When a packet is being transmitted, the Symmetric Port Database is checked to see if the requested port is already allocated. If it has been allocated, it is checked if the source computer entry in the database matches the computer requesting the port. If this is true, the port listed in the Symmetric Port Database is used and the packet is sent.
If the computers do not match or if the requested port is not in the Symmetric Port Database, the feature continues checks to the NAT Port database for an entry matching the requested port. If no entry is found, this means that the port is available. A new entry is added to the NAT Port database, and to the existing NAT database, allocating the port to the requesting computer, and the packet is sent.
If no matching entry in the NAT Port database is found, it means that the port is busy, or otherwise unavailable. The next available port is found, which is allocated to the requesting computer. An entry is added to the NAT Port database with the requesting computer and the available port. An entry is added to the Symmetric Port database, with the requesting computer, the allocated port and the requested port and the packet is sent.
This feature is only required if you need to configure NAT with pool overload or interface overload. This feature is not applicable for other NAT configurations.