Performing atomic lookup rule set transitions. A method involves performing lookups using a first set of rules, flagging a subset of the first set of rules as old rules, adding a second set of rules while continuing to perform lookups using the first set of rules, and atomically transitioning to perform lookups using the first set of rules plus the second set of rules minus the old rules. A system implementing the method includes a lookup table and a lookup table management module that is configured to make atomic lookup rule set transitions in the lookup table.