Route
poisoning attempts to improve convergence time and eliminate routing loops caused by
inconsistent updates. With this technique, when a router loses a link, the router advertises the loss
of a route to its neighbor device. Route poisoning enables the receiving router to advertise a route
back toward the source with a metric higher than the maximum. The advertisement back seems to
violate split horizon, but it lets the router know that the update about the down network was
received. The router that received the update also sets a table entry that keeps the network state
consistent while other routers gradually converge correctly on the topology change. This
mechanism allows the router to learn quickly of the down route and to ignore other updates that
might be wrong for the hold-down period. This prevents routing loops.
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
10.2.0.0
10.3.0.0
10.4.0.0
10.1.0.0
S0
S1
S1
S0
0
0
1
1
Routing Table
10.3.0.0
10.4.0.0
10.2.0.0
10.1.0.0
S0
E0
S0
S0
0
0
1
2
X X
110 Chapter 3: Medium-Sized Routed Network Construction
Figure 3-16 illustrates the following example.
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