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Overview of sFlow Technology


The sFlow technology is a monitoring technology for high-speed switched or routed networks. sFlow monitoring technology collects samples of network packets and sends them in a UDP datagram to a monitoring station called a collector. You can configure sFlow technology on a device to monitor traffic continuously at wire speed on all interfaces simultaneously. You must enable sFlow monitoring on each interface individually; you cannot globally enable sFlow monitoring on all interfaces with a single configuration statement. Junos OS supports the sFlow technology standard described in RFC 3176, InMon Corporation's sFlow: A Method for Monitoring Traffic in Switched and Routed Networks.

sFlow technology implements 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. Only the first 128 bytes of each packet are sent to the collector. Data collected include the Ethernet, IP, and TCP headers, along with other application-level headers (if present). Although this type of sampling might not capture infrequent packet flows, the majority of flows are reported over time, allowing the collector to generate a reasonably accurate representation of network activity. You configure packet-based sampling when you specify a sample rate.

  • Time-based sampling—Samples interface statistics (counters) at a specified interval from an interface enabled for sFlow technology. Statistics such as Ethernet interface errors are captured. You configure time-based sampling when you specify a polling interval.

An sFlow monitoring system consists of an sFlow agent embedded in the device and up to four external collectors. On a QFX Series standalone switch, the sFlow agent performs packet sampling and gathers interface statistics, and then combines the information into UDP datagrams that are sent to the sFlow collectors. An sFlow collector can be connected to the switch through the management network or data network. The software forwarding infrastructure daemon (SFID) on the switch looks up the next-hop address for the specified collector IP address to determine whether the collector is reachable by way of the management network or data network.


On the QFX Series standalone switches, if you configure sFlow technology monitoring on multiple interfaces and a high sampling rate, we recommend that you specify a collector that is on the data network instead of the management network. Having a high volume of sFlow technology monitoring traffic on the management network might interfere with other management interface traffic.

On a QFabric system, the sFlow technology architecture is distributed. The global sFlow technology configuration defined on the QFabric system Director device is distributed to Node groups that have sFlow sampling configured on their interfaces. The sFlow agent has a separate sampling entity, known as a subagent, running on each Node device. Each subagent has its own independent state and forwards its own sample information (datagrams) directly to the sFlow collectors.

On the QFabric system, an sFlow collector must be reachable through the data network. Because each Node device has all routes stored in the default routing instance, the collector IP address should be included in the default routing instance to ensure the collector’s reachability from the Node device.

Regardless of the rate of traffic or the configured sampling interval, a datagram is sent whenever its size reaches the maximum Ethernet transmission unit (MTU) of 1500 bytes, or whenever a 250-ms timer expires, whichever occurs first. The timer ensures that a collector receives regularly sampled data.

To ensure sampling accuracy and efficiency, QFX Series devices use adaptive sFlow sampling. Adaptive sampling monitors the overall incoming traffic rate on the device and provides feedback to the interfaces to dynamically adapt their sampling rate to traffic conditions. The sFlow agent reads the statistics on the interfaces every 5 seconds and identifies five interfaces with the highest number of samples. On a standalone switch, when the CPU processing limit is reached, a binary backoff algorithm is implemented to reduce the sampling load of the top five interfaces by half. The adapted sampling rate is then to those top five interfaces.

On a QFabric system, sFlow technology monitors the interfaces on each Node device as a group, and implements the binary backoff algorithm based on the traffic on that group of interfaces.

Using adaptive sampling prevents overloading of the CPU and keeps the device operating at its optimum level even when there is a change in traffic patterns on the interfaces. The reduced sampling rate is used until the device is rebooted or when a new sampling rate is configured.

The sFlow collector uses the IP address of the sFlow agent 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 assign an IP address to the agent, an IP address will be assigned to the agent using the IP address of a configured interface.

On the QFX Series standalone switches, the following priority is used to determine which interface will be used:

  1. Management Ethernet interface me0 IP address

  2. Any Layer 3 interface if the me0 IP address is not available

If a particular interface is not configured, the IP address of the next interface in the priority list is used as the IP address for the agent. Once an IP address is assigned to the agent, the agent ID is not modified until the sFlow service is restarted. At least one interface has to be configured for an IP address to be assigned to the agent.

In addition, you can explicitly configure the IP address for the source data (sFlow datagrams). On the QFX Series standalone switches, if you do not configure that address, the following priority is used:

  • Any Layer 3 interface IP address

  • The me0 IP address if no Layer 3 interface IP address is available

On the QFabric system, the following default values are used if the optional parameters are not configured:

  • Agent ID is the management IP address of the default partition.

  • Source IP is the management IP address of the default partition.

In addition, the QFabric system subagent ID (which is included in the sFlow datagrams) is the ID of the Node group from which the datagram is sent to the collector.


On QFX5100 standalone switches and the QFX Series Virtual Chassis (with QFX3500 and QFX3600 switches), egress firewall filters are not applied to sFlow sampling packets. On these platforms, the software architecture is different from that on other QFX Series devices, and sFlow packets are sent by the Routing Engine (not the line card on the host) and are not transiting the switch. Egress firewall filters affect data packets that are transiting a switch but do not affect packets sent by the Routing Engine. As a result, sFlow sampling packets are always sent to the sFlow collector.

On the QFX Series, limitations of sFlow traffic sampling include:

  • sFlow sampling on ingress interfaces does not capture CPU-bound traffic.

  • sFlow sampling on egress interfaces does not support broadcast and multicast packets.

  • Egress samples do not contain modifications made to the packet in the egress pipeline.

  • If a packet is discarded because of a firewall filter, the reason code for discarding the packet is not sent to the collector.

  • The out-priority field for a VLAN is always set to 0 (zero) on ingress and egress samples.

  • You cannot configure sFlow monitoring on a link aggregation group (LAG), but you can configure it individually on a LAG member interface.