EIGRP header - ACK field
The ACK field found inside the header is used to identify the type of packet that is being sent as well as to keep track of communications between specific EIGRP neighbors.
A hello packet with an ACK of zero is treated as a simple hello packet.
A hello packet with a non-zero ACK value is treated as an ACK packet, that is, an acknowledgment to an EIGRP update. The ACK value found in the field contains the sequence number last heard from the neighbor to which the packet is being sent. That sequence number can be found in the sequence header of the EIGRP update.