The Length of the IP NetMask for this summary address.
The values for the index objects isisSummAddress and
isisSummAddrPrefixLen must be consistent. When the value
of isisSummAddress (excluding the zone index, if one
is present) is x, then the bitwise logical-AND
of x with the value of the mask formed from the
corresponding index object isisSummAddrPrefixLen MUST be
equal to x. If not, then the index pair is not
consistent, and an inconsistentName error must be
returned on SET or CREATE requests.