Solved: output show me the same MAC address twice

AdamBudzinski · ‎07-30-2015

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 · ‎07-30-2015

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

View solution in original post

Rolf Fischer · ‎07-30-2015

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 · ‎07-30-2015

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 · ‎07-30-2015

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...