Where does a switch get its MAC addresses for VLAN and IF?

Inc

Where does a switch get its MAC addresses for VLAN and IF?

This question arose in a discussion today. Only thing I found out after searching cisco web was following:

By default, Catalyst switches come with the same MAC address configured on all interfaces. The diagram in this section shows a Catalyst 6500 with Supervisor Engine 2 and MSFC2. However, the MAC address on all three VLAN interfaces is the same, even though the IP addresses are different.

MAC Addresses on Layer 2 Interfaces

MAC addresses of Layer 2 Interfaces (Switchports) are unique and are assigned to that particular line module. In Cisco 6500/6000, 4500/4000, 3750, 3560, 3550, and 2970 series switches, you are not able to change the MAC address on a switchport. In Cisco 2940, and 2950/2955 series switches you can change the MAC address of switch ports using the command mac-address, under the interface configuration mode.

Can anyone shed the light on this topic?

Main uncertainty was, if the MAC address gets assigned from official OUI listed address pool or is just bogus.

Netwurk

The MAC address is hard-coded on the switch. Cisco refers to this as the default burned-in-address (BIA). You'll see references to the BIA in your show interface commands.

There is an overall MAC for the switch, something like 000b.5fa6.0000

Then the ports get MACs by incrementing the overall MAC, for instance port 1 getting 000b.5fa6.0001 and port 2 getting 000b.5fa6.0002, etc.

Some switches let you change the MAC, some don't.

To tell if a MAC address has been changed, look at the output from your show interface command

A line like "address is 0007.0e8f.088a (bia 0007.0e8f.088a)" shows us that the switch is using its hard-coded MAC.

If you see something like "address is 0007.0001.0001 (bia 0007.0e8f.088a)", then the MAC has been changed via software.

It works a little differently for the big switches with supervisor engines, but the basic concepts are the same.