Traffic Sampling, Forwarding, and Monitoring Overview

Traffic sampling allows you to sample IP traffic based on particular input interfaces and various fields in the packet header. You can also use traffic sampling to monitor any combination of specific logical interfaces, specific protocols on one or more interfaces, a range of addresses on a logical interface, or individual IP addresses. Information about the sampled packets is saved to files on the router's hard disk.

Traffic sampling is not meant to capture all packets received by a router. We do not recommend excessive sampling (a rate greater than 1/1000 packets), because it can increase the load on your processor. If you need to set a higher sampling rate to diagnose a particular problem or type of traffic received, we recommend that you revert to a lower sampling rate after you discover the problem or troublesome traffic. In addition, traffic sampling and forwarding are supported only on routers equipped with an Internet Processor II application-specific integrated circuit (ASIC). To determine whether a routing platform has an Internet Processor II ASIC, use the show chassis hardware command.

Junos OS supports both per-packet and per-flow load balancing. In Junos OS Release 9.0 and later, you can configure per-prefix load balancing. This feature enables the router to elect the next hop independent of the route chosen by other routers. The result is a better utilization of available links. Likewise, you can configure Junos OS so that, for the active route, all next-hop addresses for a destination are installed in the forwarding table. This is called per-packet load balancing, which you can use to spread traffic across multiple paths between routers.

With forwarding policies, you can configure per-flow load balancing, port mirroring, and domain name system (DNS) or Trivial File Transfer Protocol (TFTP) forwarding.

Note:

On SRX devices, sampling/J-Flow cannot be done for the packets that are handled by PowerMode IPsec (PMI) or Services Offload (SOF). Sampling/J-Flow counters will not provide the exact number of packets that are received on the interface.