3.4 Recovery Options

---

The LAN is a data-intensive environment requiring special precautions to safeguard one of the organization’s most valuable assets—information. The procedural aspect of minimizing data loss entails the implementation of manual or automated methods for backing up all data on the LAN to avoid the tedious and costly process of recreating vast amounts of information. The equipment aspect of minimizing data loss entails the use of redundant circuitry, as well as components and subsystems that are activated automatically upon the failure of various LAN devices to prevent data loss and maintain network availability.

In addition to the ability to respond to errors in transmissions by detection and correction, other important aspects of LAN operation are recovery and reconfiguration. Recovery deals with bringing the LAN back to a stable condition after an error, and reconfiguration is the mechanism by which the network is restored to its previous condition after a failure.

LAN reconfigurations involve mechanisms to restore service upon loss of a link or network interface unit. To recover or reconfigure the network after failures or faults requires that the network possess mechanisms to detect that an error or fault has occurred and to determine how to minimize the effect on the system’s performance. Generally, these mechanisms provide the following:

• Performance monitoring;

• Fault location;

• Network management;

• System availability management;

• Configuration management.

These mechanisms work in concert to detect and isolate faults, determine their effects on the system, and remedy these conditions to bring the network to a stable state with minimal impact on network availability.

Reconfiguration is a fault management scheme used to bypass major failures of network components. This process entails detecting that a fault condition has occurred that cannot be corrected by merely restarting the equipment. Once it is determined that a fault has occurred, its impact on the network is assessed so that an appropriate reconfiguration can be formulated and implemented. In this way, normal operations can continue under a new configuration until the problem can be fixed and the network restored to its primary configuration.

Fault detection is augmented by logging systems that keep track of failures over a period of time. This information is examined to determine trends that adversely affect network performance. This information, for example, might reveal that a particular component is continually causing problems on the network, or the monitoring system might detect that a component on the network has a higher-than-normal failure rate.

The configuration assessment component of the reconfiguration system uses information about the current system configuration—including connectivity, component placement, paths, and traffic flows—and maps it against the failed component. This information is analyzed to indicate how that particular failure is affecting the system and to isolate the cause of the failure. Once this assessment has been performed, a solution can be worked out and implemented.

The solution may consist of reconfiguring most of the operational processes to avoid the source of the fault. The solution determination component examines the configuration and the affected hardware or software components, determines how to move resources around to bring the network back to an operational state or indicates what must be eliminated because of the failure, and identifies network components that must be serviced.

Determining the most effective course of action is based on the criticality of keeping certain functions of the network operating and maintaining the resources available to do this. In some environments, nothing can be done to restore service because of device limitations (e.g., lack of redundant subsystems) or the lack of spare bandwidth. In such cases, about all that can be done is to indicate to the servicing agent what must be corrected and keep users informed of the situation.

Once an alternate configuration has been determined, the reconfiguration system-implements it. In most cases, this means rerouting transmissions, moving and restarting processes from failed devices, or reinitializing software that has failed because of some intermittent error condition. In some cases, nothing may need to be done except notify affected users that the failure is not severe enough to warrant system reconfiguration.

Geographically distributed LANs can be inter-networked over the WAN using such devices as bridges and routers connected to leased lines and/or switched services. An advantage of using routers for this purpose is that they permit the building of large mesh networks. With mesh networks, the routers can steer traffic around points of congestion or failure and balance the traffic load across the remaining links. In addition, routers have flow control and more comprehensive error protection than bridges.

Bridges are useful for reducing the size of sprawling LANs into discrete subnetworks that are easier to control and manage. Through the use of bridges, similar devices, protocols, and transmission media can be grouped together into communities of interest. Such partitioning can yield many advantages, such as eliminating congestion and improving the response time of the entire network. Subnetworks are also useful for testing new applications before making them available over the enterprise network.

---