A string of octets containing one bit per VLAN for
VLANS with VlanIndex values of 0 through 2047. The
first octet corresponds to VLANs with VlanIndex values
of 0 through 7; the second octet to VLANs 8 through
15; etc. The most significant bit of each octet
corresponds to the lowest value VlanIndex in that octet.
For each VLAN, if it is mapped to this MST instance,
then the bit corresponding to that VLAN is set to '1'.
If the length of this string is less than 256 octets,
any 'missing' octets are assumed to contain the value
of zero.