In many data centers, latency and jitter are the most important metrics. Even the small amounts of delay (latency) or delay variation (jitter) introduced by a switch can have a profound impact on application performance. In the data center and inside the LAN where devices are only a few meters apart every microsecond counts. Latency is also important limiting factor for high frequency computing (HPC): The more transactions an organization can process in a given unit of time, the more revenue it can expect to realize. In other words the more it takes to process packets, fewer transactions will happen.
To help reduce latency and jitter a cut-through switching architecture is required. Cut-through architecture is one method of design for packet-switching systems.
When a packet arrives at a switch, the switch starts forwarding the packet almost immediately, reading only the first few bytes in the packet to learn the destination address. The opposite approach is the store-and-forward technique that is commonly used in traditional bridges and routers. In this approach, the entire packet is received in a buffer before it is forwarded. The device performs error checking on the packet as well. The entire store-and-forward process takes time and adds latency. Some might say that it store-and-forward reduces error rate but IMO this is just an implementation issue.
As network throughput has increased, the cut-through method became an essential part of switch design. It is no longer possible to hold packets up using the store-and-forward method. Both store-and-forward and cut-through Layer 2 switches base their forwarding decisions on the destination MAC address of data packets. They also learn MAC addresses as they examine the source MAC (SMAC) fields of packets as stations communicate with other nodes on the network.
When a Layer 2 Ethernet switch initiates the forwarding decision, the series of steps that a switch undergoes to determine whether to forward or drop a packet is what differentiates the cut-through methodology from its store-and-forward counterpart.
Whereas a store-and-forward switch makes a forwarding decision on a data packet after it has received the whole frame and checked its integrity, a cut-through switch engages in the forwarding process soon after it has examined the destination MAC (DMAC) address of an incoming frame.
Director xStream and iLinkAgg xStream are based on cut-through architecture providing the necessary ultra low latency and predictable jitter that enables the modern data center architects and designers to build monitoring solutions that meet the business needs of today and tomorrow.
The following diagram illustrates the architecture advantages very nicely:
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