Switch -- MAC address

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May 25th, 2008
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What is a good document to read about the different kinds of MAC addresses supported by a switch?

For example...to determine the root bridge for STP -- MAC address of the switch is one of the parameters. How is it possible for two switches to have identical MAC addresses associated with them?

Thank you.

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joseph.derrick Sun, 05/25/2008 - 09:41
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No 2 networking devices will have the same MAC addresses. These addresses are unique and is a combination of Organizational Unique Identifier (OUI) + serial numbers of the company manufacturing the device.

For a discussion regarding STP, you may visit CCNA Prep Center since it includes helpful presentations on networking basics.

Please rate if it helps.



perkins.t Sun, 05/25/2008 - 10:23
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Hi Deejay!

Thanks for your response...but I thought that when I was watching one of the CCNA Prep videos...they were talking about STP priorities and it was stated that different Switches could have the same MAC address and the priority could be changed to break the tie.

Also, there is documentation that refers to VLAN 1 having the same MAC address on different switches.

Thanks again!


mahmoodmkl Sun, 05/25/2008 - 10:32
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I think there is some confusion for u in STP.

If the priorities are same then mac-address is used as a tie breaker i.e the lowest mac-address will be the root bridge.

I think u need to have a closer look for the sh spanning-tree vlan (vlan id) command it shows u the brigde ID of the root for that particular vlan and in the second line u will have u r local switch bridge id for that vlan.



perkins.t Sun, 05/25/2008 - 11:12
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Thanks again to everyone for responding...I understand the mechanics of STP...I am confused on the use of MAC addresses on switches. I have cut and pasted from some Cisco documentation and perhaps that will help identify where my confusion is originating.

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.

MAC Addresses for Spanning Tree Computation

MAC addresses used for Spanning Tree calculations are stored in an EEPROM present in the Supervisor module. Regardless of the types of line modules installed, the Layer 2 MAC addresses for VLANs do not change unless you replace the Supervisor module. If you do replace the Supervisor module, the Layer 2 MAC addresses of all VLANs change to those specified in the address allocator on the new Supervisor module. In the fixed configuration Catalyst switches, MAC addresses for VLANs cannot be changed.

Catalyst Switch with Support for CLI Configuration of a Unique MAC Address per Interface

This section discusses switches that support a change in MAC addresses per interface.

Catalyst 6500/6000 Supervisor Engine 720 and Supervisor Engine I with MSFC1, MSFC2, or MSFC3 That Runs CatOS System Software

This output is from the MSFC1 in which the default MAC address is the same for all interfaces:

cs-6506-24a#show interfaces | include line | address

Vlan1 is down, line protocol is down

Hardware is Cat6k RP Virtual Ethernet, address is 00d0.bcf1.ee5c (bia 00d0.bcf1.ee5c)

Internet address is

Vlan2 is down, line protocol is down

Hardware is Cat6k RP Virtual Ethernet, address is 00d0.bcf1.ee5c (bia 00d0.bcf1.ee5c)

cs-6506-24a#Use the mac-address mac_address interface configuration command in order to change the MAC address. Here is an example:

cs-6506-24a#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

cs-6506-24a(config)#interface vlan 1

cs-6506-24a(config-if)#mac-address 0007.0001.0001


cs-6506-24a(config)#interface vlan 2

cs-6506-24a(config-if)#mac-address 0007.0001.0002


cs-6506-24a#Verify the change in the MAC address in this way:

cs-6506-24a#show interfaces | include line | address

Vlan1 is down, line protocol is down

Hardware is Cat6k RP Virtual Ethernet, address is 0007.0001.0001 (bia 00d0.bcf1.ee5c)

Internet address is

Vlan2 is down, line protocol is down

Hardware is Cat6k RP Virtual Ethernet, address is 0007.0001.0002 (bia 00d0.bcf1.ee5c)

cs-6506-24a#When you run Cisco IOS system software, the same commands apply for the VLAN (SVI) and L3 interfaces. The Layer 2 (L2) interfaces, however, have MAC addresses assigned from the range of MAC addresses in each module. Issue the show module command in order to see this range:

joseph.derrick Sun, 05/25/2008 - 11:21
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Mahmood was right. If the IDs are the same then mac addresses will break the tie in which case is unique for each device.

Hope it helps.



perkins.t Sun, 05/25/2008 - 11:57
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I understand that he was right with regards to STP.

My question is not about STP...I just used that as an example from some Cisco documentation which indicated that MAC addresses could be changed.

My question is about the different kinds of MAC addresses associated with a switch.

There are MAC addresses that are "assigned" to the CPU...there are VLAN MACs that can be changed et cetera. The fact that you can change a MAC address as suggested by the Cisco document that I partially copied into my last response, indicates that there are some MAC addresses that are not "burned in" but can actually be changed.

I am looking for a document that can describe the different kinds of MAC address that can be and are utilized on switches.

Again...thanks to everyone for their assistance. I really appreciate it!


sarahr202 Sun, 05/25/2008 - 12:12
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mac address can be modified either using third party programme or using utility that comes with network card, provided that netork card permits such modification. Some nic does not allow any changes in mac address

perkins.t Sun, 05/25/2008 - 12:44
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Hi back at you!

Thanks...do you know of a Cisco document that explains the different kinds of MAC addresses used by Cisco switches?




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