If you associate two VMware Cloud Director sites that are configured with the same installation ID, you might encounter MAC address conflicts in stretched networks across these sites. To avoid such conflicts, you must regenerate the MAC addresses in one of the sites based on a custom seed that is different from the installation ID.

During the initial VMware Cloud Director setup, you set an installation ID. VMware Cloud Director uses the installation ID to generate MAC addresses for the virtual machine network interfaces. Two VMware Cloud Director installations that are configured with the same installation ID might generate identical MAC addresses. Duplicate MAC addresses might cause conflicts in stretched networks between two associated sites.

Before creating stretched networks between associated sites that are configured with the same installation ID, you must regenerate the MAC addresses in one of the sites by using the mac-address-management subcommand of the cell management tool. 

cell-management-tool mac-address-management options

To generate new MAC addresses, you set a custom seed that is different from the installation ID. The seed does not overwrite the installation ID, but the database stores the latest seed as a second configuration parameter, which overrides the installation ID.

You run the mac-address-management subcommand from an arbitrary VMware Cloud Director member of the server group. The command runs against the VMware Cloud Director database, so you run the command once for a server group.

Important: The MAC addresses regeneration requires some downtime of VMware Cloud Director. Before starting the regeneration, you must quiesce the activities on all cells in the server group.
Table 1. Cell Management Tool Options and Arguments, mac-address-management Subcommand
Option Argument Description
--help

(-h)

None Provides a summary of available commands in this category.
--regenerate None Deletes all MAC addresses that are not in use and generates new MAC addresses based on the current seed. If there is no a previously set seed, the MAC addresses are regenerated based on the installation ID. The MAC addresses that are in use are retained.
Note: All cells in the server group must be inactive. For information about quiescing the activities on a cell, see Managing a Cell.
--regenerate-with-seed A seed number from 0 to 63 Sets a new custom seed in the database, deletes all MAC addresses that are not in use, and generates new MAC addresses based on the newly set seed. The MAC addresses that are in use are retained.
Note: All cells in the server group must be inactive. For information about quiescing the activities on a cell, see Managing a Cell.
--show-seed None Returns the current seed and the number of MAC addresses that are in use for each seed.
Important: The MAC addresses that are in use are retained. To change a MAC address that is in use to a regenerated MAC address, you must reset the network interface MAC address. For information about editing virtual machine properties, see the VMware Cloud Director Tenant Portal Guide.

Regenerating the MAC Addresses Based on a New Custom Seed

The following command sets the current seed to 9 and regenerates all MAC addresses that are not use based on the newly set seed: 
[root@cell1 /opt/vmware/vcloud-director/bin]#./cell-management-tool mac-address-management --regenerate-with-seed 9
Successfully removed 65,535 unused MAC addresses.
Successfully generated new MAC addresses.

Viewing the Current Seed and the Number of MAC Addresses in Use for Each Seed

The following command returns information about the current seed and number of MAC addresses per seed: 
[root@cell1 /opt/vmware/vcloud-director/bin]#./cell-management-tool mac-address-management --show-seed
Current MAC address seed is '9' and based on MacAddressSeed config.
MAC address seed    9 is in use by     12 MAC addresses
MAC address seed    1 is in use by      1 MAC addresses
In this example, the system output shows that the current seed is 9, based on which there are 12 MAC addresses. In addition, there is one MAC address that is based on a previous seed or installation ID of 1.