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I'm studying STP protocol and how to configure STP on switches. I know each switch has several ports and each port has one MAC address. but when I read the below article I noticed that it allocate one MAC for its own switch device and uses that MAC in Bridge ID.

https://www.cisco.com/assets/sol/sb/Switches_Emulators_v2_3_5_xx/help/250/index.html#page/tesla_250_olh%2Fstp_status.html%23ww1188490

my question is does we have a MAC address only for the whole switch additional to port's MAC? if yes what is its purpose?

3 Answers 3

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Cisco switches have a "base" MAC address for, among other things, sending spanning tree BPDUs.

You can see the base address in the output of show version.

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  • Why does the switch itself have a STP procedure separate from the ports? The ports themselves have all these settings.
    – hamed
    Mar 15 at 18:05
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    So you know the BPDUs come from the same switch.
    – Ron Trunk
    Mar 15 at 18:25
  • thanks, dear Ron, but now another question comes to my mind, I found that we have a switch bridge ID In order for the switch's 'Bridge ID' to be unique from the point of view of the rest of the switches, so we have a different Bridge ID and priority for each port, what is the purpose of those values?
    – hamed
    Mar 23 at 7:54
  • I think you need to study STP a little more. Then the answer will come to you.
    – Ron Trunk
    Mar 23 at 17:41
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As per IEEE 802.1Q, STP switches are required to use a unique group MAC address for the bridge entity (see 802.1Q Clause 13.2) for unique identification.

Many switches use a unique MAC address for each port based on the switch's "base MAC" but that is not required.

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  • But it is used in stp configuration
    – hamed
    Mar 15 at 18:45
  • @hamed That's what you're question and my answer is about.
    – Zac67
    Mar 15 at 19:15
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Yes, switch will gave base Mac address additional to each switch ports ..

Base Mac address of switch is used for various process. One of the example is when (STP) Spaning tree protocal is configured. Bride ID will be calculated on respective switch .

Bridge id = priority value + Mac address of base switch

By default lowest bridge ID switch will become root switch among all connected switches

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  • thanks, dear Sagar, but now another question comes to my mind, I found that we have a switch bridge ID In order for the switch's 'Bridge ID' to be unique from the point of view of the rest of the switches, so we have a different Bridge ID and priority for each port, what is the purpose of those values?
    – hamed
    Mar 23 at 7:53

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