2009-04-16 by 

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Overview

The Internet has become so pervasive and integral for conducting business and communicating with customers, partners and employees that network performance, high-availability, and uptime are absolute requirements for running the day-to-day operations of an organization. Network downtime not only costs money and loss of productivity, it can also adversely affect a company’s reputation among customers and partners. For many companies, their entire business strategy depends on how well the network performs.

There are many events that can cause a network or site to go down ,ranging from natural disasters and security attacks to a backhoe cutting a network line, or failing network infrastructure. Few organizations plan for, and have the budget to implement appropriate network infrastructure to ensure their datacenters and remote offices have the protection they need in anticipation of disaster.  For many companies, it’s a case of closing the barn door after the horse runs away. 

According to market research firm Infonetics, large enterprises typically lose between 2 and 16 percent of their annual revenues due to losses associated with network downtime. The more distributed a company’s network is, the more likely it is to suffer service-provider interruptions. According to a recent Infonetics survey, retailers are affected the most, with service providers accounting for more than 30 percent of their downtime costs. Another cause of downtime is human error, which accounts for about one-fifth of the downtime costs. For financial institutions, this percentage jumps to nearly one-third.  Thus, network survivability is key to a business’s productivity and profitability. 

Failover

Failover within a communications network is the process of instantly transferring tasks from a failed component to a similar redundant component to avoid disruption and maintain ongoing operations. Automated failover is the ability to quickly reroute data automatically from a failed component such as a server or network connection to a functioning component and is essential for mission-critical systems.

Different components may be configured for either cold standby (requiring human intervention), warm standby (automatic but delayed) or hot standby (automatic) failover. The three critical elements requiring failover configuration are power, network connectivity and server capacity.

This article describes the different types of failover, the requirements of failover design and strategies for successful failover implementation.

Device Failover

In a failover situation, such as a firewall, router, WAN controller, server load balancer, disk drive, web server, etc., data is transferred to the same type of redundant component to ensure there is limited interruption in data flow and operation.  If a primary component becomes unavailable because of either failure or scheduled downtime, the secondary component serves as a backup and takes over for its troubled counterpart. 

The capability to switch automatically to a redundant or standby system or network upon failure happens without human intervention (see Failover Hierarchy for other types of failover).  Automated failover is essential in servers, systems or networks requiring continuous availability and a high degree of reliability — those that are responsible for mission-critical processes and data (see examples below).  There are few IT managers who want to be responsible for putting a server back online on a holiday weekend.

Failover Hierarchy

As mentioned above, there are different types of failover—some that are not entirely automatic by intention and require manual intervention.  This is called "automated with manual approval"—activity is automatic once approval is given.  When hardware is on “cold standby,” failover must be performed manually, which invites error.  In contrast, when hardware is on “warm standby,” the backup system runs in the background, so the transfer takes place automatically.  The data on both systems is automatically synchronized.  To the user, failover resembles a very fast automatic service reboot. 

The most reliable failover scenario is “hot standby,” in which both systems permanently run in parallel — data on both systems is 100% synchronized at all times.  Users will not be aware of any failures.  This level of failover protection usually requires a corresponding modification to the client.  To run both with systems complete synchronicity, the connections to the client must be mirrored 100 percent.  This normally requires clients that have connections with two or more servers at the same time and can communicate with all of them.  A typical Web browser cannot do this.  Some enterprises implement both hot failover and cold failover for disaster recovery.  It is important to differentiate between failover and disaster recovery.  Failover is a methodology through which to resume system availability in an acceptable period of time while disaster recovery is used to resume system availability when all failover strategies have failed.

The Critical Role of Failover

The convergence of voice, data and video over a single IP network is making the network infrastructure one of the most critical elements in operational success.  These voice, video and data services are increasingly integrated with business-critical applications such as VoIP, e-mail, customer relationship management (CRM), etc.  Therefore, all forms of communication with customers, suppliers and employees are inextricably tied to network operation.  If the network fails, access to critical information can be lost or potentially compromised, with potentially calamitous results: for example, an airport risks massive delays that impact passengers; or patients’ health may be compromised by a major medical center experiencing application delivery delays. There are many organizations for which network failure is not an option.

Examples of Organizations that Need Failover

·     Small and medium-sized businesses need both incoming and outgoing network link aggregation and failover for an increasing assortment of critical-business traffic, from VoIP to email.  For example, the local corner store that does online banking and bill-pay over the Internet, or a manufacturing company that needs email, web services, hosted ERP, and ecommerce applications available 100% of the time.

·     Companies with a central headquarters and a number of branch offices or remote employees need secure and reliable data communications.  They need reliable performance and high-availability of their VPN data, including the ability of the tunnel to automatically failover if a WAN link goes down.

·     Web hosting companies, MSPs, ASPs and small ISPs need incoming link aggregation and failover to ensure that their services are reliable, with extra bandwidth and redundancy available to their servers.  Their mission-critical applications need to be up and running 24/7.  If a WAN link goes down, the failover process has to be smooth and transparent to users.

·     Many of these companies are deploying VoIP applications to cut expenses and enhance productivity. These companies now need quality of service levels and traffic-shaping for guaranteed bandwidth to critical services and applications such as VoIP; and

·     Companies that have ERP, CRM or any other software accessed over the Internet. etc.

Failover Requirements

Most corporate and government networks are comprised of three main elements — LAN, WAN and network infrastructure devices and services.  The LAN provides interconnectivity around a single organizational location.  The WAN provides interconnectivity between these locations (interconnecting specific geographical sites), other business partners, and access to public networks such as the public switched telephone network in the case of voice traffic, and the Internet for data traffic.  The network infrastructure services element provides the services that allow control of the network and flow of data (DNS, DHCP, WINS, FTP), and contain access to the network using Active Directory, RADIUS, etc.

These three elements of network infrastructure services have several requirements for creating a failover environment, the most basic of which is a connecting cable between the two devices.  The second device initiates its systems only when it detects a problem in the first device. Some systems have the ability to page or send a message to a specific technician or support center.  There may also be a third "spare parts" device that has running spare components for "hot" switching to prevent downtime.

The following are other critical elements that comprise a failover environment:

Power

With power failures being one of the most common reasons for network and systems failures, all critical network components at either the primary datacenter, call center or failover site must be connected to a power source that has very high-availability — 99.999% in the case of a datacenter.

A LAN that provides critical services such as a hospital or bank should be equipped with uninterruptible power supplies (UPS) for each component of its distribution and access portions.  These should be connected to emergency power sources to maintain internal communication. The WAN routers, switches, firewalls, etc. need the same form of protection to provide continuous communication and interconnection to external sites and other public networks.

Large datacenters and critical operations, such as call centers, must rely on multiple electric power companies to provide utility power to their locations. The power is brought into the critical site from different geographical locations.  So, if power is interrupted by a car hitting a utility pole accident that severs electric lines at a particular location, the other utility can continue to provide uninterrupted power.

Emergency power generators may be used instead of alternate utilities.  These generators, together with UPS equipment, can provide a continuous stream of electrical power for days if necessary, while utility power is being restored.  

Network Redundancy

Levels of redundancy should be determined for the primary and backup networks based on the identification of critical network components, impact analyses, and established recovery objectives.  There should be consideration for redundancy of network devices such as switches, routers, gateways, etc.  There should also be consideration given to redundant components such as power supplies, CPUs, and circuit cards for the network switches and routers.

WAN Link Aggregation

Consideration must be given to the redundancy and diversity of WAN links in conjunction with automated failover.  Redundancy can be achieved by providing multiple links and multiple types of links for a single site, and between multiple sites.  For example, if the WAN network utilizes MPLS or ATM, it might be prudent to provide different links such as via frame relay, so that if a carrier’s entire service goes down, the organization can have a backup strategy, which many include satellite or microwave links.

Diversity of links can be accomplished either by link diversity — two or more links travel different routes to your locations — or through carrier diversity.  Multiple carriers are used to provide Internet access diversity and redundancy to companies that rely heavily on Internet connectivity.

WAN Bandwidth Capacity

Several capacity factors associated with the alternate sites must be properly assessed in order to avoid failures caused by unanticipated high traffic volumes from a primary site.  One factor is the peak capacity coming from the primary site that failed.  The second factor is the peak capacity of the secondary site to which the traffic will be rerouted.  The size of the WAN links should allow for both peak capacities, plus an additional 25-40% accommodating new peak traffic volumes. Additional traffic may come from new applications such as VoIP, and/or traffic congestion caused by customers, suppliers, and employees.

Aggregated bandwidth must be ample enough to provide ISP failover and redundancy. If one link were to fail, enough bandwidth would still be needed for users to be productive. Intelligent link load balancing monitors bandwidth availability throughout the network, and priority-assigns traffic to the link with the greatest available bandwidth in order to guarantee that time-sensitive traffic (i.e. voice and video — and other critical applications) receive the bandwidth required for smooth performance.

In addition to the availability of WAN links, there is a need for a load balancer to connect users to available servers.  If a server to which the user is connected suddenly becomes unavailable, the load balancer redirects the request to one of the other replicated servers.  This action causes the loss of the original session-to-credential mapping where the user is new to this substitute server, and is normally forced to login again.

WAN Link Load Balancing and Failover

Many companies deploy a specialized WAN optimization controller to merge WAN link load balancing and failover to cost-effectively eliminate downtime for business-critical, time-sensitive applications and ensure network performance.  These devices enable redundant WAN and ISP access, and can provide both outbound and inbound WAN/ISP load balancing and failover.

Bandwidth aggregation combines multiple WAN links into what is effectively one large network connection. Alternately, it can use bandwidth aggregation to maintain these links separately and allocate Internet traffic across them. Both techniques result in larger pools of available bandwidth, and greater reliability.

Site-to-site Channel Bonding

For site-to-site channel bonding, WAN optimization controllers with intelligent link load balancing are installed at both a local and remote site and direct traffic over the Internet between the two sites using the combined (or bonded) bandwidth of multiple ISP or WAN connections. Each site connected by such a bonded link is assigned a unique identifier that allows it to be differentiated from other sites. Each site is also configured with addressing information for both the local and remote end of the bonded link. This allows the WAN optimization controller at the local site to identify traffic that should be sent across the bonded link and direct it to the specified IP addresses on the WAN link(s) of the remote site. When the WAN optimization controller identifies such outgoing traffic, it is disassembled at the packet level into separate streams of data, then encapsulated for transmission through the bonded channels and sent over all available WAN links. Since each encapsulated packet contains addressing information for a specific remote location, data is easily reassembled at that location.

Summary

The Internet has become so pervasive and integral for conducting business and communicating with customers, partners and employees, that network performance, reliability and uptime are becoming required for running the day-to-day operations of many organizations. Network downtime not only costs money and loss of productivity, it can also adversely affect a company’s reputation among customers and partners. There are many events that can cause a network or site to go down, such as natural disasters, security attacks, human errors, and other network infrastructure elements that can fail. When evaluating how to avoid network failures, it is important to evaluate the many options available to ensure high-availability, network uptime and optimal network performance. It is also critical to examine the solutions that will not only help avoid network failures, but are also affordable, and when deployed, will be operationally cost-effective.

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