What's the inline monitoring?

I searched wikipedia and google but do not find the meaning of inline monitoring for network.

who can tell me the definition of it?

I can get the definition of passive monitoring .

whether the inline monitoring is opposite of passive monitoring?


The inline-monitoring provenance.

  • We need to see the term in context, as there is not a set definition of it. Please edit your question to include a link to the source, or quote the paragraph where you found it.
    – Ron Trunk
    Mar 13, 2020 at 12:11
  • I have updated my post, please check it.
    – aircraft
    Mar 13, 2020 at 14:58

2 Answers 2


With regard to Juniper devices, the answer is right in the document you linked to:

When you enable inline monitoring, you can monitor actual IPv4 and IPv6 packets at different sampling rates, and export the actual packet up to the configured clip length. By default, Junos OS supports a maximum packet length of 126 byte starting with the Ethernet header. The monitored packets are exported to an collector for further processing. The packets are exported in an IPFIX format, which includes information on the original packet size, and incoming or outgoing interface.

Juniper inline monitoring is roughly equivalent to Cisco span port.

  • So, the inline monitoring is not a universal concept in networking as same as passive monitoring? it's just in Juniper vendor's concept ?
    – aircraft
    Mar 13, 2020 at 15:07
  • This is how Juniper uses the term. Other companies may use it differently. There is no "universal" meaning.
    – Ron Trunk
    Mar 13, 2020 at 15:35

Inline monitoring would typically imply that some type of traffic capturing device is added in the middle of two points (inline) that traffic traverses. A passive monitoring implementation instead receives a copy of the traffic from some device that is already inline.

An example of passive would be configuring a switch with SPAN ports to replicate traffic to another set of ports that attach to a dedicated network sniffer appliance.

An example of inline would be cabling the sniffer to be in the path of the traffic, e.g., PC->Switch->Sniffer->Firewall. Traffic passes through the sniffer as if it were just another switch.

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