Troubleshooting Routing Loops with Maximum Metric Settings
IP packets have inherent limits via the Time-To-Live (TTL) value in the IP header. In other words,
a router must reduce the TTL ?¬?eld by at least 1 each time it gets the packet. If the TTL value
becomes 0, the router discards that packet. However, this does not stop the router from continuing
to attempt to send the packet to a network that is down.
To avoid this prolonged problem, distance vector protocols de?¬?ne in?¬?nity as some maximum
number. This number refers to a routing metric, such as a hop count.
With this approach, the routing protocol permits the routing loop until the metric exceeds its
maximum allowed value. Figure 3-14 shows this unreachable value as 16 hops. After the metric
value exceeds the maximum, network 10.4.0.0 is considered unreachable.
Figure 3-14 Maximum Metric
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
6
Routing Table
10.2.0.0
10.3.0.0
10.4.0.0
10.1.0.0
S0
S1
S1
S0
0
0
5
1
Routing Table
10.3.0.0
10.4.0.0
10.2.0.0
10.1.0.0
S0
S0
S0
S0
0
4
1
2
X
A C B E0
S0
S0
10.
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