Snooping protocol ensures memory cache coherency in symmetric multiprocessing (SMP) systems. Each processor cache on a bus monitors, or snoops, the bus to verify whether it has a copy of a requested data block. Before a processor writes data, other processor cache copies must be invalidated or updated.
Snooping protocol is also known as bus-snooping protocol.
Techopedia explains Snooping Protocol
The two snooping protocol types are:
Write-Invalidate: Before changing its copy, a data-writing processor invalidates copies of data in all other system processor caches. This action is communicated by an invalidation signal sent over the bus.
Write-Update: A data-writing processor announces new data through the bus. All affected caches are updated with the new data.