I may be missing the point here, but in a situation
where the net won't
route longer prefixes than /20, doesn't punching a /24 hole in a /20
render the entire /20 unusable, not just the /24 - because the carrier who
formally advertised the /20 now has to advertise a bunch of /21,/22,/23
and /24 prefixes to cover the remainder?
No. The /20 supernet continues to be advertised, and the new /24 net
is also advertised. Since the /24 advertisement is more specific,
traffic is routed to it; if it's not inside the /24 but inside the /20,
the less specific route is used.
For example, if carrier A advertises 192.168.0.0/20 and carrier B
advertises 192.168.4.0/24, carrier C would have two routing table entries:
192.168.0.0/20 -> a.a.a.a
192.168.4.0/24 -> b.b.b.b
and traffic for 192.168.4.2 would go to the more specific route b.b.b.b,
while traffic for 192.168.5.4 wouldn't match the /24, and instead takes
the less specific /20 route to a.a.a.a. It doesn't cause the /20 to be
split into individual routes, but does create one extra routing table
entry for each "hole".
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