-----Original Message-----
From: K. Graham [mailto:kgraham@rogers.com]
Sent: Saturday, April 06, 2002 6:26 AM
To: nanog@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