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Understanding How to Use sFlow Technology for Network Monitoring on an EX Series Switch

Sampling Mechanism and Architecture of sFlow Technology on EX Series Switches

sFlow technology uses the following two sampling mechanisms:

• Packet-based sampling: Samples one packet out of a specified number of packets from an interface enabled for sFlow technology.
• Time-based sampling: Samples interface statistics at a specified interval from an interface enabled for sFlow technology.

The sampling information is used to create a network traffic visibility picture. The Juniper Networks Junos operating system (Junos OS) fully supports the sFlow standard described in RFC 3176, InMon Corporation's sFlow: 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 in UDP datagrams, directing those datagrams to the IP address and UDP destination port of the collector. Each datagram contains the following information:

• The IP address of the sFlow agent
• The number of samples
• The interface through which the packets entered the agent
• The interface through which the packets exited the agent
• The source and destination interface for the packets
• The source and destination VLAN for the packets

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.

Note: 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 user-configured 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.

Note: sFlow technology on EX Series switches does not support graceful restart. When a graceful restart occurs, the adaptive sampling rate is set to the user-configured sampling rate.

sFlow Agent Address Assignment

The sFlow collector uses the sFlow agent’s IP address to determine the source of the sFlow data. You can configure the IP address of the sFlow agent to ensure that the agent ID for the sFlow agent remains constant. If you do not specify the IP address to be assigned to the agent, an IP address is automatically assigned to the agent based on the following order of priority of interfaces configured on the switch:

1. Virtual management Ethernet (VME) interface

2. Management Ethernet interface

If neither of the preceding interfaces has been configured, the IP address of any Layer 3 interface or the routed VLAN interface (RVI) is assigned to the agent. At least one interface must be configured on the switch for an IP address to be automatically assigned to the agent. When the agent’s IP address is assigned automatically, the IP address is dynamic and changes when the switch reboots.

sFlow data can be used to provide network traffic visibility information. You can explicitly configure the IP address to be assigned to source data (sFlow datagrams). If you do not explicitly configure that address, the IP address of the configured Gigabit Ethernet interface, 10-Gigabit Ethernet interface, or the routed VLAN interface (RVI) is used as the source IP address.