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Cisco 850 Series and Cisco 870 Series Access Routers Software Configuration Guide
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Appendix B
Concepts
NAT
line is up, the backup interface is placed in standby mode. In standby mode, the backup interface is
effectively shut down until it is enabled. Any route associated with the backup interface does not appear
in the routing table.
Because the backup interface command is dependent on the router’s identifying that an interface is
physically down, it is commonly used to back up ISDN BRI connections, asynchronous lines, and leased
lines. The interfaces to such connections go down when the primary line fails, and the backup interface
quickly identifies such failures.
Floating Static Routes
Floating static routes are static routes that have an administrative distance greater than the administrative
distance of dynamic routes. Administrative distances can be configured on a static route so that the static
route is less desirable than a dynamic route. In this manner, the static route is not used when the dynamic
route is available. However, if the dynamic route is lost, the static route can take over, and the traffic can
be sent through this alternative route. If this alternative route uses a dial-on-demand routing (DDR)
interface, then that interface can be used as a backup feature.
Dialer Watch
Dialer watch is a backup feature that integrates dial backup with routing capabilities. Dialer watch
provides reliable connectivity without having to define traffic of interest to trigger outgoing calls at the
central router. Hence, dialer watch can be considered regular DDR with no requirement for traffic of
interest. By configuring a set of watched routes that define the primary interface, you are able to monitor
and track the status of the primary interface as watched routes are added and deleted.
When a watched route is deleted, dialer watch checks for at least one valid route for any of the IP
addresses or networks being watched. If there is no valid route, the primary line is considered down and
unusable. If there is a valid route for at least one of the watched IP networks defined and the route is
pointing to an interface other than the backup interface configured for dialer watch, the primary link is
considered up and dialer watch does not initiate the backup link.
NAT
Network Address Translation (NAT) provides a mechanism for a privately addressed network to access
registered networks, such as the Internet, without requiring a registered subnet address. This mechanism
eliminates the need for host renumbering and allows the same IP address range to be used in multiple
intranets.
NAT is configured on the router at the border of an
inside network
(a network that uses nonregistered IP
addresses) and an
outside network
(a network that uses a globally unique IP address; in this case, the
Internet). NAT translates the inside local addresses (the nonregistered IP addresses assigned to hosts on
the inside network) into globally unique IP addresses before sending packets to the outside network.
With NAT, the inside network continues to use its existing private or obsolete addresses. These addresses
are converted into legal addresses before packets are forwarded onto the outside network. The translation
function is compatible with standard routing; the feature is required only on the router connecting the
inside network to the outside domain.