support each segment with different hardware. If 100 stations are attached to a single switch,
some of the stations can be moved to other switches. This reduces the effect of a hub failure and
some subnetwork failures. If a router is placed between segments, the CCDP can protect against
additional problems and cut subnetwork traffic. NICs, hub ports, and interface cables can be
redundant.
usage on all internetworking devices and is therefore recommended only in situations in which
complete redundancy is required. It also assumes that end-station software, including both the
network and the application subsystems, can handle and effectively use the redundant
components. The application software or the networking software or both must be able to detect
network failures and initiate the use of the other network. Hot Standby Router Protocol (HSRP)
is an example of an application that can detect a router failure. For IP, HSRP allows one router
to automatically assume the function of the second router if the second router fails.
multistation access units (MAUs) can detect certain media connection failures and bypass the
failure internally. FDDI dual rings can wrap traffic onto the backup ring to avoid portions of the
network that have problems.
are available. Using various hardware detection techniques, routers can sense certain media-
level problems. If routing updates or routing keepalive messages have not been received from
devices that would normally be reached through a particular router port, the router will soon
declare that route down and will look for alternate routes. Meshed networks provide these
alternate paths, allowing the router to compensate for media failures.
exceed a company's capacity to stay within budget. The key is striking a happy medium
between cost and disaster protection.
problems. When catastrophic failures occur, the use of dual devices can effectively reduce the
adverse effects of a show-stopping hardware failure. After a failure, discovery protocols help
end stations choose new paths with which to communicate across the network. If each network
connected to the failed device has an alternative path out of the local area, complete
connectivity is still possible.
not be used unless the primary routers are not functioning.