1. High Availability Design
Ram Dantu Hi
Slides are adopted from various sources from Cisco and
Interwork Inc.,

2. Agenda
Definitions
Concepts / Calculations
Examples
Challenges

3. Availability as a percentage
1 year = minutes
Availability
Unscheduled downtime per year
99%
3 days 15 hours 36 minutes
99.9%
8 hours 45 min.
99.99%
52 min. 36 sec.
99.999%
5 min. 15 sec. %
32 sec.

4. Getting downtime from availability
1 year = 525,960 minutes
Uptime = availability * Time
Annual Uptime = Availability * 525,960
Annual Downtime = 525,960 – Annual Uptime
Availability =
Annual Uptime = * 525,960
Annual Uptime = 525,907.4
Annual Downtime = 525,960 – 525,907.4 =
Downtime = (1-Availability) * Time

5. Availability vs. Reliability
Availability = users’ perception
Reliability = individual component failures
Reliability impacts maintenance costs but doesn’t necessarily have to impact availability

6. Defects Per Million Availability DPM 99% 10000 99.9% 1000 99.99% 100
99.999% 10 % 1

7. Calculating Availability
MTBF
MTBF + MTTR
Example: 6500 Chassis
MTBF = hours (about 42 years)
MTTR = 4 hours
Availability = / ( )
= /
= = %

8. 6500 Availability Module Availability Chassis 99.99892% Power Supplies%
Supervisor - including software %
GBIC Line Card %
GBIC %
10/100 Line Card

9. Availability Formulas
Serial availability
Availability = AvailA × AvailB × AvailC
= % × % × %
= % A C B
99.999%

10. Availability Formulas
99.9% B A
Parallel availability
Availability = 1 – ((1 – AvailA) × (1 – AvailB))
= 1 – ((1 – 99.9%) × (1 – 99.9%))
= %

11. Availability Formulas
Parallel-series availability
99.9% B A D C F E   % % %
= %

12. Availability Formulas
Series-parallel availability
99.9% B A D C F E
99.7%
= 1 – ((1 – 99.7%) × (1 – 99.7%))
= %

13. Core
Distribution
Access

14. One Core / Distribution 6500
Chassis %
Dual power %
Supervisor + software %
Dual GBIC Line Cards
Dual GBICs
Single switch availability %

15. Pair of Core / Distribution 6500’s
Series-Parallel Availability
% series availability each
Two switches in parallel
1 – ((1 – %) × (1 – %))
= %

16. Access Layer 6500 Chassis 99.99892% Dual power 99.99999%
Dual Supervisors
Dual GBIC Line Cards
Dual GBICs
10/100 Line Card %
Switch availability %
Access port availability %

17. Challenges Improving availability at the access layer
“NIC Teaming” in servers
Reduce MTTR
MTBF = hours
Availability with MTTR of 4 hours = %
Availability with MTTR of 2 hours = %

18. Challenges Long convergence times Spanning tree
Eliminate layer-2 links where possible
Avoid layer-2 loops
Use STP enhancements where appropriate
Routing protocols
Use a link-state (or EIGRP) routing protocol
Use routing convergence enhancements
Minimize routing table sizes
Limit convergence scope