An LSA/LSP is flushed (purged) when the contents carried by the LSA/LSP are no longer valid. In OSPF when an LSA is flushed the age is set to MaxAge and the LSA is flooded. In IS-IS when an LSP is purged (flushed) the header alone is flooded with the Remaining Lifetime set to 0, and the value of checksum set to 0.
OSPF only allows self originated LSA to be flushed, IS-IS spec allows in certain cases for non-self originated the LSP to be purged (though new implementations don’t support this and the update RFC has changed it) which can lead to problems.
In OSPF a flushed LSA is not removed unless the LSA is not on any of the retransmit lists and none of the adjacencies on the router are in state Exchange or loading. This ensures that an LSA that an LSA is flooded to all its neighbors before it is removed from the domain. In IS-IS an LSP purged is kept for ZeroAge lifetime if the LSP purged is a self originated LSP and the LSP is kept for MaxAge if the LSP is non self-originated before the LSP is deleted.
When purging an IS-IS LSP the header and authentication data is kept while purging (certain OSPF implementations do the same). However for those LSP’s that don’t support authentication, because the checksum is set to 0 for purged LSP’s, the integrity of the contents cannot be verified. In OSPF the entire content of the LSA is intact while flushing leading to unnecessary data sending.