nanog mailing list archives
RE: gigabit router (was Re: Getting a "portable" /19 or /20)
From: "David Schwartz" <davids () webmaster com>
Date: Wed, 11 Apr 2001 00:26:54 -0700
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. Suppose a router received a packet that causes it to modify its routing table in some way. If another packet is received in close time proximity to the first packet, it can be reasonably routed by either policy. Even a router with a central table could still route it either way, depending upon when the routing process get scheduled in relation to when the interface interrupt is services. (Or for other reasons, depending upon the hardware you are dealing with.) The only way to sure this type of consistency is to centrally process every single packet in strict sequence, fully applying all routing changes the packet may require. There is no benefit to this added effort, after all, the router would still have to work even if the packet with the routing data was dropped. 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. 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. DS
Current thread:
- Re: Getting a "portable" /19 or /20, (continued)
- Re: Getting a "portable" /19 or /20 Henry Yen (Apr 10)
- Re: Getting a "portable" /19 or /20 Valdis . Kletnieks (Apr 10)
- Re: Getting a "portable" /19 or /20 Richard A. Steenbergen (Apr 10)
- Re: Getting a "portable" /19 or /20 Greg Maxwell (Apr 10)
- Re: Getting a "portable" /19 or /20 Richard A. Steenbergen (Apr 10)
- gigabit router (was Re: Getting a "portable" /19 or /20) Craig Partridge (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Richard A. Steenbergen (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) alex (Apr 10)
- RE: gigabit router (was Re: Getting a "portable" /19 or /20) David Schwartz (Apr 10)
- RE: gigabit router (was Re: Getting a "portable" /19 or /20) alex (Apr 10)
- RE: gigabit router (was Re: Getting a "portable" /19 or /20) David Schwartz (Apr 11)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Matt Zimmerman (Apr 11)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Craig Partridge (Apr 11)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Brett Frankenberger (Apr 11)
- Re: Getting a "portable" /19 or /20 Greg Maxwell (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Richard A. Steenbergen (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Rafi Sadowsky (Apr 11)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Craig Partridge (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Greg Maxwell (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Richard A. Steenbergen (Apr 10)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Peter Galbavy (Apr 11)
- Re: gigabit router (was Re: Getting a "portable" /19 or /20) Bora Akyol (Apr 10)