RouterA will advertise the 172.16.0.0/16 network to RouterB. When the auto-summary command has been used to enable automatic summarization on a router, Enhanced Interior Gateway Routing Protocol (EIGRP) automatically summarizes networks on classfulboundaries. Summarization, which is also referred to as aggregation, minimizes and optimizes the size of routing tables and advertisements and reduces a router's processor and memory requirements. Summarization is also useful in limiting the scope of EIGRP queries.
The 172.16.0.0/24, 172.16.1.0/24, and 172.16.3.0/24 networks in this scenario use Class B addresses. Therefore, these network ranges are summarized to the Class B boundary, which is /16.
To disable automatic summarization, you should issue the no autosummary command. The no autosummary command enables EIGRP to advertise the actual networks, not the classful summary. You should use the no autosummary command when a classful network is divided and portions of the same classful network exist in different parts of the network topology. If you were to issue the no autosummary command on RouterA, RouterA would advertise the individual network ranges and subnet mask information to RouterB.