Quick Question on Industry Standard

Sat Apr 6 14:33:47 UTC 2002

-----Original Message-----
From: K. Graham [mailto:kgraham at rogers.com]
Sent: Saturday, April 06, 2002 6:26 AM
To: nanog at merit.edu
Subject: Quick Question on Industry Standard

>From my understanding there is a 99.97% up time value that most companies
try 
and match.  Is this a hard and fast rule or is this a value that we all try 
and emulate as best as we can?  Do I have the value incorrect?  Is it higher

or lower?  I had always thought that it was 99.97% but have not found 
anywhere to reference that figure, 

Cisco references that figure in their MTBF/MTTR (mean time before failure)
calculations, however they also reference 99.9%.  Our organization does not
include scheduled maintenance in our HA (high availability) calculations,
and I expect most organizations don't either.

Quote, "Availability is calculated using statistical models for all the
system components, the simplest model for a component being binary. The
component is either in or out of service. Availability can be calculated
from failure rates, measured in mean time between failures (MTBF), and
repair times, measured in mean time to repair (MTTR)."

Also, "The average downtime contribution by any component is calculated by
amortizing the MTTR time over the MTBF period. For example, if a component
critical to the operation of the platform has an MTBF of 250,000 hours and a
MTTR of 1 hour, it contributes 2.1 minutes (60 min/250,000 hr/8760 hr/yr) of
unavailability to the system per year"

http://www.cisco.com/warp/public/cc/pd/rt/7500/prodlit/haibd_ov.htm

So to calculate the physical HA stats, you need to reduce your network to
component levels and do the calculations.  Cables are not usually included
in these calculations.

HTH

-- Tim
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