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Issue of February 2004 

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Improving Network Availability

Here's a quick guide to defining, measuring, and improving availability in your network. by Sudhir Narang

Analyzing and predicting availability for your entire network, not individual devices, is complex, but it's certainly workable—and essential. A highly available network has become a business necessity. But today's networks carry multiple applications and incorporate a wide variety of devices and paths, so they may appear difficult to assess. Today, vendors deliver various products & solutions that enable network managers to get very detailed and precise information on the problem areas, and the availability of networks. This article formulates a practical definition for High Availability along with metrics and methods for measuring network availability. It also examines four areas to assess and plan remedies: network design, infrastructure, operations and maintenance, and support.

Define First, Then Measure

Network availability is more important today than ever before. The heat is on because users expect much more from networks compared to the early days of e-mail and file sharing. Today, enterprises maintain on their networks a multitude of mission-critical applications that drive their businesses: videoconferencing with partners, Web-based order entry, payment processing, and customer relationship management, to name a few. The network simply cannot go down.

These types of applications are a cornerstone to making operations much more efficient—and are absolutely critical to an enterprise's viability and productivity. Meanwhile, they also make networks more complex, and availability becomes an issue. The crucial concept to remember here is that availability must be defined for the network as a whole. The probability that users can get to applications is the sum of the availability of individual devices and paths.

Fortunately, the data for analyzing the availability of multipurpose data networks are all there for the asking: the Simple Network Management Protocol (SNMP), ping, trap, and trouble ticket reports that most enterprises already collect, as well as from published statistics by network device vendors. Additionally, the software exists to put this data together in useful ways, so network managers can see the level of availability their users are experiencing—and also analyze and project problems to be fixed before they affect users. Having five nines—99.999 percent availability, or just a few minutes of downtime a year—has been the traditional aim for most large enterprise-wide and service-provider networks.

In a multi-application network, availability isn't an issue of how many 'nines' you have. It's having a highly available network where you need it—for your mission-critical applications. Clearly, the network manager's perspective should encompass the long-term, so as to spot and fix potential problems before they affect users.

Calculating Theoretical Network Availability

Most consultants on high-availability usually begin an analysis of a network's availability by calculating the Theoretical Availability of three categories of components—hardware, software, and power supplies—in each of the three network segments: access, distribution, and core. The first step that network consultants take is to use information from product data sheets of the various components of the network. This approach is well regarded since it defines what is theoretically possible for a company to attain, and helps set expectations.

In our experience, the math always yields the lowest common denominator. It's common sense, that a chain is only as strong as its weakest link. The "six nines" of reliability in the core and distribution portions are due to the redundancy that permeates both areas in this network. For many organizations, a lower rating is acceptable in the access layer because it affects fewer users. When a company collects enough of its own data, it can replace the theoreticals with actuals.

This Theoretical Availability assessment is important, as it can point the way to areas that need work. In addition, planners can use it to determine if the network, as functioning or with planned upgrades, can meet the needs of the business. And the path availability estimates can help predict the cost of downtime and the return on investment likely from improvements to the network. Importantly, any network availability study needs to study network behavior in the long term. Managers need to typically study the trend for at least three months, preferably even longer.

Getting to the Availability You Need

The real challenge comes after an enterprise has baselined availability and identified problem areas. Network managers now need to start measuring how the network measures up in reality. To this effect, managers need to assess four areas to pinpoint and plan remedies: network design, infrastructure, operations & maintenance, and support.

Network Design

The most important aspect of network design is a rigid hierarchy of core, distribution, and access. Corporations usually want the highest availability and resilience in the core—the five nines. In the Distribution layer, managers may be able to go down a nine, and perhaps only need three nines in the access layer. This approach also tracks the economics, as you have the fewest devices in the core, with most at the edge, perhaps even orders of magnitude more.

Device-level resilience can be provided by redundant processors and uninterruptible power, as well as with technologies rooted in products that comprise the network. The availability delivered by each of the network components really determines the total network availability of an organization. The organization needs to invest in products that can deliver optimal application-level resilience.

Then there are overall design principles such as avoiding single points of failure wherever possible—for example, having only one logical or physical circuit out of a router. When any link is down, the router is out of business. Enterprises need to invest in creating redundant paths to every device. If you have a single point of failure in your core, it's only a matter of time before it comes back to haunt you.


A network design is only as good as the systems it comprises. In the physical infrastructure, both hardware and software play key roles in achieving availability. Core systems should have full redundancy built in. Core systems should also have sufficient throughput and be scalable so they can accommodate spikes on demand, particularly as next-generation IP-based services such as videophone conferencing and public wireless access are added to the network. And, of course, every vital system needs UPS.

One way to ensure high reliability in the network is what is called the 'cookie-cutter approach.' That is, every branch office, data center, and network node that performs the same functions should have identical hardware and software. This way, you can make fixes faster and more accurately; cut down on your spares inventory, and compare apples to apples when monitoring your network.

Operations and Maintenance

Once you know how your network is arranged and base-lined, you can tweak operations for maximum efficiency. In many cases, you might not need new hardware, just operational changes. Efficient operations depend on rigorous adherence to best practices as well as network maintenance, which, again, requires monitoring to catch problems before they affect users. Appropriate performance analysis and metrics tools are important. It is also wise to map your maintenance contracts to your business and availability goals.


The final component in improving availability is to evaluate your support structures to make sure they match your needs. Know the capabilities of your own personnel and of your vendors, and how they fit the needs of your network. Do your people have the skill sets they need? Does your vendor have the tools and the required support service? Can they provide the training that your staff needs?


Many enterprise managers and executives are beginning to understand that network availability management is fundamental to successful network operations—just as the accounting department is integral to controlling costs.

The relationship between availability baselines and corporate bottom lines is not surprising. Networks are rapidly increasing in complexity and criticality to an organization's success. And as voice and video services are folded into existing data networks, this relationship will become even more profound. All this points to the need for knowing how your network is performing and supporting organizational goals. Armed with metrics such as network availability, decision-making is improved and allocation of resources is more precise.

The author is Vice President, Cisco Systems India & SAARC

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