Some people seem to view CIDR as an opportunity to
subdivide their traditional 'Class B' networks. I would classify
most of these announcements as 'mistakes' because they usually also
announce the supernet, and have the same path.
I could see the intentional announcement of both if you've got diverse --
but ultimately equal -- paths into your B. So site X might advertise
B.0/17 to normally receive the low half of the address range, site Y
might advertise B.128/17 to normally receive the high half, and both
sites would advertise B/16 so that if one site went down in that
scenario, the other's B/16 advertisement would take over for the failed
site's /17. In a normal case, though, you'd see the /16 and /17 with the
Does this pattern seem to apply to any of the announcements you see?