BIA MAC Address vs Alias MAC Address

[vlad.vlaicu]

If you make an alias for your computer's MAC address, the Cisco switch will learn the avertised MAC and in parenthesis you will also see the BIA MAC Address. How does it detect the BIA Address?

[rsimoni]

can you attach some outputs showing that please?

from what I remember the bia in parenthesis shows up when you change a local interface mac addresses, but I don't recall any output from cam showing both addresses for a connected device.

Riccardo

[vlad.vlaicu]

Unfortunately, I don't have a real Cisco equipment, only GNS3 where it seems i cannot reproduce. But I remember doing it in a lab environment once.

[rsimoni · Accepted solution]

Hi Vlad,

I have the impression that you got confused with something else 

when you change the local address of a host (which by default use the BIA of the NIC or port ASIC for switches and routers) that is the only source MAC address which will be used on all L2 frames sent by that host.

So a Cisco switch can only learn such source MAC address and will not be able to determine if it is a 'forged' one or a BIA. A port might learn multiple MAC addresses anyway, one being a BIA and one being a local address (or alias as you defined it) used by another host or some application or protocol.

Feel free to attach an output showing a different behavior.

Riccardo

[vlad.vlaicu]

You are right. I was confusing it with the output of "sh int". Rockie mistake

Router#sh  int f0/0 

FastEthernet0/0 is up, line protocol is up

  Hardware is Gt96k FE, address is c000.09d0.0000 (bia c000.09d0.0000)

  Internet address is 12.12.12.1/24

  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

Since we brought up, how do you make the two addresses be different?

[cadet alain]

Hi,

enter interface configuration and type mac-address xxxx.xxxx.xxxx

Regards.

Alain

Don't forget to rate helpful posts.

[rsimoni]

that is the one.

Vlad can you please close and rate this question?

thanks

Riccardo