Adding New Links between Existing Stack Units

Adding new links between existing units causes loops if STP is not configured. Any link, excluding stacking ports, between two units in the stack causes loops.

Use one of the following methods to add links successfully between existing stack units.

1. Configure STP and run the stack interactive-setup utility.
   1. Configure spanning tree protocols (for example, STP, RSTP, MSTP, and PVST) on the active controller before making connections. You can enable either global STP or STP only on the default VLAN. If there are already loops before STP is configured and if the looping packets have higher priority than the STP BPDU packets, STP may not converge. In this case, disable or disconnect the looping links to break loops and allow STP to converge. Then reenable or reconnect the links. For example, suppose a link from data port 1/2/4 to 2/2/4 creates a loop. You can configure disable under interface ethernet 1/2/4 to break the loop. Then configure enable after STP converges.
   2. Make all new physical connections.
   3. Wait for STP to converge before entering the stack interactive-setup command. When STP converges, it places some looping ports in the BLOCKING state.
   4. Run the stack interactive-setup utility. Either option 2 or 3 can discover new links within the stack and configure proper stack-ports or stack-trunks. A linear stack may become a ring. The utility can discover multiple sets of links between unit pairs. For example, it can convert a linear stack to a ring and discover the links among units in one run.
   5. Be sure to enter the write memory command if the system prompts you with a "write memory" message.
2. Use stack-port, stack-trunk, multi-stack-port, or multi-stack-trunk commands to create links.
   1. This approach does not require configuring STP.
   2. Configure the stack-port or stack-trunk command on both edge units to change a linear stack to a ring. Then add new links to connect both edge units.
   3. To enlarge the connections between two units, configure the multi-stack-trunk command to change both ends of a live link at the same time. Then connect the new links. The links won't cause loops because they have proper stack-trunk configuration.
   4. To reduce the live connections between two units, physically remove the unwanted cables first. Then configure the multi-stack-port or multi-stack-trunk command to change both ends of the live link at the same time.
      Following example needs to be revised to discuss a model other than the ICX 7750.
      For example, suppose you have an ICX 7750 stack with a live link stack-trunk 1/2/1 to 1/2/3 connected to stack-trunk 2/2/4 to 2/2/6. After removing the unwanted cables between ports 1/2/3 to 2/2/6 and/or between ports 1/2/2 to 2/2/5, you can enter the command multi-stack-port 1/2/1 and 2/2/4 or the command multi-stack-trunk 1/2/1 to 1/2/2 and 2/2/4 to 2/2/5 to reduce the number of ports in the live link.
   5. Changing stacking ports to data ports may cause loops. It is better to disconnect the unwanted cables before reducing live connections as explained in the previous point. The other option is to reduce live connections and then disable the data ports after reducing live connections using the disable port-number command in interface configuration mode.