output show me the same MAC address twice

[AdamBudzinski]

hi all,

and easy one for sure. When i run the show interfaces G0/1 command on a switch , why does the output show me the same MAC address twice? 

address is 00d0.58c0.4519 (bia 00d0.58c0.4519)

I know, BIA stands for Burned-in-address and is located on ROM but what's the idea behind it? 

EDIT: also would like to ask, why does a router maintain an arp entry for it's own interface, it looks like this is not the case on for example a Windows laptop ... 

Best Regards

Adam 

[Rolf Fischer · Accepted solution]

Adam,

some interfaces allow you to configure user-defined MAC addresses:

R1#show int f0/0 | i bia
  Hardware is AmdFE, address is cc00.0fac.0000 (bia cc00.0fac.0000)

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int f0/0
R1(config-if)#mac-address 0200.0000.0001
R1(config-if)#do show int f0/0 | i bia
  Hardware is AmdFE, address is 0200.0000.0001 (bia cc00.0fac.0000)

HTH

Rolf

[AdamBudzinski]

Hi Rolf, 

thank you for posting. So basically the "user defined" MAC will "override" the BIA , correct ?

What would be a reason / scenario to do so ? 

Best Regards

Adam

[Rolf Fischer]

So basically the "user defined" MAC will "override" the BIA , correct ?

Correct, yes.

What would be a reason / scenario to do so ? 

Good question, I can't think of a good reason.

In labs I sometimes like to give some interfaces handy MAC addresses but there must be a better reason for the existence of this command...