Agree that this rumor surfaces pretty much every year. Given NetApp's current market cap of 11.5 B it would be by far the largest acquisition in the history of Cisco. https://en.wikipedia.org/wiki/List_of_acquisitions_by_Cisco_Systems
Very unlikely in my opinion. NetApp was much cheaper in early 2016 and that would have been the time for Cisco to acquire NetApp when the stock was close to $20/share. Rumor was that NetApp was shopping itself like Pure is doing now. Things may be better for NetApp, but why buy a company with single digit growth when you can just continue to partner with them??
What @PmfvdBs-5cme said. This is neither the first and it will not be the last time this rumor appeared. Most likely, nothing will come of it once again.