infinite; IGRP uses a delay value of more than 4 billion to imply an infinite distance route.
through Router C. Router C claimed to have a metric 2 route to 162.11.7.0 at the same time that
Router C was receiving the update that the route to 162.11.7.0 was no longer valid. (Note:
Infinity is shown as the value 16 in Table 6-9, which is RIP's value for infinity.) So, Router B
thinks 162.11.7.0 is reachable through Router C, and Router C thinks 162.11.7.0 is
unreachable. The process repeats itself with the next routing update, except Router B advertises
metric 3 and Router C advertises an infinite (bad) metric for subnet 162.11.7.0. This will
continue until both numbers reach infinity.
for which the number is considered to be infinite. For example, 16 is infinite for RIP, and
4,294,967,295 is infinite for IGRP.
includes two related concepts that affect what routes are included in a routing update:
same interface x.
out Serial1 does not advertise that subnet. B's update also does not include subnet 162.11.9.0,
presumably because B's route to that subnet also points out Serial1. However, because B's route
to 162.11.5.0 points out Serial0 to Router A, B advertises about that subnet out Serial1.
horizon. Instead of not advertising a route out the same interface in which the route was learned,
poison reverse means that the routes are advertised but with a poison (infinite) metric. In other
words, in Figure 6-6, Router B would also advertise routes to 162.11.6.0, 162.11.9.0, and
162.11.10.0, all with infinite metric.
infinity can occur in redundant networks (networks with multiple paths) even with split horizon
enabled. The holddown timer is part of a solution to the counting to infinity problem when
networks have multiple paths to many subnets. Split horizon does not defeat the counting to
infinity problem in all topologies. An additional solution is required, which includes a
holddown timer and a routing update feature called route poisoning. Figure 6-7 shows an
example topology showing counting to infinity.