[36614] in North American Network Operators' Group
Re: gigabit router (was Re: Getting a "portable" /19 or /20)
daemon@ATHENA.MIT.EDU (Craig Partridge)
Wed Apr 11 17:15:48 2001
Message-Id: <200104112059.f3BKxtZ30620@aland.bbn.com>
To: Matt Zimmerman <mdz@csh.rit.edu>
Cc: nanog@merit.edu
In-Reply-To: Your message of "Wed, 11 Apr 2001 16:47:28 EDT."
<20010411164727.B647@alcor.net>
Date: Wed, 11 Apr 2001 16:59:54 -0400
From: Craig Partridge <craig@aland.bbn.com>
Errors-To: owner-nanog-outgoing@merit.edu
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
