[c-nsp] Conditional advertise-map

[Heath Jones]

I completely agree with the problem of tcam overflow if an aggregated prefix
dissapears! I did overlook that though, thanks for pointing it out!

I think it is still certainly an advantage to do aggregation pre-tcam. We
are only better off than where we are now. Even if the FIB gets hammered
because of some change on the wider internet (if the tcam overflows), it
could revert to software forwarding for some prefixes / catchall type
arrangement. Obviously the selection of prefixes to catch in software is
important...

I wonder if someone has modelled this - see just how much aggregation could
realistically be done at each AS. I'd imagine its similar to the info you
got about 1/2ish of the prefixes out there being deaggregates..?


On 15 September 2010 18:43, Gert Doering <gert at greenie.muc.de> wrote:

> Hi,
>
> On Wed, Sep 15, 2010 at 06:38:26PM +0100, Heath Jones wrote:
> > I thought you'd say that...
> >
> > There is absolutely *NO* reason why an additional entry needs to occupy
> > space in TCAM in this case.
> > If you have 2 contiguous prefixes that can be directly aggregated to a
> > single prefix on the next subnet boundary, and that share the same
> > next-stage-treatment (egress, queueing etc), they can take up only 1
> spot.
> > If a router isn't preprocessing routing information before it goes to the
> > forwarding plane, then its not efficiently using the expensive
> resources..
>
> Well, the theory agrees with you.  Unfortunately, routers out there in
> the market (and remember, this is *cisco*-nsp) don't do this.
>
> I have asked for it for a number of times, but it's a tricky problem
> (like: you have a /16 that encompasses 256 /24, the RIB barely fits
> into the FIB space, and then the /16 goes away -> 253 extra FIB entries
> needed -> non-deterministic boom) *and* Cisco has no commercial interest
> in spending lots of engineering effort to the goal of selling *less*
> expensive hardware...
>
> gert
>
> --
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>