Dell PowerConnect J-8216 Software Guide for JUNOS version 10.3 - Page 3356

Dell PowerConnect J-Series Ethernet Switch Complete Software Guide for Junos OS 3284 A Method for Monitoring Traffic in Switched and Routed Networks (see http://faqs.org/rfcs/rfc3176.html). NOTE: sFlow technology on the switches samples only raw packet headers. A raw Ethernet packet is the complete Layer 2 network frame. An sFlow monitoring system consists of an sFlow agent embedded in the switch and a centralized collector. The sFlow agent's two main activities are random sampling and statistics gathering. It combines interface counters and flow samples and sends them across the network to the sFlow collector. J-EX Series switches adopt the distributed sFlow architecture. The sFlow agent has two separate sampling entities that are associated with each Packet Forwarding Engine. These sampling entities are known as subagents. Each subagent has a unique ID that is used by the collector to identify the data source. A subagent has its own independent state and forwards its own sample messages to the sFlow agent. The sFlow agent is responsible for packaging the samples into datagrams and sending them to the sFlow collector. Because sampling is distributed across subagents, the protocol overhead associated with sFlow technology is significantly reduced at the collector. If the mastership assignment changes in a Virtual Chassis setup, sFlow technology continues to function. Adaptive Sampling The switches use adaptive sampling to ensure both sampling accuracy and efficiency. Adaptive sampling is a process of monitoring the overall incoming traffic rate on the network device and providing intelligent feedback to interfaces to dynamically adapt their sampling rate to the traffic conditions. Interfaces on which incoming traffic exceeds the system threshold are checked so that all violations can be regulated without affecting the traffic on other interfaces. Every 5 seconds the agent checks interfaces to get the number of samples, and interfaces are grouped based on the slot that they belong to. The top five interfaces that produce the highest number of samples are selected. Using the binary backoff algorithm, the sampling load on these interfaces is reduced by half and allotted to interfaces that have a lower sampling rate. Therefore when the processor limit is reached, the sampling rate is adapted such that it does not load the processor any further. If the switch is rebooted, the adaptive sampling rate is reset to the user-configured sampling rate. Also, if you modify the sampling rate, the adaptive sampling rate changes. The advantage of adaptive sampling is that the switch continues to operate at its optimum level even when there is a change in the traffic patterns in the interfaces. You do not need to make any changes. Because the sampling rate adapts dynamically to changing network conditions, the resources are utilized optimally resulting in a high performance network. Infrequent sampling flows are not reported in the sFlow information, but over time the majority of flows are reported. Based on a defined sampling rate, 1 out of N packets is captured and sent to the collector. This type of sampling does not provide a 100 percent accurate result in the analysis, but it does provide a result with quantifiable accuracy. A polling interval defines how often the sFlow data for a specific interface are sent to the collector, but an sFlow agent can also schedule polling.