OC-768. After that, who knows. Memory needs to me accessed in the picosecond speed range. I think that optical switches will be in place before core OC-3072 and higher links come online. my $.02, chris

You misunderstand. Getting multiple forwarding tables synchronized on one box IS simple, if the architecture considered it from the start. Trying to bolt it on later can cause problems, however. These problems are an implementation issue on a particular platform.

As a counterpoint to what you say, consider that all commonly deployed routers that can handle OC-192 rates do NOT have a single centralized forwarding engine.

Or do you know something about KISS that was not apparent to those who designed these working products?

Prabhu

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-----Original Message----- From: alex@yuriev.com [mailto:alex@yuriev.com] Sent: Wednesday, April 11, 2001 4:41 PM To: nanog@merit.edu Subject: RE: gigabit router (was Re: Getting a "portable" /19 or /20)

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Vendors have known how to solve this problem for many years. Failure to do so is a poor implementation and has nothing to do with centralized forwarding being better/worse than distributed forwarding.

Yet another person who does not understand the KISS principle. I am sure in theory it all works great, though I am seeing way too many comments similiar to:

"The connectivity issues have been resolved. This appears to be the same CEF related issues we experienced Monday evening, and we have a case open with Cisco. As we get more information from Cisco, we will be passing it along."

Alex