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I have a Juniper EX2200 switch running JunOS 10.4R4.5.

Under Configure -> System Properties -> Management Access, I can configure which interface allow management access. There is a mystery interface call jsrv.1.

EX2200 Screen Shot

jsrv.1 is not listed in the configuration anywhere. It does not show up on this list of Juniper interface names. I cannot find an explicit mention of what this interface is used for online, although I see it in other people's show interfaces output.

What is this interface and what is it used for?

EDIT: We upgraded the EX2200 switches to JunOS 12.3R3.4 and the Jsrv.1 interface is gone now. There is only "me" and "lo" now.

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  • 2
    Everything I've found says it's used for an internal loopback, but I can't find details. Bottom line, it's not something that you need to configure.
    – Mike Pennington
    Commented Jan 28, 2014 at 18:13
  • Do you have any sources you can share?
    – myron-semack
    Commented Jan 28, 2014 at 19:35
  • 1
    This NOC project bug, which lists jsrv among all the other known internal interfaces on juniper switches
    – Mike Pennington
    Commented Jan 28, 2014 at 20:52

1 Answer 1

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Interface jsrv.1 is always associated to IP address 128.0.0.127. 

jsrv                    up    up
jsrv.1                  up    up   inet     128.0.0.127/2

An archive at SourceForge has the below command output snippet that shows a connection between 128.0.0.127 and port 6343 (sFlow port)1:

tcpdump -v udp port 6343  -s 1500:
11:52:07.624977 IP (tos 0x0, ttl 254, id 5728, offset 0, flags [none],
 proto UDP (17), length 1484)
   10.1.1.1.60578 > collector.6343: sFlowv5, IPv4 agent 128.0.0.127,
 agent-id 17, seqnum 38496, uptime 1963373233, samples 8, length 1456
    flow sample (1), length 196,
<...snipped...>

There is a Juniper Knowledge Center (cached google link) article that indicates there are 2 separate programs supporting the sFlow function, an agent and a collector.

The sFlow monitoring system consists of an sFlow agent embedded in the EX Switch and has 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. The sFlow collector uses the sFlow agent’s IP address to determine the source of the sFlow data. This KB article explains how the EX Switch assigns the IP address to the sFlow agent.

The jsrv interface appears to be a built-in Juniper sFlow sub-system that allows communication between the internal collector and agent.

1Note, this is the only connection to 128.0.0.127 I've been able to locate.

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6
  • Could you add details that show how you concluded there is a relationship between jsrv and sFlow? That would be worth an upvote :-)
    – Mike Pennington
    Commented Jan 28, 2014 at 20:57
  • @MikePennington The only real thing I have to go on is that tcpdump output, which is why I worded this with "looks like". I can't imagine Juniper would publish documentation on how their internal sFlow components function behind the scenes. So unfortunately, I can't; the above is pretty much everything I've found about the jsrv interface and it's about as good as a shot in the dark.
    – Ryan Foley
    Commented Jan 28, 2014 at 21:04
  • ok, normally we discourage speculation in answers...
    – Mike Pennington
    Commented Jan 28, 2014 at 21:12
  • @MikePennington You're right, I didn't really connect that IP address to jsrv, I'll edit to add clarity.
    – Ryan Foley
    Commented Jan 28, 2014 at 21:20
  • I'm still surprised this is not officially documented anywhere.
    – myron-semack
    Commented Jan 29, 2014 at 14:10

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