Overview:
 
Received IPv6 prefix via Prefix Information Option in RA message is not processed (Even when presented code seems to have this ability).
There is function "in6_ifadd()" referenced in the: source/netinet6/nd6_rtr.c Line: 1624
This function seems to allow configuration of the prefix and IID with length !=64.
The code responsible for setting prefix and IID with length !=64 is presented inline and below: source/netinet6/nd6_rtr.c Lines: 2233-2244

Actual Results:
When FreeBSD receives an RA message with prefix len == 80, no ipv6 address with plen = 80 is autoconfigured on the interfaces.

Expected Results: 
Ipv6 address with plen == 80 autoconfigured on the interfaces.

Code:
 2232         /* make ifaddr */
 2233         in6_prepare_ifra(&ifra, &pr->ndpr_prefix.sin6_addr, &mask);
 2234 
 2235         IN6_MASK_ADDR(&ifra.ifra_addr.sin6_addr, &mask);
 2236         /* interface ID */
 2237         ifra.ifra_addr.sin6_addr.s6_addr32[0] |=
 2238             (ib->ia_addr.sin6_addr.s6_addr32[0] & ~mask.s6_addr32[0]);
 2239         ifra.ifra_addr.sin6_addr.s6_addr32[1] |=
 2240             (ib->ia_addr.sin6_addr.s6_addr32[1] & ~mask.s6_addr32[1]);
 2241         ifra.ifra_addr.sin6_addr.s6_addr32[2] |=
 2242             (ib->ia_addr.sin6_addr.s6_addr32[2] & ~mask.s6_addr32[2]);
 2243         ifra.ifra_addr.sin6_addr.s6_addr32[3] |=
 2244             (ib->ia_addr.sin6_addr.s6_addr32[3] & ~mask.s6_addr32[3]);