what is the difference between the eth.src Address and the Bridge-System-Id? Sometimes they are the same but mostly not. Attached you will find a screenshot where they are the same.
eth.src is the media access control address (MAC address) of the device which is a unique identifier assigned to network interfaces for communications at the data link layer of a network segment.
In terms of STP Bridge system identifier (BID) is a unique identifier composed by priority and MAC address.
So depending on the priority(32768 is the default value) which is in configured in increments of 4096 you will have different values showing on the BID.
In regards to your comment concern:
as you see in the screenshot the priority isn't zero but the mac and bridge-id are the same. How could that be if you are correct?
Highlight Bridge Identifier: 32768 / 0 / 00:17:ef:74:62:00 to check hex.
In the hex values you look for your MAC and in front of that you would see the 2byte for the decimal value of 32768 (32768+0).
So something like: 80 00 00 17 ef 74 62 00