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I saw a post say Switch do not need its MAC address, unless it's an administrable Switch for managing.

but I found the spanning-tree protocol need a MAC address of Switch to form STP Root Bridge ID.

so, is the switch always need a MAC address? nevertheless in a two layer circuit, all the Switch need a self MAC address at least?

three-blocks
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3 Answers3

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The basic function of a switch is transparent bridging - for this, it doesn't need any MAC address of its own.

However, if you need to talk to a switch - ie. a managed switch - then that switch requires an address; usually that is a MAC address and an IP address [*]. STP is only supported on managed switches and in addition, an STP bridge is required to have a unique bridge ID which usually is identical to the management MAC address.

[*] A managed network device could just have a serial console which doesn't require any addressing. However, this was only used on very early devices. Today, most devices feature both, in-band network management and out-of-band console access.

Zac67
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  • I have a question, in a switch STP circuit, can there have a non-manage Switch? – three-blocks Nov 11 '18 at 04:08
  • Theoretically yes, but I have never seen an unmanaged switch supporting STP. Without a console, you wouldn't be able to set the STP priorities. With STP, that unmanaged switch would be required to have a MAC address as well. – Zac67 Nov 11 '18 at 08:17
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Yes, if your switch supports spanning-tree protocol(either legacy spanning tree protocol, rapid-pvst+ or MST) your switch will have a mac address since spanning-tree protocol uses bridgeID to elect the root bridge and bridgeID consists of the bridge-Priority, a unique mac address and system-id which is basically the vlan number.

Also if a switch has a management ip address, it will need to have a mac address so that ethernet frames can be forwarded to it at layer 2.

Muhammad Nasir Akram
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The MAC address used by the spanning tree algorithm (STA) implemented in a given switch does not belong to any of its Ethernet interfaces. At least in Cisco implementations, this MAC address is known as base MAC or universal (burned-in) MAC and its purpose is to identify the switch itself, not some of its Ethernet interfaces. For this reason, this MAC address is printed on a label attached on the back of the device.

You can check a device's base MAC by issuing the show version command:

SW1#show version Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2(25)FX, RELEASE SOFTWARE (fc1) Copyright (c) 1986-2005 by Cisco Systems, Inc. Compiled Wed 12-Oct-05 22:05 by pt_team

ROM: C2960 Boot Loader (C2960-HBOOT-M) Version 12.2(25r)FX, RELEASE SOFTWARE (fc4)

System returned to ROM by power-on

Cisco WS-C2960-24TT (RC32300) processor (revision C0) with 21039K bytes of memory.

24 FastEthernet/IEEE 802.3 interface(s) 2 Gigabit Ethernet/IEEE 802.3 interface(s)

63488K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address : 0001.43BA.DE15

Motherboard assembly number : 73-9832-06 Power supply part number :

341-0097-02 Motherboard serial number : FOC103248MJ Power supply

serial number : DCA102133JA Model revision number : B0

Motherboard revision number : C0 Model number :

WS-C2960-24TT System serial number : FOC1033Z1EY Top

Assembly Part Number : 800-26671-02 Top Assembly Revision

Number : B0 Version ID : V02 CLEI Code Number : COM3K00BRA

Hardware Board Revision Number : 0x01

You can read more on this here

Humberto Fioravante Ferro
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