In the context of the IEEE 802.1AX standard, link aggregation provides methods to combine multiple Ethernet interfaces, forming a single link layer interface, specific to a switch or server. Therefore, link aggregation is implemented between a single switch and a PowerScale node, not across PowerScale nodes.

Implementing link aggregation is neither mandatory nor is it necessary, rather it is based on workload requirements. Implementing link aggregation is recommended if a transparent failover or switch port redundancy is required.

Link aggregation assumes all links are full duplex, point to point, and at the same data rate, providing graceful recovery from link failures. If a link fails, traffic is automatically sent to the next available link without disruption.

It is imperative to understand that link aggregation is not a substitute for a higher bandwidth link. Although link aggregation combines multiple interfaces, applying it to multiply bandwidth by the number of interfaces for a single session is incorrect. Link aggregation distributes traffic across links. However, a single session only uses a single physical link to ensure packets are delivered in order without duplication of frames.

As part of the IEEE 802.1AX standard, the Frame Distributor does not specify a distribution algorithm across aggregated links but enforces that frames must be sent in order without duplication. Frame order is maintained by ensuring that all frames of a given session are transmitted on a single link in the order that they are generated by the client. The mandate does not allow for additions or modifications to the MAC frame, buffering, or processing to re-order frames by the Frame Distributor or Collector.

Thus, the bandwidth for a single client is not increased, but the aggregate bandwidth of all clients increases in an active/active configuration. The aggregate bandwidth is realized when carrying multiple simultaneous sessions. It might not provide a linear multiple of each link’s data rate because each individual session uses a single link.

Another factor to consider depends on the workload--certain protocols might or might not benefit from link aggregation. Stateful protocols such as NFSv4 and SMBv2 benefit from link aggregation as a failover mechanism. On the contrary, SMBv3 Multichannel automatically detects multiple links, using each for maximum throughput and link resilience.

Table 1.      Link aggregation

OneFS supports round robin, failover, load-balance, and LACP link aggregation methods. In previous releases, FEC was also listed as an option. However, FEC was simply the naming convention for load-balance. In OneFS 8.2, load-balance replaces the FEC option.

Multi-chassis link aggregation

As discussed in the previous section, the IEEE 802.1AX standard does not define Link Aggregation between multiple switches and a PowerScale node. However, many vendors provide this functionality through proprietary features. Multiple switches are connected with an Inter-Switch link or other proprietary cable and communicate by a proprietary protocol forming a virtual switch. A virtual switch is perceived as a single switch to a PowerScale node, with links terminating on a single switch. The ability to have link aggregation split with multiple chassis provides network redundancy if a single chassis were to fail.

Each vendor has a proprietary implementation of Multi-Chassis Link Aggregation, but externally the virtual switch created is compliant with the IEEE 802.1AX standard.

Regarding bandwidth, the concepts discussed for single switch Link Aggregation still apply to Multi-Chassis Link Aggregation. Also, because the multiple switches form a single virtual switch, it is important to understand what happens if the switch hosting the control plane fails. Those effects vary by the vendor’s implementation but will affect the network redundancy gained through Multi-Chassis Link Aggregation.