I'd argue that a route caching should have the property that the
cached route, whenever used, is the same route that would be found
from a fresh lookup.  This is a pretty strong requirement, and I do
not believe that the current code does this.  Clearing all cached
routes on every routing table change (where ARP renewal isn't a
change) would be adequate but excessive.

If one added code to do a new lookup in the udp output path, and log
every time the cached/fresh routes don't match, printing both routes,
it would probably help a lot to understand what's going on.

-- 
        Greg Troxel <gdt@ir.bbn.com>