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 in found in the ExtremeSwitching and Summit Switches: Hardware Installation Guide for Switches Using ExtremeXOS 16 or Earlier.

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 (with one exception), 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. The exception is if there is an equal distance between the nodes. In this case, if both nodes are 48-port nodes, the nodes are grouped into two groups of 24 ports (by the hardware architecture), and thus it is possible to use both directions around the stack.