Assigning a MAC Address to a Stack

Each 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.

Note

This task is not necessary when you configure the stack using Easy Setup. With Easy Setup, the MAC address is assigned by default.

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.

When 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 conditions. 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 was 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.

To assign a MAC address to a stack, follow these steps.

Use the show stacking configuration command to display the stack MAC address configuration.

Slot-1 stack.3 # show stacking configuration
Stack MAC in use: 02:04:96:9c:e4:39
Node               Slot         Alternate          Alternate
MAC Address        Cfg Cur Prio Mgmt IP / Mask     Gateway         Flags     Lic
------------------ --- --- ---- ------------------ --------------- --------- ---
*00:04:96:9c:e4:39 1   1   Auto <none>             <none>          CcEeMm-Nn --
 00:04:96:9b:c1:34 2   2   Auto <none>             <none>          CcEeMm-Nn --
 00:04:96:9e:5c:76 3   3   Auto <none>             <none>          --EeMm-Nn --
 00:04:96:9c:53:b6 4   4   Auto <none>             <none>          --EeMm-Nn --
* - Indicates this node
Flags:  (C) master-Capable in use, (c) master-capable is configured,
        (E) Stacking is currently Enabled, (e) Stacking is configured Enabled,
        (M) Stack MAC in use, (m) Stack MACs configured and in use are the same,
        (i) Stack MACs configured and in use are not the same or unknown,
        (N) Enhanced protocol is in use, (n) Enhanced protocol is configured,
        (-) Not in use or not configured
License level restrictions: (C) Core, (A) Advanced edge, or (E) Edge in use,
        (c) Core, (a) Advanced edge, or (e) Edge configured,
        (-) Not in use or not configured

The MAC Address column displays the factory MAC address for the node. The stack MAC address configuration information appears in the last three positions of the Flags column. As shown in the key at the bottom of the command display, the stack MAC configuration is displayed with the letters capital M, lower-case m, and lower-case i. If the flags read ---, the stack MAC address needs to be configured. If the flags read Mm-, the stack MAC address is already configured and in use.

To configure the stack to use the MAC address of the master node, follow these steps:
1. Log in to the master node.
2. Issue the command configure stacking mac-address.
3. To verify that the command executed properly, enter the command show stacking.
  
  The Flags column should show --i, indicating that the stack MAC is configured but is not in use. After you restart the stack, the i will no longer appear in the Flags column.
4. To verify that the stack MAC is configured consistently, enter the command show stacking detail.
  Verify that all of the stack MAC addresses are the same.
To configure the stack to use a MAC address from a non-master node, log in to the master console and enter the command configure stacking {node-address node-address | slot slot-number} mac-address .
For example:
```
Slot-1 stack.4 # configure stacking slot 2 mac-address
This command will take effect at the next reboot of the specified node(s).
```
The stack is configured to use the MAC address of the node in slot 2.
Reboot the stack.

Verify the new stack MAC address using the command show stacking configuration .

For example:

Slot-1 stack.5 # show stacking configuration
Stack MAC in use: 02:04:96:9b:c1:34

Node               Slot         Alternate          Alternate
MAC Address        Cfg Cur Prio Mgmt IP / Mask     Gateway         Flags     Lic
------------------ --- --- ---- ------------------ --------------- --------- ---
*00:04:96:9c:e4:39 1   1   Auto <none>             <none>          CcEeMm-Nn --
 00:04:96:9b:c1:34 2   2   Auto <none>             <none>          CcEeMm-Nn --
 00:04:96:9e:5c:76 3   3   Auto <none>             <none>          --EeMm-Nn --
 00:04:96:9c:53:b6 4   4   Auto <none>             <none>          --EeMm-Nn --
* - Indicates this node