Protocol data unit (PDU) and Service data unit (SDU):
At any particular layer N of OSI model, a PDU is a complete message that implements the protocol at that layer. When this “layer N PDU” is passed down to layer N-1, it becomes the data that the layer N-1 protocol is supposed to service. Thus, the layer N protocol data unit (PDU) is called the layer N-1 service data unit (SDU).
AD (Administrative distance):
Multihome: When your network have more then one ISP connection then it is called as multihome.