Switching fabrics and control protocols for them

A network unit for use in a switching fabric stores for each network unit in the switching fabric a respective change identification number. In each unit this number is incremented on each change of state. The unit broadcasts protocol packets which indicates incremented change identification numbers; receives corresponding packets from the other units; and determines when the last change identification numbers in the table are the same for all the units. This allows a control based on whether all the units know that all the other units have been updated in response to all the changes of state in the fabric. The network unit can also participate in a routing algorithm, and particularly a shortest path algorithm, within a fabric that constitutes a single network entity.