Floating static
routes are
static routes that are always in the configuration of a router but
are installed in a routing table only when a dynamic route to the
same network is lost. The dynamic route overrides the static route
because the static route is configured with a greater distance.
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Lab
Activity |
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In this lab,
you will learn how to use a floating static route to setup
a backup route that will only appear in the routing table when the
link advertised by the routing protocol fails. |
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This type of static
route can be used when the alternative link is a dialup connection.
Dynamic routing information is usually sent or received over a
dialup link only if the link is intended to be up for a long period
of time. Remember that dynamic routing protocols send updates at
regular intervals, causing the link to go up and down on a regular
basis.
This scenario
provides a real-life example of a floating static route being
installed in a routing table after the lower-cost (shorter-path or
lower-metric) routes are removed.
In the main figure, the
link between router A and router C is now an ISDN connection. This
link is used when the primary link between router A and router B
goes down.
The partial
configuration from router A shows how to configure a floating static
route. The 130 is the distance parameter. It overrides the distance
of 120 that RIP uses. (see RouterA#show
running-config
command output).
In the
routing table from router A, you can see that router A has installed
dynamic RIP routes for networks 10.0.0.0 and 0.0.0.0 via router B
(see RouterA#show ip route
(1) command
output).
In the
routing table from router A, you can see that router A has not
received an update for networks 10.0.0.0 and 0.0.0.0 for almost two
minutes. These routes will time out soon. (see RouterA#show ip route
(2) command output).
In the
output of the debug ip routing command from router A, you can
see the old routes being removed and the new ones being accepted.
Note the higher distances (130) in the new routes. (see RouterA#debug ip
routing command
output).
In the
routing table from router A, you can see the new routes installed.
(see RouterA#show ip route
(3) command output).
The dial-on-demand
routing (DDR) commands necessary to actually cause router A to make
an ISDN call to router C have not been included. DDR is beyond the
scope of this semester. The key factor needed for DDR to work is that
the router must decide that the dial-up interface is the correct
interface to use for forwarding a packet to its next hop.
In this scenario,
router A has just installed routes that will cause it to forward
packets destined to 10.1.0.0 via its ISDN interface. If we assume
that the DDR commands are properly configured, the call should be
made and a connection should be established.
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