redundancy [was: something about arrogance]

Tue Jul 30 21:25:12 UTC 2002

At 3:23 AM -0700 2002/07/30, Pedro R Marques wrote:

>  It is my impression, from reading this list and tidbits of gossip,
>  that the most common causes of failure are:
>  - link failure
>  - equipment failure (routers mostly), both software and hardware
>  - configuration errors

	Most likely true.

>  To do so, one can look at:
>  - 2 external links to distinct providers
>  - 2 external links to the same provider

	The latter doesn't protect you from a mis-configuration problem 
from the same provider, upstream of their redundant links to you. 
Moreover, it also doesn't protect you if they have a SPOF above your 
redundant links, even if logically they have two (or more) separate 
outward links, if they are over the same fiber, or the fibers in 
question are physically close to each other, then a single backhoe 
could take you out.

	A second provider doesn't necessarily protect you against the 
backhoe problem, but it would reduce the chances of a problem caused 
by an upstream misconfiguration.

>  While i can't speak to the economics part of the equation (although
>  i would expect it to be cheaper to buy an additional link than connect
>  to a different provider) from a point of view of restoration,
>  protecting a path with an alternate path from the same provider
>  is certainly an aproach that gives you much better convengence times.

	Perhaps, perhaps not.  I would be willing to bet that there are 
at least a few large providers that effectively run each city as a 
separate business, and they'll rape you just as much or more for two 
connections as you would pay to get one connection each from two 
companies.

>  Unless the main concern is that the upstream ISP fails entirely...
>  which given the fact that it tends to have frontpage honors on the
>  NYTimes this days does not apear to be an all to common occurence
>  (i mean operationally, not financially - clarification added to
>  dispel potential humorous remarks).

	Again, I think that this is at least partly dependant on who the 
upstreams are.  If they're small enough, then a single backhoe could 
take out all the fiber (or cause the remaining fiber to be loaded 
well past capacity and practically useless) or cause a power loss 
across the entire facility.

	Even if you buy connectivity from a pretty big upstream, what 
with WorldCom and Qwest both being in serious trouble (and KPN/Qwest 
having completely shut down operations), I would indeed be very 
concerned about complete failure of my upstream.

-- 
Brad Knowles, <brad.knowles at skynet.be>

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
     -Benjamin Franklin, Historical Review of Pennsylvania.

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