MAC Address Pool on CISCO Switches

[daudparvez]

I have read in the CCNP Book STP Topic that a Catalyst Switch may have a pool of either 64 or 1024 MAC Address.

Since there was just a single paragraph stating this feature and no further deail, I searched the net to get this thing clear.

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On CISCO site I found the following information:

MAC Address on a VLAN (SVI) or L3 Interface on Catalyst Switches

"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.

Catalyst switches have varied support for the ability to change the MAC address for a VLAN (SVI) or L3 interface. You do not need to change the burned-in MAC address if the network devices support multiple IPs to a single MAC Address Resolution Protocol (ARP) table, which is common. Also, you do not need to change the MAC address if the switches support a per-VLAN MAC address table. Cisco switches support a per-VLAN MAC address table or content-addressable memory (CAM) table. This support allows the switches to maintain a MAC address table per VLAN. Therefore, the switches can have the same MAC address on multiple VLAN interfaces without issue."

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.

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But this further pointed out at the lack of my knowledge.There were so many things which I had not heard about. Can someone please explaing the following things:

1..Supervisor Engine 2 and MSFC2 ???

2.. How many types of MAC Addresses exist on a CISCO Swithch and their use?

3..Wether these MAC Addresses are unique on every Switch and  why such a large no. of MAC Addresses are provided on each swithch? Isn't it a waste of MAC Addresses?

4..Also, does each port of the switch has a Separate hard coded MAC Addresses or not? (Although in my view, there is no need of assigning a MAC Address to a switch port since it will work for  the MAC Address of the device attached to it.)

Greatly confused in this MAC Addresses puzzle. Would be thankful if someone can explain in detail.

[Phillip Remaker]

The MAC address pools of every switch are unique.  Usually the addresses are assigned in "blocks" starting at some "burned in" MAC address on the supervisor or line card, as appropriate.  Those burned in starting addresses are managed to be unique in manufacturing, and the block is sized by the maximum mumber of physical ports ever expected on the device with that starting MAC address.

As for a "waste" of MAC addresses: he processes adhere to the guidelines set out in

http://standards.ieee.org/develop/regauth/tut/eui.pdf

Each port gets a unqiue MAC address only if it needs it, like if it gets configured as a routing port.

[paul driver]

Hi Daud,

MFSC cards are intelligent hardware devices that can be installed into mainly large  4500/6500 catalyst switching chassis  for optimising lan/wan services – they have inbuilt hardware which can perform fast switching and routing services.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html

res

Paul

[Giuseppe Larosa]

Hello Daud,

>> 4..Also, does each port of the switch has a Separate hard coded MAC Addresses or not? (Although in my view, there is no need of assigning a MAC Address to a switch port since it will work for  the MAC Address of the device attached to it.)

when a switch has to send out an STP BPDU it has to use a different source MAC address on each port, the same it happens when sending out CDP or other L2 signaling frames.

even if the user devices are not going to use those MAC addresses they are needed to build and mantain a loop free topology and for other signalling needs.

the use of a single source MAC addresss for STP would not allow for loop detection when connecting two ports of the same switch with a LAN cable.

So it is not a waste of MAC addresses after all.

Hope to help

Giuseppe

[SvenArneSteffensen]

Giuseppe

I dont agree with you there.

STP do NOT need unique MAC for each port in order to detect switching loops.

It uses the BID. When it come to detect a loop it also looks on the BID, the port priority and the port # not if it has different MAC adresses.

Sven Arne