Is there a convention for modifying Mac-Address ?

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Jan 25th, 2010
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Hi there, i had to modify MAC Adress on a VRF SVI in order to bridge my vlan to another one in the global router (bridge with FWSM). It resolved the issue i had (thanks to people here).    On another setup, i also had to modify mac adress in two pair of Cisco ASA 5550 running each multiple context in failover.


On th VRF mapped svi, i only modify a digit of the mac adress (replacing the first 0 with a 1....)  In the ASA, we made an internal definition mac Structure (relative to context id, context vlan etc etc).


My question is : Is there a range of usable MAC adress we can use for this kind of purpose ? Or a convention for modifying it relative to the kind of equipment ?


Thanks !

Correct Answer by peter.mainwaring about 7 years 4 months ago

Have a look at :-


http://en.wikipedia.org/wiki/MAC_address


Be careful about the least significant bit of the most significant byte though. If it is set to 1 the MAC address will be treated as a Multicast. Make sure it is zero so that the MAC address will be treated as a Unicast.


The link suggests that the second least significant bit should be set to 1 for a Locally Administered Address as it is always zero for a Universally Administered Address.


The link explains it all.


Pete

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Correct Answer
peter.mainwaring Mon, 01/25/2010 - 07:52
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Have a look at :-


http://en.wikipedia.org/wiki/MAC_address


Be careful about the least significant bit of the most significant byte though. If it is set to 1 the MAC address will be treated as a Multicast. Make sure it is zero so that the MAC address will be treated as a Unicast.


The link suggests that the second least significant bit should be set to 1 for a Locally Administered Address as it is always zero for a Universally Administered Address.


The link explains it all.


Pete

Giuseppe Larosa Mon, 01/25/2010 - 11:22
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Hello Gizmo,

in the output of sh module in a multilayer switch you can find a block of contiguous MAC addresses that are associated to the system.

Picking up from this block may be a possible choice.

They should be 1024.

(note: they are probably used by CDP, STP and other L2 protocols to differentiate source ports on same device)

Changing less significant two octets (rightmost) to code the vlan number is a possible choice too.

What counts is to avoid to have in the same subnet the same MAC address used twice as you have found in your tests.

In theory a single alternate MAC address value could be used in all VRFs mapped SVIs


Hope to help

Giuseppe

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