When network 10.4.0.0 goes down, Router C
poisons its link to network 10.4.0.0 by entering a table entry for that link as having in?¬?nite cost
(that is, being unreachable). By poisoning its route to network 10.4.0.0, Router C is not susceptible
to incorrect updates from neighboring routers, which may still have an outdated entry for network
10.4.0.0.
Figure 3-16 Route Poisoning
When Router B sees the metric to 10.4.0.0 jump to in?¬?nity, it sends an update called a poison
reverse to Router C, stating that network 10.4.0.0 is inaccessible, as illustrated in Figure 3-17. This
is a speci?¬?c circumstance overriding split horizon, which occurs to make sure that all routers on
that segment have received information about the poisoned route.
Figure 3-17 Poison Reverse
Route Maintenance Using Hold-Down Timers
Hold-down timers prevent regular update messages from inappropriately reinstating a route that
might have gone bad. Hold-downs tell routers to hold any changes that might affect routes for
A C B E0
S0
S0
10.1.0.0 10.4.0.0 10.2.0.0
S0
S1 E0
10.3.0.0
Routing Table
10.1.0.0
10.2.0.0
10.3.0.0
10.4.0.0
E0
S0
S0
S0
0
0
1
2
Routing Table
10.
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