16.64.0/20
172.16.32.128/26
172.16.32.64/26
172.16.64.0/20
C A
B
D
172.16.128.0/20
172.16.32.0/24 172.16.0.0/16
Corporate
Network
132 Chapter 3: Medium-Sized Routed Network Construction
?– Selecting routes from route summaries: If more than one entry in the routing table matches
a particular destination, the longest pre?¬?x match in the routing table is used. Several routes
might match one destination, but the longest matching pre?¬?x is used.
For example, if a routing table has different paths to 192.16.0.0/16 and to 192.16.5.0/24, packets
addressed to 192.16.5.99 would be routed through the 192.16.5.0/24 path because that address has
the longest match with the destination address.
Classful routing protocols summarize automatically at network boundaries. This behavior, which
cannot be changed with RIP-1 and IGRP, has important results, as follows:
?– Subnets are not advertised to a different major network.
?– Discontiguous subnets are not visible to each other.
In Figure 3-36, RIP-1 does not advertise the 172.16.5.0 255.255.255.0 and 172.16.6.0
255.255.255.0 subnets because RIPv1 cannot advertise subnets; both Router A and Router B
advertise 172.
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