In message <20010411164727.B647@alcor.net>, Matt Zimmerman writes:
I think Alex was referring to internal consistency within the router (between linecards), not external consistency. For example, if linecard X believes tha t 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.
I used to give a course on building high speed routers. A section of the course was entitled "common mistakes to avoid." One of them was precisely this problem. The rule is forward once within a box -- if linecard X has decided how a packet is to be forwarded, linecard Y shouldn't be reconsidering the decision. (And, in fact, if you look at most linecard designs, the output path, from switch to transmission, does not include a forwarding engine -- there's only a forwarding engine on the inbound path). Craig