North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: gigabit router (was Re: Getting a "portable" /19 or /20)
On Wed, Apr 11, 2001 at 12:26:54AM -0700, David Schwartz wrote: > > > Why do you think central fowarding is superior to distributed > > > forwarding? > > > > Because you will have consistency problem. You are nearly 100% guaranteed > > to have them. > > > > Alex > > Ahh, so that's what you're thinking. > > If you have forwarding table F(X) at time X and forwarding table F(X+1) > at time X+1, a packet that arrives between times X and X+2 can > reasonably be forwarded by any of the tables. There is no special > sequencing present or required between the packets that involve routing > protocols and the data packets. I think Alex was referring to internal consistency within the router (between linecards), not external consistency. For example, if linecard X believes that a packet should be forwarded to linecard Y, but linecard Y's forwarding table is older than X's, Y could misforward the packet, causing a forwarding loop or a dropped packet. Thus, it can be the case that neither the old path nor the new path is taken. Yes, there are ways to approach this problem, but it is a problem that central-forwarding systems will not have. > We misroute packets between routers because routing table updates don't > happen fast enough. It's not a problem -- IP is designed to tolerate > packet losses and has never guaranteed sequencing. It is true that IP does not make guarantees about delivery, but packet loss has a detrimental effect on performance nonetheless. > The added occasional misroutes due to inconsistency will be > proportional to the ratio of the average network transport time for a > routing protocol packet to the average delay in propogating forwarding > table changes to a linecard. You do the math. I think a more useful model is this: S(X) = (% of time that a router X spends in a consistent state) * (packets/sec through router X) For the percentage of packets which will be successfully routed. The total end-to-end loss is 1 - S(X)^N for N identical routers. N >= 20 is not uncommon these days, and packets/sec gets higher all the time. -- - mdz
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