I wanted to follow up on my query below. I found a real-world case
where a company advertises /16, /19 and /24 nets from the same Class
B space....yet all are reachable from Verio's network (I tested this with
Verio's Traceroute Server). A phone call to Verio verified for me why
all 3 nets are reachable. (interestingly, the traceroutes themselves
sort of masked how this was working--hence, the need to talk to Verio)
Basically, it was as I had thought--because at least one of the sites
advertises /16, this is enough to get traffic off of Verio's net, ie,
all traffic destined for any piece of the /16 is passed to its "nearest"
neighbor. If that neighbor in turn has the /19 & /24 in addition to the
/16, the routing from that point on will be optimal. If not, it
passes it to the neighbor from which it received the /16...and so on, until
has the full /19 or /24 net.
So, when I originally posted the note below, I was worried that Verio
not be able to reach certain networks with prefixes longer than /16, ie,
sort of blackholing the routes to those destinations. (Remember, I'm
new to Internet routing.) That would indeed seem to be the case if there
were no /16 advertisement in addition to the more specific announcements.
That's the generic BGP/Verio lesson I drew. The more specific lesson is that
if your working with a large provider like Internap or Genuity in all your
sites, traffic destined for your location is going to get back to you as
long as they are advertising the /16 in addition to the more specific nets.
...even if the source hosts are on networks like Verio's.
Any further comments appreciated.