About Assigning a MAC Address for the Stack

The stack must use a single MAC address. When the master node fails over to the backup node, the backup node must continue to use the same MAC address that the master node was using.

Each stackable switch is assigned a single unique MAC address during production. By default, no stack MAC address is configured. You can choose any node to supply its factory assigned MAC address to form the stack MAC address.

When you assign a MAC address to a stack, one of the stackable switches is designated as the node whose factory-assigned MAC address is used to form the stack MAC address. Once this is done, all nodes receive and store this formed MAC address in their own NVRAM. Whenever the stack boots up, this MAC address is used, regardless of which node is the master.

Note

If new nodes are added to the stack, the new nodes must be configured with the stack MAC address. The easiest way to do this is to use the synchronize stacking {node-address node_address | slot slot_number} command.

Before being stored as the stack MAC address, the chosen node‘s factory assigned MAC address is converted to a locally administered MAC address. This prevents duplicate MAC address problems which lead to dual master situations. The chosen MAC address is put into effect only at node boot time. If the address needs to be changed on a single node, rebooting that node results in usage of the same address stack-wide.

If you do not configure the stack MAC address or it is not the same on all nodes, a warning message appears in the log.

Each node operates with whatever address is available (the configured stack MAC address or the node‘s factory-assigned MAC address). If a master node fails over to the backup node, and the backup node‘s address is different than the one the former master node was using, the address is inconsistent with the addresses programmed into the packet forwarding hardware. The MAC address related to the management IP address changes to the one in use by the new master, but no gratuitous ARP requests are sent. In this case, it takes some time for hosts on the management network to flush the related ARP entry.

Note

If the node whose MAC address is chosen is removed from the stack with the intention of using the node elsewhere in the network, and that node is selected to supply the stack MAC in its new stack, the stack MAC of the original stack must be reconfigured to prevent a duplicate MAC address in the network.

Published March 2020prev | next

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