Verification is never ending process! You can never be sure that you have verified everything. The aim of verification is risk reduction to the level of practical perfection.
The increase in chip complexity coupled with pressure to shorten time to market, are pushing chip design companies towards adoption of third party IPs. Let us consider you have weighed in all pros and cons of IP outsourcing and decided have to go for a third party IP for your next project. Then the question is - Does the externally bought IP need re-verification?
Let us answer this question by first examining the reasons that third party vendors may give you to persuade you not to go for re-verification: