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
S0
S0
S0
0
2
1
2
X
Reviewing Dynamic Routing 107
Figure 3-11 Inconsistent Data Continues to Propagate
Because Routers A, B, and C conclude that the best path to network 10.4.0.0 is through each other,
packets from Router A destined to network 10.4.0.0 continue to bounce between Routers B and C,
as illustrated in Figure 3-12.
Figure 3-12 Routing Loop Exists Because of Erroneous Hop Count
Continuing the example in Figure 3-12, the invalid updates about network 10.4.0.0 continue to
loop. Until some other process can stop the looping, the routers update each other inappropriately,
considering that network 10.4.0.0 is down.
This condition, called count-to-in?¬?nity, causes the routing protocol to continually increase its
metric and route packets back and forth between the devices, despite the fundamental fact that the
destination network, 10.4.0.0, is down. While the routing protocol counts to in?¬?nity, the invalid
information enables a routing loop to exist, as illustrated in Figure 3-13.
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