[f-nsp] static vs dynamic cam mode on MLX/XMR (turning foundry direct routing off)

[Richard A Steenbergen]

On Sat, Jan 05, 2008 at 05:05:26PM +0100, Gunther Stammwitz wrote:
> Hello Niels,
> 
> > You basically turn your router into a flow-based one from about a
> > decade
> > ago.  I haven't tested how that works out but I imagine it will not run
> > well at all.
> 
> I think you aren't right: cam entries are not added per flow but per 
> destination ip, so it's not that bad..

Per destination route actually. What Niels meant is that its like the 
decade+ old Cisco FAST switching, where you have an accelerated forwarding 
path but it is not pre-populated and is not capable of holding the entire 
routing table. Every time you get a packet to a new destination which is 
not in the fast path, it has to go to the slow path for lookup and 
programming into the fast path. This is not to be confused with FLOW 
switching, which actual src/dst layer 3 and/or 4 information is stored, 
thats entirely different.

There is nothing inheriently wrong with going non-prepopulated for certain 
applications, the trade-off is that routing becomes non-desterministic. 
You are no longer guaranteed that forwarding any packet to any destination 
will behave exactly the same as any other packet under all circumstances, 
some destinations may require more system resources than others, and some 
traffic patterns will require different route-cache loads. The danger is 
from people who don't understand how this works when they are one day 
confronted with a new traffic pattern and their device stops working. :)

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
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