Port-Based Load Sharing
NoteThis feature is supported on all ExtremeSwitching Universal platforms.
index = key value % N
where N = the number of ports in the aggregator
The resulting behavior is that ports with a key value of 0 distribute to the lowest numbered port in an aggregator, ports with a key value of 1 distribute to the second lowest numbered port in an aggregator, etc.
A port-based load sharing group contains aggregator ports 2,4,6 and 8. If the zero-based load sharing key 7 is assigned to port 1, then traffic received on port 1 and forwarded to the group will be transmitted on port 8 according to the following calculation:
7 (key) modulo 4 (number of ports in the aggregator) = 3 (index), which corresponds to port 8 which has zero-based index 3 in the sorted array of aggregator ports as shown in Aggregator Ports Array:
|0||0, 4, 8, 12||2|
|1||1, 5, 9, 13||4|
|2||2, 6, 10, 14||6|
|3||3, 7, 11, 15||8|
Port-based Load Sharing Limitations
When considering the selection of a LAG algorithm, even distribution over member ports is usually the goal. Full utilization of the LAG‘s bandwidth requires even distribution. In the absence of even distribution, a single member port may become oversubscribed while other member ports are undersubscribed resulting in traffic loss when the LAG, viewed as an aggregate of member ports, is undersubscribed. LAGs distribute best when the diversity of flows destined for the LAG is large relative to the number of ports in the aggregator. For example, when many thousands of L2 flows are destined to a LAG using the “L2” algorithm, distribution on the LAG is typically good (even). Since the number of ports which will switch to a LAG is unlikely to be much larger (orders of magnitude) than the number of ports in the aggregator, extra care may be required from a network administrator when configuring and/or provisioning a switch using port-based LAGs.