Resolution
In order to display the product inventory listing of all Cisco products installed in the networking device, use the show inventory command in user EXEC or privileged EXEC mode.
In some scenarios, it is seen that after you upgrade the Cisco IOS on the Catalyst 6500 switch to Cisco IOS Software Release 12.2(18)SXF5 and later, the show inventory command does not display the serial number of the GBICS or it is blank as shown here:
NAME: "Gigabit Port Container 3/5", DESCR: "Gigabit Port Container"
PID: , VID: , SN:
This reason this occurs is that GBICs and SFPs suppress the output because they are not yet Unique Device Identifier (UDI) compliant.
In such cases, issue the show inventory raw command, which retrieves information about all of the Cisco products installed in the networking device, even if the entities do not have a product ID (PID) value.
Refer to the How to Retrieve the Unique Device Identifier section of Unique Device Identifier Retrieval for more information.