High Availability - Networks (I)

Redundant devices

 
Today’s networks are high-tech and most times high speed. Common to most Wide Area Network (WAN) designs is the need for a backup to take over in case of any type of failure to your main link. A simple scenario would be if you had a single T1 connection from your core site to each remote office or branch office you connect with. What if that link went down? How would you continue your operations if it did?
 
Adding redundancy is the most common way to increase your uptime. First, make sure there's redundancy within your core router; redundant CPU cards, power supplies and fans usually can be added to chassis-based routers and switches, and some router and switch vendors have equipment with dual backplanes. With redundant CPU cards, you can force a failover to one card while you upgrade the second one, instead of having to bring the whole router down for the upgrade.
 
The goal of redundant topologies is to eliminate network downtime caused by a single point of failure. All networks need redundancy for enhanced reliability and this is achieved through reliable equipment and network designs that are tolerant to failures and faults and networks should be designed to reconverge rapidly so that the fault is bypassed.
 
Network redundancy is a simple concept to understand. If you have a single point of failure and it fails you, then you have nothing to rely on. If you put in a secondary (or tertiary) method of access, then when the main connection goes down, you will have a way to connect to resources and keep the business operational.
 
The critical point is that highly reliable network equipment is expensive because it is designed not to break and this typically includes things like dual power supplies, watchdog processors and redundant disk systems.
 
A highly available system may be built out of less expensive network products but these components may lack the redundant power supplies or other features of high-reliability equipment, and therefore, they may fail more often than the more expensive equipment. However, if the overall network design takes into account the fact that equipment may fail, then end users will still be able to access the network even if something goes wrong.
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