If a daisy chain is broken, or if a ring is broken in two places, it is possible to form two separate active stack topologies. This results in a dual primary situation, as shown in the following example .
| P6 | Node 6 is powered off |
| M | Primary nodes |
| B | Backup nodes |
| S | Standby nodes |
| X | Indicates the broken link |
In the example, a link was broken while a node in the ring was powered off. The broken link formerly connected the original primary (M1) and backup (M2) nodes of a single active topology.
All nodes in the stack except the powered-off node are in the active topology and all nodes are configured to be primary-capable. Nodes 1, 7 and 8 form an active topology and nodes 2, 3, 4, and 5 form another active topology. Node M2 immediately transitions from backup to primary node role. Nodes B8 and B3 are elected in their respective active topologies as backup nodes.
If the backup node is on one stack and the primary node is on the other, the backup node becomes a primary node because the situation is similar to that of primary failure. Because both stacks are configured to operate as a single stack, there is confusion in your networks. For example, all of the switch‘s configured IP addresses appear to be duplicated. The management IP address also appears to be duplicated since that address applies to the entire original stack.
To help mitigate the dual primary problem, you can configure primary-capability so as to prevent some nodes in the stack from operating in backup or primary node roles. In addition, you can force all nodes in the (broken) stack topology to restart and come up as not primary-capable for the life of that restart. The save configuration {primary | secondary | existing-config | new-config} command saves the configuration on all nodes in the active topology.
Standby nodes that exist in a severed stack segment that does not contain either the original primary or backup node do not attempt to become the primary node. Instead, these nodes reboot. After rebooting, however, a primary election process occurs among the nodes on this broken segment, resulting in a dual primary situation.
Dual primary conditions are also possible when two non-adjacent nodes in a ring or a single (middle) node in a daisy chain reboot.
When the rebooting nodes have sufficiently recovered, or when a severed stack is rejoined, the dual primary condition is resolved, resulting in the reboot of one of the primary nodes. All standby and backup nodes that had been acquired by the losing primary node also reboot.