Cisco Switch – Why Does a Switch Have Different MAC Addresses?

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I am working with Cisco Nexus 3000 switch. I was under the assumption that a switch will always have a single MAC address but when I ran the following command, it displayed two different MAC addresses though only the first two bytes were different.

show interface ethernet 1/17 mac-address

Output:
MAC-Address              Burn-in-Mac-Address
ab:cd:ef:gh:ij:12        ab:cd:ef:gh:ij:78

Each interface on the switch has a different MAC. Can someone explain to me the importance of this? When does the common MAC address and interface specific addresses be used?

Best Answer

Cisco assigns a large block, e.g. 1024, MAC addresses to a switch supervisor for use in STP because Cisco defaults to PVST+ and needs a MAC address for each VLAN. Each interface that may send ethernet traffic needs a unique MAC address. Logical interfaces, e.g. SVIs, will also get MAC addresses.

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