An indication of which Virtual LANs are allowed
on this Inter-Switch Link. This is an octet string
value with bits set to indicate allowed VLANs. It
can be interpreted as a sum of f(x) as x goes from
0 to 1023, where f(x) = 0 for VLAN x not allowed
and f(x) = exp(2, x) for VLAN x allowed.