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

Wed Apr 11 20:59:54 UTC 2001

In message <20010411164727.B647 at 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