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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)
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. Prabhu ---------------------------------------------------------------------- Prabhu Kavi Phone: 1-978-264-4900 x125 Director, Adv. Prod. Planning Fax: 1-978-264-0671 Tenor Networks Email: [email protected] 100 Nagog Park WWW: www.tenornetworks.com Acton, MA 01720 > -----Original Message----- > From: Matt Zimmerman [mailto:[email protected]] > Sent: Wednesday, April 11, 2001 4:47 PM > To: [email protected] > Subject: 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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