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MAC Addresses used by the Cisco 3750

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Introduction

The CPU on a 3560 or 3750 has a stash of 128 MAC addresses allocated when the device is built, for physical ports and SVI (vlan) interfaces.

Explanation of MAC Address allocation

You can get the "base" MAC address by issueing a show version. The first 64 addresses (inclusive) from that address are used for physical ports. The next 64 after that are used for SVI (vlan) interfaces.

The CPU allocates MAC addresses to vlan interfaces in the order which the interfaces are created, so they can change after a reboot of the switch.

If you use more than 64 vlan interfaces, the additional interfaces will all get the same MAC address as the 64th interface.

For example:

base address: xx:xx:xx:xx:3e:00

first vlan address: xx:xx:xx:xx:3e:40 (this is base + 64)
last vlan address: xx:xx:xx:xx:3e:7f (this is base + 127)

Further Information

You might also find the following page interesting: Supported Platforms for Unique MAC Address Configuration on VLAN or L3 Interfaces for Catalyst Switches

Comments
Taste66
Beginner
Thank you for this explanation. I arrived here because we see an issue with the 3750 switch in combination with multicast routing. The IGMP querier uses the MAC address of the VLAN and the IP address for the VLAN (192.168.64.254 in our case) However the query for a specific multicast group send to the specific multicast address when a receiver decides to leave the multicast group uses the MAC address of the physical port and the same IP address as all other IP traffic to this VLAN (192.168.65.254). Therefore there are 2 MAC addresses for the same IP address. This causes confusion for the ARP tables of the remaining devices which still receive this multicast group. Is there a way to change this behavior by configuration?