Routing vs Switching (was Re: Another UUNET Explanation)

Wed Jul 2 05:49:25 UTC 1997

Alex Rubenstein <alex at nac.net> writes:

> Ummm.. Maybe you are missing my point. Those of us that do not have
> bottomless pockets to pay for a "EVERY ROUTET HAS A CONNECTION TO EVERY
> OTHER ROUTER (VIA PPP -- added by me). 

Hmmm... a router that is connected to every other router
via PPP has precisely the same problems as a router
connected to every other router by PVCs using some kind of
CBR-like service.

If I had bottomless pockets I would still design a
maximally hierarchical network.  Some routers would be
connected to long-haul point-to-point circuits.  Other
routers would talk either via some sort of LAN thing or
if money were no object, probably via POSIP.

The idea would be to create a tree-like fan-out from the
"backbone" routers (the ones with the long-haul circuits)
to the "customer aggregation" routers, aggregating traffic
and reachability information upwards from many customers
towards a set of crunchy boxes with relatively few (but
very fat) interfaces.

The idea is to conserve the amount of work any given
router has to do with respect to convergence, since that's
a poorly-scalable hot-spot.

In the past the hot-spot may have been the amount of
traffic through a box, such that so few fat interfaces
could be used that it was economically compelling to move
that particular load into some sort of L2 switch and take
the lumps wrt inherent routing scalability problems and
the lack of conservation of configuration effort.

Since there are existence proofs that this hot spot is now
no longer economically insurmountable, and some much
crunchier boxes are on the near horizon, the argument for
using smart L2 fabrics at all is becoming weaker.

> I never said it was the catch all / fix all. But, it is cheaper, and n our
> environment, works well and affordably. Backhoes can only do so much
> damage, and that is why one has backup ISDN or some
> other means. 

ISDN does a lousy job of backing up 140Mbps worth of
traffic...

However, as I said in another message, I buy the argument
that tariffs and vendor pricing (particularly cost/port
for low-speed interfaces, modulo things like the CT-3
card) make fast-packet L2 fabrics attractive.

What I don't understand is the "why route when you can
switch?" assertion, and I'd love someone to explain it to
me in simple terms.

> In a high scale network (like BlueBlue Net), they have
> several DS3 Frame Relay trunks leaving each PoP, so that
> dismisses the backhow bit also.

Something has to do the converging when the underlying
physical topology changes, no?

	Sean. (out-of-touch former network hack)
- --
Sean Doran <smd at ab.use.net>
"Boy, you obviously don't have a clue!" -- Chad Skidmore, Datasource L.L.C.