An indication of the format of Device-Id contained in the
corresponding instance of cdpGlobalDeviceId. User can only
specify the formats that the device is capable of as
denoted in cdpGlobalDeviceIdFormatCpb object.
serialNumber(1) indicates that the value of cdpGlobalDeviceId
object is in the form of an ASCII string contain the device
serial number.
macAddress(2) indicates that the value of cdpGlobalDeviceId
object is in the form of Layer 2 MAC address.
other(3) indicates that the value of cdpGlobalDeviceId object
is in the form of a platform specific ASCII string contain
info that identifies the device. For example: ASCII string
contains serialNumber appended/prepened with system name.