When clouds have a large number of hosts, the default size of the ARP
cache is too small. The cache can overflow, which means that the
system has no way to reach some ip addresses.
Setting the threshold limits higher addresses the situation, in a
reasonably safe way (the maximum impact is 5MB or so of additional RAM
used). Docs on ARP at http://man7.org/linux/man-pages/man7/arp.7.html,
and more discussion of the issue in the bug.
Change-Id: I701141784224f5f870f6da73a24bed8015694409
Closes-Bug: 1780348
53b58388d3