gigabit router (was Re: Getting a "portable" /19 or /20)

Thu Apr 12 00:03:14 UTC 2001

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