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Logical unit number / sub interface

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  • 1.  Logical unit number / sub interface

    Posted 03-14-2019 02:12

    Hi,

     

    Sorry to sound absolutely green.

    Just started on Juniper platform and is not really used to the concept of logical unit number of the physical interface.

     

    Been seeing these below output in my switch config

     

    em2 up up
    em2.32768 up up inet 192.168.1.3/24
    
    vme up up
    vme.0 up up inet 192.168.0.203/24


    q1) How does one decide to use a logical unit number of 0 ? and a logical unit number of 32768 ? - if the purpose is to just give the interface an IP address.

    I also see some logical unit number of 16385, in the loopback interface

     

    lo0 up up
    lo0.16385 up up inet

     

    Regards,

    Alan



  • 2.  RE: Logical unit number / sub interface

     
    Posted 03-14-2019 02:47

    Hi alankoh,

     

    Please use unit 0 in these cases, the em2.32768 and lo0.16385 are system generated and reserved for internal/control and not for user configuration.  Mostly, unit numbers above 16K are all reserved for special purposes.

     

    Hope this helps.

     

    Regards,
    -r.

    --------------------------------------------------

    If this solves your problem, please mark this post as "Accepted Solution."
    Kudos are always appreciated :).



  • 3.  RE: Logical unit number / sub interface
    Best Answer

    Posted 03-14-2019 02:52

    A physical interface can consists of multiple logical interfaces, each called a unit. Each unit has a number as a unique identifier. The 0 just refers to the first logical interface.
    If you are configuring a point-to-point circuit or non-VLAN-tagged Ethernet, the logical unit number must be zero.
    Logical unit 32767  is created to accept untagged (control packets like LACP) packets that come in on an interface.
    Logical interface lo0.16385 is the loopback interface for the internal routing instance. Created by the internal routing service process, this interface facilitates internal traffic. It prevents any filter created on loopback lo0.0 from blocking internal traffic.
    The logical unit number can be in the range 0 through 65,535 for demux and PPPoE static interfaces only. The logical unit number can be in the range 0 through 16,385 for all other static interface types.

     

     



  • 4.  RE: Logical unit number / sub interface

    Posted 03-19-2019 09:12

    Hi Both,

     

    How do we actually know which interfaces/logical units are system-generated and can be ignored ? Is there any documentation for this ?

     

    E.g. I got this em2 interface that is up with logical unit 32768 (even higher then 32767)  and it has an IP assigned .

    But i cannot find this interface in my configuration.

    admin> show interfaces terse
    
    em2 up up
    em2.32768 up up inet 192.168.1.3/24

    Any idea how/can i remove this ? How do i even disable this interface ?

    I am using qfx5100-48s-6q

     

    Regards,

    Alan



  • 5.  RE: Logical unit number / sub interface

    Posted 03-19-2019 09:31

    Hi Alan,

     

    em2 used exclusevely for communication between two Junos VMs, for example for control-plane protocols - NRS, NSB, GRES.

    There is no need to remove or disable this interface.

     

    Regards,

    Roman



  • 6.  RE: Logical unit number / sub interface

    Posted 03-20-2019 10:22

    Hi Roman,

     

    So it is consider normal to "not" find em2 interface inside the configuration ?

    My point is - how do we verify/know if an interface is a "system generated" and can be ignored  ?

     

    Regards,

    Alan



  • 7.  RE: Logical unit number / sub interface

    Posted 03-20-2019 10:53
    Please refer the url given below to understand the type of interfaces available on Junos. Also refer the links given in Related Documentation Section for more details:
    https://www.juniper.net/documentation/en_US/junos/topics/concept/interfaces-types-of-interfaces-overview.html