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3710 Views 3 Replies Latest reply: Oct 10, 2008 4:01 PM by Nicolas MICHEL RSS

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show interfaces

Oct 10, 2008 12:18 AM

Chetan 539 posts since
Jul 26, 2008


Hi Friends,



I have a little query.



Here is the selected output of show interfaces command on a router




Router# show interfaces



Ethernet 0 is up, line protocol is up

Hardware is MCI Ethernet, address is 0000.0c00.750c (bia 0000.0c00.750c)

Internet address is, subnet mask is









Now what is this 0000.0c00.750c (bia 0000.0c00.750c) ?



I understand that 0000.0c00.750c is the hardware address of ethernet 0,



But why is it written twice ?



What is the significance of adding prefix "bia" to h/w address, which is written in parenthesis ?



All comments are most welcomed.



  • Conwyn 9,683 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Oct 10, 2008 1:46 AM (in response to Chetan)
    Re: show interfaces


    Hi Chetan



    History lesson I am afraid. Token Ring had a burnt in address (bia) and a local administrated address (laa) which began with a 4. So a Token Ring Card would have two addresses. The LAA would be used to indicate location. For example 4000.0001.xxxx would be allocated to all PC on site 0001. Remember we are in the mid 80's so IP is not widespread. Each site would have a bridge which connected to other sites. The Token Ring Bridge inspected the site code and passed traffic along the link. Please remember this links would be very small such as 64Kbs without the site code the bridge would have to build the MAC table from traffic on the 64Kb link. Hence we have two addresses.



    Regards Conwyn






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  • Kerry LeBlanc 340 posts since
    Jul 29, 2008
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    2. Oct 10, 2008 4:11 AM (in response to Chetan)
    Re: show interfaces

    Another reason that it shows twice is due to the fact that the first one can be changed.


    Changing your MAC address from the default is what we call MAC spoofing. This term has a negative connotation because its more popular uses are for improper activities, particularly wireless network hacking. However, MAC spoofing does have legitimate uses, such as testing MAC filtering.



    To change your MAC address on a Cisco router, use the mac-address command while in Interface Configuration Mode. Just use the command with the new MAC address-it's that simple.



    Once you change it, the BIA gives you a way to see the original when you need to change it back.



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  • Nicolas MICHEL 164 posts since
    Jul 3, 2008
    Currently Being Moderated
    3. Oct 10, 2008 4:01 PM (in response to Kerry LeBlanc)
    Re: show interfaces


    thanks for the infos guys












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