A "high availability" system comprises multiple switches under the control of multiple control processors ("CPs"). One of the CPs is "active," while the other CP is in a "standby" mode. Upon detection of a failure of the active CP, fail-over occurs to the standby CP. The standby CP re-issues all messages that the previously active CP had issued and which had not yet completed when the fail-over occurred. The newly active CP uses a different range of transaction identifiers ("XIDs") than its predecessor CP. By using a different range of XID values, the newly active CP determines whether an incoming response message is responsive to a message that originated from the previously active CP or the newly activated CP. The currently active CP ignores all response messages that do not have an XID in its range, thereby ensuring proper and reliable messaging.