Stacking Link Overcommitment
The stack is formed by each node supplying a pair of full-duplex, logical stacking ports. Each node can operate on a stack with full duplex throughput up to the limits found in the hardware installation guide for your switch.
Even though two links are available, the links might not be fully utilized. For example, suppose there is a ring of eight nodes and the nodes are numbered clockwise from 1 to 8. The stacking port limit in this example is 10 Gbps in each direction for a total stack throughput of 20 Gbps for each port, or 40 Gbps total. Suppose node 1 wants to send 10 Gbps of unicast traffic to each of node 2 and node 3. The shortest path topology forces all traffic from node 1 over the link to node 2. Traffic from node 1 to node 3 passes through node 2. Thus, there is only 10 Gbps link available. However, if node 1 wanted to send 10 Gbps to node 2 and node 8, there would be 20 Gbps available because both links connected to node 1 would be used.
In a ring of eight nodes, between any two nodes, only one link is used. If the devices provide 48 1-Gbps Ethernet ports, the overcommitment ratio between two such nodes is approximately 5:1.