This octet-string bitmask is the extended-width equivalent of rcIngressMirrorPorts, selecting which ports have their ingress traffic mirrored, used on switches with more ports than the standard PortList type can address. Admins use it instead of rcIngressMirrorPorts when the switch has enough ports to require the wider bitmask representation. On a high-port-count switch, an admin would set the appropriate bits in this octet string to select ingress mirror source ports beyond what the standard PortList mask can represent.