DHCPv6 PD oddity with client changing prefix size.

[Tim DeNike]

Running 4.3.5 in an ISP environment.  Delivering prefixes via DHCPv6-PD.

By default we give users a /64 but will allow them to request a /60 or /56.

Everything works fine with the /64s and /60s EXCEPT if a user changes
prefix lengths.

IE:  User plugs in and enables DHCPv6, they get a /64.  They decide they
want a /60 so change their prefix hint to /60 release/renew, they get the
same /64 back again.

The only way to get them to acquire a /60 is to either manually remove the
/64 lease from the leases file or wait out until the default-lease-time has
expired.

Apparently when the client releases the prefix, it is still held for them
so they can get it back.  No problem with that functionality.  Makes
sense.  But if the client releases and requests a prefix that doesn't have
a length that matches the previous lease then they should be allocated a
prefix from the pool that does match based on the rules of
"prefix-length-mode".

We do have "prefix-length-mode exact;" set.

Seems like a simple oversight/unintended side effect.

Thoughts?

Tim
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