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MAC address

Posted on 2003-02-24
Medium Priority
Last Modified: 2013-12-27
hello all,

I have two NIC cards on an Ultra 5, but for some reason they have the same MAC address. This is a second hand box but I reinstalled Solaris when I got it.

Two questions:

How do I make a permanent change for one of the MAC addresses?

Would this prevent DHCP from working correctly on either interface?

Thanks for any help!
Question by:foxtrotzulu

Expert Comment

ID: 8011828
How did check that they have the same mac address?
did you use ifconfig -a?

The 'real' mac addresses can be found on the NIC itself. Each nic should contain alabel with it's factory assigned address. The first 2/4 number indicate the vendor of the nic.

Author Comment

ID: 8012319
elfie: I did use ifconfig -a and refered to the 'ether' variable. i agree that the true MAC address is on the card itself, but what are you saying though? They are not supposed to be the same through 'ifconfig', right.


Expert Comment

ID: 8014761
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Expert Comment

ID: 8014981
Good morning,

Foxtrotzulu,  when you do :-

#ifconfig -a

What do u see ?

I have several sun servers with two NIC.  When I enter the above commands, I notice that both cards have the same MAC address or ether address as "# man ifconfig" describes it.

So, the only thing I can deduce is that the NIC takes the MAC address of the server they are installed in.

The man pages suggest the following to change it :-

# ifconfig le0 ether aa:1:2:3:4:5

Obviously you've looked at the man pages like Ive just done.  To be honest, I thought that each server has one MAC address, I didnt realise, that one server can have multiple MAC's. Are you sure your not confusing IP's with MAC's ??  Dont get me wrong, I could be way out, MACS isnt an area I often venture into.

LVL 10

Expert Comment

ID: 8015083
You can set the MAC address via the openboot environment, on SPARC boxes (MAC address is stored in EEPROM).
It's generally a bad idea to have the same MAC on two seperate interfaces on the same network segment, as you can not be sure which interface will be receiving which packets.  So, some bits of a conversation may end up at (say) eth0 and the other bits may end up at eth1!  Very bad scenario.  There are times when you may wish to have the same MAC for all interfaces (i.e.: some fail-over products), but that's not what you're asking.

Good luck.


Expert Comment

ID: 8016123
Sun-Managers mailing list has a FAQ on this :

12. Networking
12.1)   Why do both my net interfaces have the same ethernet address?
12. Networking
Subject: 12.1) Why do both my net interfaces have the same ethernet address?

  The Ethernet version 2.0 specification (November 1982) states:  The physical address of each station is set by network management to a unique value associated with the station, and distinct from the address of any other station on any Ethernet.  The setting of the station's physical address by network management allows multiple multiple data link controllers connected to a single station to respond to the same physical address.

  This doesn't normally constitute a problem because each interface will typically be on a different subnet. If, for some reason, different ethernet addresses are required on different interfaces (for example, to attach two interfaces to the same subnet), a new one may be assigned using the ifconfig command.

Accepted Solution

colsey79 earned 200 total points
ID: 8017859
If you're really concerned about both interfaces having the same MAC address, you can set the interfaces to provide the MAC address rather than the EEPROM.  This is done by running the following command at the ok prompt:

ok> setenv local-mac-address?=true

Reboot and the interfaces will have different MAC addresses.

Then you don't need to worry about ensuring the interfaces are on different networks or how your DHCP is configured.

Author Comment

ID: 8019017
This is all very interesting.

I suspect that a problem I have with DHCP may be related to this MAC address problem that we have been discusing.

The DHCP server that assigns my IP address also assigns the hostname for my interface based on its MAC address. Therefore I'm guessing that one MAC = one IP. That is the reason I was looking to make the MAC change.

I will give the above suggestions a shot and post any news. Thanks to all for the help.


Author Comment

ID: 8030726
Here is a very helpful summary of my problem for future reference. Thanks to everyone for the help.
SUMMARY: Configure multiple network interface within
same subnet-

Thanks for all the answers. Here is the summary.

The IEEE ethernet standards state that the Media
Access Code (mac)
is assigned per node, and Sun writes its drivers per
the standard.
If your run 'ifconfig -a' as root, you will see that
all NIC's have
the same ethernet address.

Having multiple interfaces from the same host with the
same ethernet
address on the same network can confuse the other
equipment on the

Anyway, depending on the requirement, the following
steps can be
used to get both the interfaces worked.

You have to issue the following commands from the OBP
before you
boot the system
OK> setenv local-mac-address? true
OK> reset-all
You could also make this change by issuing the
following commands
from the OS before rebooting.

#eeprom local-mac-address?=true

This will ensure that each NIC has a unique mac
address as opposed
to the default configuration where the mac address is
selected from
the IDPROM and is therefore the same for each NIC on
the system
which also means that the same machine can not have
two NICs on
the same subnet without making the above change.


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