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
16
Routing Table
10.2.0.0
10.3.0.0
10.4.0.0
10.1.0.0
S0
S1
S1
S0
0
0
16
1
Routing Table
10.3.0.0
10.4.0.0
10.2.0.0
10.1.0.0
S0
S0
S0
S0
0
16
1
2
X
Reviewing Dynamic Routing 109
Preventing Routing Loops with Split Horizon
One way to eliminate routing loops and speed up convergence is through the technique called split
horizon. The split horizon rule is that sending information about a route back in the direction from
which the original update came is never useful. For example, Figure 3-15 illustrates the following:
?– Router B has access to network 10.4.0.0 through Router C. It makes no sense for Router B to
announce to Router C that Router B has access to network 10.4.0.0 through Router C.
?– Given that Router B passed the announcement of its route to network 10.4.0.0 to Router A, it
makes no sense for Router A to announce its distance from network 10.4.0.0 to Router B.
?– Having no alternative path to network 10.4.0.0, Router B concludes that network 10.4.0.0 is
inaccessible.
Figure 3-15 Split Horizon
Preventing Routing Loops with Route Poisoning
Another operation complementary to split horizon is a technique called route poisoning.
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