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What do interfaces em0 and em1 refer to when SRX/vSRX devices are in a Chassis Cluster?



Article ID: KB21706 KB Last Updated: 18 May 2021Version: 4.0

This article explains what the em0 and em1 interfaces refer to when SRX/vSRX devices are in a Chassis Cluster.



Chassis cluster moves into a bad state due to a miss of internal communication heart-beats.

In order to understand the issue better, we need to understand the role of the em0 link in a chassis cluster.

  • Both em0 and em1 are internal interfaces that connect between the Routing Engine (RE) and the Control Board (CB). There is no correlation between em0/em1 and any physical interfaces.
  • The em0 in VSRX is an internal link that is enabled by default. In some of the Juniper boxes, em0 is another link useful for management like fxp0. But in the case of vsrx, don't get confused with it because of the same name.
  • A  user does NOT have an option to enable/disable this em0 link in case of a VSRX cluster. 
  • If you check interface extensive output for em0, you will see that it has an internal IP address e.g which confirms that its an internal link.
sroot@rick-sa-vsrx-vSRX> show interfaces em0 extensive 
Physical interface: em0, Enabled, Physical link is Up
  Interface index: 64, SNMP ifIndex: 17, Generation: 1
  Type: Ethernet, Link-level type: Ethernet, MTU: 1514, Clocking: Unspecified, Speed: 1000mbps
  Device flags   : Present Running
  Interface flags: SNMP-Traps
  Link type      : Full-Duplex
  Physical info  : Unspecified
  Hold-times     : Up 0 ms, Down 0 ms
  Current address: 02:00:00:00:00:04, Hardware address: 02:00:00:00:00:04
  Alternate link address: Unspecified
  Last flapped   : 2020-12-09 23:44:04 UTC (15w0d 05:08 ago)
  Statistics last cleared: Never
  Traffic statistics:
   Input  bytes  :          23061704421
   Output bytes  :          15576899386
   Input  packets:            231316513
   Output packets:            183694433
  Input errors:
    Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Giants: 0, Policed discards: 0, Resource errors: 0


  Logical interface em0.0 (Index 4) (SNMP ifIndex 18) (Generation 3)
    Flags: Up SNMP-Traps 0x4000000 Encapsulation: ENET2
    Traffic statistics:
     Input  bytes  :          23061706977
     Output bytes  :          15576902368
     Input  packets:            231316541
     Output packets:            183694460
    Local statistics:
     Input  bytes  :          23061706977
     Output bytes  :          15576902368
     Input  packets:            231316541
     Output packets:            183694460
    Protocol inet, MTU: 1500
    Max nh cache: 100000, New hold nh limit: 100000, Curr nh cnt: 1, Curr new hold cnt: 0, NH drop cnt: 0
    Generation: 143, Route table: 1
      Flags: Is-Primary
      Addresses, Flags: Is-Preferred Is-Primary
        Destination: 128/2, Local:, Broadcast:, Generation: 1   


These are the main functionalities of the em0 link in a chassis cluster of a VSRX:
RE on Primary node will communicates with:
  • Local vSRX PFE (data plane) running on WRL of Primary node.
  • Remote vSRXPFE running on WRL of Secondary node, and
  • Remote RE running on Secondary node. 
via em0 internal link.
Modification History:

2021-04-19: Detailed information about em0 link role in VSRX node is added.
2018-12-21: Article reviewed for accuracy. Very minor modifications (non-technical) made. Article is correct and complete.


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