Inter-exchange media types

Tue May 7 02:53:27 UTC 1996

In message <199605040558.AAA03102 at freeside.fc.net>, Jeremy Porter writes:
> 
> Paul, with all due respect, there is a lot of apparent need and
> demand for 100mbps interconnects, one could argue
> that 100 100mbps interconnects are better than 3 or 4 622mbps
> interconnects.  I think 100 interconnects are at least as likely
> in 2 years as 4 622mbps interconnects.

That brings up another scaling problem.  If one AS is attached to 100
or so interconnects, that's 100 border routers.  That's not so bad
with one backbone (ie: national provider in the US).  If there are 2
or more (there are and will be more than 2), and each reaches X% of
the 100 or so interconnects (where X% is 100% minus epsilon, say 95%
for example), then they will announce the routes they hear from one
interconnect to all other 100 or so minus 1 interconnects.  The other
backbones will hear the same set of routes 100 or so times.  

If we continue to do shortest path out, that's 100 paths (I'll drop
the "or so") to each prefix.  That means we need to fix the "MED
latching" feature and use MED.  With any good IBGP implementation, if
MEDs are different the routers with less preferred routes with
withhold their announcements.  If so, we have another slight problem.
If the primary route is withdrawn, most or all of those less preferred
routes will get announces and gradually slosh around until the next
best route is installed everywhere and all others are again
suppressed.

This is not impossible for a router to handle, just a lot of route
flap to deal with and coding had better be very carefully done.  There
is existance proof that suggests that handling high levels of route
flap can be done less than perfectly in ways that can promote
sustained instability.

The problem is that 100 interconnects will amplify route flap problems
by a factor of 100.  Just a heads up for router vendors - just expect
to see high route flap loads and deal with it.  Stability does not
require infinite CPU power, just algorithms which don't choke when the
load gets high, but rather converge at whatever rate they can sustain.

I suspect we will see both.  A lot more interconnects and a lot faster
interconnects.

Curtis