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Operational and Organizational Security: Policies and Disaster Recovery• Chapter 12 745

Figure 12.4 Multiple Links Used to Provide High Availability on a Network

Site A

Site B Site C

Many companies depend on Internet connectivity almost as much as network
connectivity. In some cases, such as e-commerce businesses, they depend on it
more.A redundant ISP can be used to provide connectivity when an organization’s
primary ISP’s service becomes unavailable.The link to the secondary ISP can be
configured as a low priority route, while the primary ISP is advertised as high pri-
ority. Such a configuration will have users using the primary ISP for normal usage,
but automatically switching over to the low priority connection when the first one
fails. If a secondary ISP is not desired, the administrator should ensure that the ISP
uses two different points of presence.A point of presence is an access point to the
Internet; therefore, having multiple points of presence will allow access to the
Internet if one goes down.
Because data is a commodity of any business, it is important to ensure that it is
always available to those who need it. Redundant Arrays of Inexpensive Disks
(RAID) was developed to prevent the loss of data and/or improve performance.
RAID provides several methods of writing data across multiple disks, and writing
to several disks at once. Rather than losing a single disk and all the information,
administrators can replace the damaged disk and regenerate the data quickly.When
determining which level of RAID to use, it is important to remember that some
RAID levels only increase performance, some only prevent loss of data, but not all
will do both.The different levels of RAID available include:

■ RAID 0 (Disk Striping) Data is written (striped) across two or more
disks, but no copies of the data are made.This improves performance
because data is read from multiple disks, but there is no fault tolerance if a
disk fails.

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