Routing

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Question on ISIS Router Type

  • 1.  Question on ISIS Router Type

     
    Posted 30 days ago

    Hi.

     

    I have an L2 ISIS router as follows:

    set logical-systems R2 protocols isis level 1 disable

    set logical-systems R2 protocols isis interface all

     

    However, the router is falsely advertising himself as an L1L2 router in the attributes field of its LSP:

    IS-IS level 2 link-state database:

    LSP ID                      Sequence Checksum Lifetime Attributes

    vMX-addr-0-R2.00-00              0x2   0x39e0     1098 L1 L2

     

    Shouldn't the Attributes field say 'L2' instead of 'L1 L2' ?

     

    Thanks,

    Deepak


    Juniper Business Use Only



  • 2.  RE: Question on ISIS Router Type

    Posted 29 days ago
    Take a look at the output there, only the section for level 2 on the link-state database (LSDB) is showing in the LSP, which means the IS is showing the LSP as level 2. Also, in the LSP information, you will see the 'Level 2' flag set.

    Elvin


  • 3.  RE: Question on ISIS Router Type

     
    Posted 29 days ago
    Thanks Elvin.

    But why does the Attributes field say L1L2 instead of L2? 
    (This is more of an academic question than a practical one).

    --Deepak


  • 4.  RE: Question on ISIS Router Type

    Posted 27 days ago
    I see this in some notes I had from a previous lab I did... I had disabled L1 on some interfaces, and I see this as so with "show isis interface"... where I see Level 1 is Disabled.


    set protocols isis interface lo0.4 level 1 disable

    set protocols isis interface lt-0/1/0.34 level 1 disable

    set protocols isis interface lt-0/1/0.35 level 1 disable

    set protocols isis interface lt-0/1/0.3000 level 1 disable


    [edit]

    P-1(r4)#run show isis interface

    IS-IS interface database:

    Interface             L CirID Level 1 DR        Level 2 DR        L1/L2 Metric

    lo0.4                 3   0x1                   Passive                 0/0

    lt-0/1/0.3000         2   0x1 Disabled          Down                   10/10

    lt-0/1/0.34           2   0x1 Disabled          Down                   10/10

    lt-0/1/0.35           2   0x1 Disabled          Down                   10/10



    ------------------------------
    Aaron Gould
    Senior Network Engineer
    aaron@gvtc.com
    https://www.linkedin.com/in/agould123/
    ------------------------------



  • 5.  RE: Question on ISIS Router Type

    Posted 27 days ago
    This academic question needs an academic answer :)
    Therefore it's necessary to have a look at the RFC INTERNATIONAL ISO/IEC STANDARD 10589.
    And what does it say about the LSP attribute describing the node capability ?

    Two things:

    Level 2 Intermediate Systems: These systems act as Level 1 Intermediate systems in addition to acting as a system in the subdomain consisting of level 2 ISs. Systems in the level 2 subdomain route towards a destination area, or another routeing domain.

    and

    IS Type – Bits 1 and 2 indicate the type of Intermediate System – One of the following values:
    x 0 – Unused value

    x 1 – ( i.e. bit 1 set) Level 1 Intermediate system
    
x 2 – Unused value
    
x 3 – (i.e. bits 1 and 2 set) Level 2 Intermediate system.


    Therefore: a «Level 2 system» is in fact is a Level1+2 system (therefore the JunOS terminology L1 L2 is correct).
    If Level 1 is «disabled» on the router, it really means that no attached links are generating L1 information (and this is the usual way to use ISIS today actually: one single L2 «area» and nothing more).

    ------------------------------
    Olivier Benghozi
    ------------------------------



  • 6.  RE: Question on ISIS Router Type

     
    Posted 27 days ago
    Thanks Olivier for your answer.

    So the IS Type field does not make any distinction between an L1L2 router and an 'only' L2 Router. Thus there is no way to get the IS Type field to say 'L2'. It can only say either 'L1' or 'L1 L2'.

    Thanks,
    Deepak