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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.

enter image description here

enter image description here

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Apr 1, 2018 at 19:49

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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.

EDIT HERE

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

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  • 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?
    – Charlie
    Jan 18, 2018 at 14:11
  • @thomas please check edited answer
    – DRP
    Jan 18, 2018 at 15:14
  • Your comment is correct, but there is also a case where this do not work. I added a screenshot of an example where it do not match.
    – Charlie
    Jan 18, 2018 at 16:19
  • @thomas first case is STP and the device itself is root bridge, second case is RSTP and device is not root bridge, my understanding per the scope of the title on the question it was about STP and not RSTP correct?
    – DRP
    Jan 18, 2018 at 17:49
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    Some bridges use the source MAC from their interface towards the link in question but the bridge ID is always (built) the same for a specific device, regardless of interface.
    – Zac67 ♦
    Jan 18, 2018 at 17:50

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