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    Understanding How to Use sFlow Technology for Network Monitoring on a MX Series Router

    The sFlow technology is a monitoring technology for high-speed switched or routed networks. sFlow monitoring technology randomly samples network packets and sends the samples to a monitoring station called a collector. You can configure sFlow technology on a Juniper Networks router to continuously monitor traffic at wire speed on all interfaces simultaneously.

    This topic describes:

    Benefits of sFlow Technology on a MX Series Router

    • sFlow can be used by software tools like a network analyzer to continuously monitor tens of thousands of switch or router ports simultaneously.
    • Since sFlow uses network sampling (forwarding one packet from ‘n’ number of total packets) for analysis, it is not resource intensive (for example processing, memory and more). The sampling is done at the hardware application-specific integrated circuits (ASICs) and hence it is simple and more accurate.

    Sampling Mechanism and Architecture of sFlow Technology on a MX Series Router

    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. 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. To configure packet-based sampling, you must specify a sample rate.
    • Time-based sampling—Samples interface statistics at a specified interval from an interface enabled for sFlow technology. Statistics such as Ethernet interface errors are captured. To configure time-based sampling, you must specify a polling interval.

    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).

    An sFlow monitoring system consists of an sFlow agent embedded in the router 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 as 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

    Caution: In case of dual VLAN, all fields may not be reported.

    MX Series devices can adopt the distributed sFlow architecture. The sFlow agent has sampling entities known as subagents. Each subagent is responsible for monitoring a set of network ports and 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: You cannot configure sFlow monitoring on a link aggregation group (LAG), but you can configure it individually on a LAG member interface.

    Infrequent sampling flows might not be reported in the sFlow information, but over time the majority of flows are reported. Based on a configured sampling rate N, 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: We recommend that you configure the same sample rate for all the ports in a line card. If you configure different sample rates, the lowest value is used for all ports on the line card..

    Note: If the mastership assignment changes in a Virtual Chassis setup, sFlow technology continues to function.

    Adaptive Sampling

    To ensure sampling accuracy and efficiency, MX Series routers 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 few seconds (12 seconds for MX Series routers).

    On a Flexible PIC Concentrator (FPC), when the flow sample limit is reached because of sflow sample processing, a binary backoff algorithm is initiated. This reduces the sampling load on that FPC by half. The backoff algorithm achieves this by doubling the sampling rate on the FPC. This process is repeated until the CPU-load due to sflow on the given FPC comes down to an acceptable level.

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

    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.

    Note: sFlow technology on MX Series routers 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 of 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 device:

    MX Series Devices

    1. Virtual Management Ethernet (VME) interface
    2. Management Ethernet interface

    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. When the agent’s IP address is assigned automatically, the IP address is dynamic and changes when the router reboots.

    sFlow data can be used to provide network traffic visibility information. You can explicitly configure the source IP address to be assigned to the sFlow datagrams. If you do not explicitly configure the IP address, the IP address of any of the configured Layer 3 network interfaces is used as the source IP address. If a Layer 3 IP address is not configured, then the agent IP address is used as the source IP address.

    sFlow Limitations on Routers

    On the MX Series, limitations of sFlow traffic sampling include the following:

    • Trio chipset cannot support different sampling rate for each family. Hence, only one sampling rate can be supported per FPC.
    • Adaptive load balancing is applied per FPC and not for per interface under the FPC.
    • True OIF (outgoing interface) is not supported with sFlow.

    MX Series routers support configuration of only one sampling rate (inclusive of ingress and egress rates) on an FPC. To support compatibility with the sflow configuration of other Juniper Networks products, MX Series routers still accept multiple rate configuration on different interfaces of the same FPC. However, the router programs the lowest rate as the sampling rate for all the interfaces of that FPC. The sFlow show command (show sflow interfaces) displays the configured rate and the actual (effective) rate. However, different rates on different FPCs is still supported on MX Series routers.

    Modified: 2018-02-19