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This Question is Answered 2 Helpful Answers available (2 pts)
15511 Views 10 Replies Latest reply: Jun 22, 2010 12:16 AM by remek gorecki RSS

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Bridge ID Clarification

Apr 28, 2009 10:58 PM

Mike.B 528 posts since
Oct 20, 2008

Hi Guys,

 

I'm confused on something so please bear with me as I'm not entirely sure I can explain this well enough. =D

 

It's my understanding that a Bridge ID consists of two parts: 1) A priority value  2) A MAC Address.

 

What specific MAC address is used here?  How does the switch decide what MAC address to use here?  Is there only one available for this specific purpose per switch? Or is this the specific hardware address of the switchport itself?

 

Sorry if this doesn't make sense.  I guess I'm wondering if the hardware addresses for the individual switch ports are uses in the bridge ID or not... for example, if I run a show interfaces, I'll see stuff like:

 

FastEthernet0/4 is down, line protocol is down (disabled)
  Hardware is Lance, address is 0002.16c5.0d04 (bia 0002.16c5.0d04)
  MTU 1500 bytes, BW 100000 Kbit, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255

If this port was used to connect to another switch (which it obviously isn't), would its hardware address be used in the bridge ID?

 

Thanks!

 

-M

  • Chris 811 posts since
    Jul 25, 2008
    Currently Being Moderated
    1. Apr 29, 2009 6:44 AM (in response to Mike.B)
    Re: Bridge ID Clarification

    Hi Mike,

     

    The MAC address used for the BID is the switch's Base MAC address.  You can see this address when rebooting an switch.

     

    HTH

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,426 posts since
    Oct 7, 2008
    Currently Being Moderated
    3. Apr 29, 2009 9:19 AM (in response to Mike.B)
    Re: Bridge ID Clarification

    Use the "show spanning-tree bridge id" command to see what BID is being used.  Remember that this is 4 bits priority + 12 bits VLAN ID + 48 bits MAC address.

     

     

    Rack1SW1#sh span bridge id
    VLAN0001         8001.0019.060c.9080
    VLAN0003         8003.0019.060c.9080
    VLAN0005         8005.0019.060c.9080
    VLAN0006         8006.0019.060c.9080
    VLAN0007         8007.0019.060c.9080
    VLAN0017         8011.0019.060c.9080
    VLAN0022         8016.0019.060c.9080
    VLAN0036         8024.0019.060c.9080
    VLAN0042         802A.0019.060c.9080
    VLAN0052         8034.0019.060c.9080
    VLAN0100         8064.0019.060c.9080
    VLAN0127         807F.0019.060c.9080
    VLAN0473         81D9.0019.060c.9080
    Rack1SW1#

     

    The MAC used is your system MAC.  This will be one bit value lower than your lowest port MAC.

     

    Rack1SW1#sh int | in bia
      Hardware is EtherSVI, address is 0019.060c.90c0 (bia 0019.060c.90c0)
      Hardware is EtherSVI, address is 0019.060c.90c1 (bia 0019.060c.90c1)
      Hardware is EtherSVI, address is 0019.060c.90c2 (bia 0019.060c.90c2)
      Hardware is Fast Ethernet, address is 0019.060c.9083 (bia 0019.060c.9083)
      Hardware is Fast Ethernet, address is 0019.060c.9084 (bia 0019.060c.9084)
      Hardware is Fast Ethernet, address is 0019.060c.9085 (bia 0019.060c.9085)
      Hardware is Fast Ethernet, address is 0019.060c.9086 (bia 0019.060c.9086)
      Hardware is Fast Ethernet, address is 0019.060c.9087 (bia 0019.060c.9087)
      Hardware is Fast Ethernet, address is 0019.060c.9088 (bia 0019.060c.9088)
      Hardware is Fast Ethernet, address is 0019.060c.9089 (bia 0019.060c.9089)
      Hardware is Fast Ethernet, address is 0019.060c.908a (bia 0019.060c.908a)
      Hardware is Fast Ethernet, address is 0019.060c.908b (bia 0019.060c.908b)
      Hardware is Fast Ethernet, address is 0019.060c.908c (bia 0019.060c.908c)
      Hardware is Fast Ethernet, address is 0019.060c.908d (bia 0019.060c.908d)
      Hardware is Fast Ethernet, address is 0019.060c.908e (bia 0019.060c.908e)
      Hardware is Fast Ethernet, address is 0019.060c.908f (bia 0019.060c.908f)
      Hardware is Fast Ethernet, address is 0019.060c.9090 (bia 0019.060c.9090)
      Hardware is Fast Ethernet, address is 0019.060c.9091 (bia 0019.060c.9091)
      Hardware is Fast Ethernet, address is 0019.060c.9092 (bia 0019.060c.9092)
      Hardware is Fast Ethernet, address is 0019.060c.9093 (bia 0019.060c.9093)
      Hardware is Fast Ethernet, address is 0019.060c.9094 (bia 0019.060c.9094)
      Hardware is Fast Ethernet, address is 0019.060c.9095 (bia 0019.060c.9095)
      Hardware is Fast Ethernet, address is 0019.060c.9096 (bia 0019.060c.9096)
      Hardware is Fast Ethernet, address is 0019.060c.9097 (bia 0019.060c.9097)
      Hardware is Fast Ethernet, address is 0019.060c.9098 (bia 0019.060c.9098)
      Hardware is Fast Ethernet, address is 0019.060c.9099 (bia 0019.060c.9099)
      Hardware is Fast Ethernet, address is 0019.060c.909a (bia 0019.060c.909a)
      Hardware is Gigabit Ethernet, address is 0019.060c.9081 (bia 0019.060c.9081)
      Hardware is Gigabit Ethernet, address is 0019.060c.9082 (bia 0019.060c.9082)
    Rack1SW1#

     

    Notice the gigabit ports have the lowest addresses.  9081 is the lowest.  So 9080 (as seen above) is the System MAC on the switch, which is used for BID values.

     

    HTH,

     

    Scott

    smorris@internetworkexpert.com

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  • JohnB352 31 posts since
    Apr 23, 2009
    Currently Being Moderated
    Re: Bridge ID Clarification

    I have a followup question to your answer.

    I'm studying for my ICND2, and I was trying to figure the same thing that the OP was asking; where does the MAC in the BID come from?

     

    I'm using Packet Tracer, and on my configuration it looks like the MAC in the BID comes from VLAN 1.  But, where is that coming from?  Here's what I have from the Root Bridge:

     

    Gig 1/1  0090.21b6.9101
    Gig 1/2 0090.21b6.9102
    VLAN 1  0090.2b47.8421

    Base ethernet MAC Address: 0090.2B47.8421   (shown on a reload)

    Show spanning-tree vlan 1
    Root ID    Priority    24577
         Address     0090.2B47.8421
         This bridge is the root


    The Base or System address on this switch isn't the lowest switch port (G 1/1) address, minus a value of 1.  It's VLAN 1, minus 1.

    Where is the MAC for VLAN 1 coming from?  And is the results here different than what you get on your switch, as far as how the BID was determined?

    Thanks

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,426 posts since
    Oct 7, 2008
    Currently Being Moderated
    5. Jun 18, 2009 9:54 AM (in response to JohnB352)
    Re: Bridge ID Clarification

    Yup.  As noted above, it's one below the lower port MAC.  Known as the system MAC.  In modular switches it's based on the supervisor card, but in fixed switches, it's one lower than lowest port MAC.

     

    HTH,

     

    Scott

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  • JohnB352 31 posts since
    Apr 23, 2009
    Currently Being Moderated
    Re: Bridge ID Clarification

    swmorris wrote:

     

    Yup.  As noted above, it's one below the lower port MAC.  Known as the system MAC.  In modular switches it's based on the supervisor card, but in fixed switches, it's one lower than lowest port MAC.

     

    HTH,

     

    Scott

     

    There is a difference in what you're seeing and what I am seeing.

     

    You show one of your Gig interfaces with this MAC address:  0019.060c.9081

    And you say the BID would be based on that:  9081-1 = 9080

     

    In my config the lowest MAC was Vlan 1, not the MAC of G1/1.

    I'm trying to understand "why the difference".

    The BID in my example is the Vlan 1 MAC minus 1.

     

    Is that because of the "supervisor card" or "fixed switch" that your'e referring to?  (I'm not familiar with those terms)

    Does that explain why your BID is based on the Gig int and mine is based on the VLAN1 int?

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,426 posts since
    Oct 7, 2008
    Currently Being Moderated
    7. Jun 18, 2009 5:02 PM (in response to JohnB352)
    Re: Bridge ID Clarification

    Actually, it depends more on which set of interfaces (chipset) powers up first.    In my switch, it happened to be the gig ports.  Depending on what hardware platform you are using, it may vary.  In modular chassis, the sup module powers first, therefore it's the base mac.

     

    HTH,

     

    Scott

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  • JohnB352 31 posts since
    Apr 23, 2009
    Currently Being Moderated
    Re: Bridge ID Clarification

    Ok, thanks for clearing that up.

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,426 posts since
    Oct 7, 2008
    Currently Being Moderated
    9. Jun 19, 2009 6:34 AM (in response to JohnB352)
    Re: Bridge ID Clarification

    No worries....

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  • remek gorecki 1 posts since
    Jul 22, 2008
    Currently Being Moderated
    10. Jun 22, 2010 12:16 AM (in response to Mike.B)
    Re: Bridge ID Clarification

    Is it possible to change this MAC?

     

    in 3750 it is  Base Ethernet Mac Addres and its 1 bit lower than lowest MAC in switch  (in this case g1/0/1)

     

    RG

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