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ISIS Attached Bit questions

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  • 1.  ISIS Attached Bit questions

    Posted 02-26-2020 13:54

    ISIS-diag.JPG

    I captured LSP packets on vMX1 interface ge-0/0/1 facing vMX3. I got the following two LSP packets. One has attached bit on, the other has attached bit off. They have differnet LSP_ID I highlighted.

    Can someone explain to me the reasons ?

    root@vMX1# run show interfaces ge-0/0/1 | match Curr
      Current address: 00:05:86:71:d8:01, Hardware address: 00:05:86:71:d8:01
    

    ISIS-attachedBit-1.JPG

    ISIS-attachedBit-2.JPG

    thanks !!



  • 2.  RE: ISIS Attached Bit questions

     
    Posted 02-26-2020 20:15

    Hello Gongyayu

     

    On the vmx nodes are you running the isis inside a vrf or in the global routing instance ?

    Also both the vmx1 and vmx3 are level 1 only and still you are seeing the attached bit right ?

    It would be helpful if you can paste the isis configuration here.



  • 3.  RE: ISIS Attached Bit questions

    Posted 02-27-2020 08:55
    vMX1
    root@vMX1# show | display set
    set version 14.1R1.10
    set system host-name vMX1
    set interfaces ge-0/0/0 unit 0 family inet address 10.10.4.11/24
    set interfaces ge-0/0/0 unit 0 family iso
    set interfaces ge-0/0/1 unit 0 family inet address 172.16.2.11/24
    set interfaces ge-0/0/1 unit 0 family iso
    set interfaces ge-0/0/2 unit 0 family inet address 10.10.6.11/24
    set interfaces ge-0/0/2 unit 0 family iso
    set interfaces ge-0/0/3 unit 0 family inet address 10.10.1.11/24
    set interfaces ge-0/0/3 unit 0 family iso
    set interfaces lo0 unit 0 family iso address 49.0124.0000.0000.0011.00
    set protocols isis export Exp2ISIS
    set protocols isis interface ge-0/0/0.0 level 1 disable
    set protocols isis interface ge-0/0/1.0 level 2 disable
    set protocols isis interface ge-0/0/2.0 level 1 disable
    set protocols isis interface ge-0/0/3.0 level 1 disable
    set protocols isis interface lo0.0
    set policy-options policy-statement Exp2ISIS term 1 from protocol isis
    set policy-options policy-statement Exp2ISIS term 1 from route-filter 172.31.2.0/24 exact
    set policy-options policy-statement Exp2ISIS term 1 then accept
    set policy-options policy-statement Exp2ISIS term 2 then reject
    set policy-options policy-statement isis-l2-into-l1 from protocol isis
    set policy-options policy-statement isis-l2-into-l1 from level 2
    set policy-options policy-statement isis-l2-into-l1 to level 1
    set policy-options policy-statement isis-l2-into-l1 then accept

    vMX3
    root@vMX3# show | display set set version 14.1R1.10 set system host-name vMX3 set logical-systems vMX-LR3 interfaces lt-0/0/0 unit 1 encapsulation ethernet set logical-systems vMX-LR3 interfaces lt-0/0/0 unit 1 peer-unit 0 set logical-systems vMX-LR3 interfaces lt-0/0/0 unit 1 family inet address 172.31.2.12/24 set logical-systems vMX-LR3 interfaces lt-0/0/0 unit 1 family iso set logical-systems vMX-LR3 interfaces lo0 unit 1 family iso address 49.0124.0000.0000.0023.00 set logical-systems vMX-LR3 protocols isis interface lt-0/0/0.1 level 2 disable set logical-systems vMX-LR3 protocols isis interface lo0.1 set chassis fpc 0 pic 0 tunnel-services bandwidth 10g set interfaces lt-0/0/0 unit 0 encapsulation ethernet set interfaces lt-0/0/0 unit 0 peer-unit 1 set interfaces lt-0/0/0 unit 0 family inet address 172.31.2.11/24 set interfaces lt-0/0/0 unit 0 family iso set interfaces ge-0/0/1 unit 0 family inet address 172.16.2.13/24 set interfaces ge-0/0/1 unit 0 family iso set interfaces lo0 unit 0 family iso address 49.0124.0000.0000.0013.00 set protocols isis interface lt-0/0/0.0 set protocols isis interface ge-0/0/1.0 level 2 disable set protocols isis interface lo0.0

    thanks for taking a look !!



  • 4.  RE: ISIS Attached Bit questions

     
    Posted 02-27-2020 22:22

    Hi Gongyayu

     

    You will see the attached bit in the cases where either you are having ISIS connections inside of a routing-instance or a logical system. If you have isis L1 only adjacency between to different nodes (no under routing instance or logical system) then you will not see the attached bit in the LSP.

    The explanation by aarseniev is the reason why this is behaving so.



  • 5.  RE: ISIS Attached Bit questions

    Posted 02-28-2020 03:42

    Hello,

     


    @raviky wrote:

    Hi Gongyayu

     

    You will see the attached bit in the cases where either you are having ISIS connections inside of a routing-instance or a logical system.

     

    It does not matter, ISIS adjacency in GRT behaves the same way.

     


    @raviky wrote:

    If you have isis L1 only adjacency between to different nodes (no under routing instance or logical system) then you will not see the attached bit in the LSP.


    Again, routing-instance/logical system or GRT does not matter. What matters is L2 IS (not L1!) being able to reach another L2 IS.

    In plain English - ATT bit is seen in L1 LSP being sent by L1/L2 IS but it's L2 part of L1/L2 IS which needs to reach (i.e. calculate the SPF towards) another L2 IS.

    ISO 10589 standard is quite authoritative in this regard, google for it.

    HTH

    Thx

    Alex

     

     

     



  • 6.  RE: ISIS Attached Bit questions

    Posted 03-09-2020 06:24

    thanks so much !



  • 7.  RE: ISIS Attached Bit questions
    Best Answer

    Posted 02-27-2020 03:33

    Hello,

     


    @gongyayu wrote:

     

    I captured LSP packets on vMX1 interface ge-0/0/1 facing vMX3. I got the following two LSP packets. One has attached bit on, the other has attached bit off. They have differnet LSP_ID I highlighted.

     

     

    The LSP with ATT==0 is from DIS/pseudonode.

    And JUNOS does not run SPF on behalf of pseudonode, it only considers pseudonode as transit node for LAN. 

    Since ISO 10589 standard says that in order to set ATT bit, the L2 IS must determine (i.e. run SPF) that it can reach other L2 IS, then DIS/pseudonode does not set ATT bit.

    To avoid this "problem" by eliminating DIS election, use point-to-point ISIS link type on Ethernet

    https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/point-to-point-edit-protocols-isis.html

    HTH

    Thx

    Alex

     

     



  • 8.  RE: ISIS Attached Bit questions

    Posted 02-27-2020 08:58

    thanks so much !!!



  • 9.  RE: ISIS Attached Bit questions

    Posted 03-09-2020 11:17

    Awesome reply

     

    I saw this once in the past with cisco, ended up reviewing the operation of ISIS for multi access circuits to find out that it was n

    needed to just set the link for point-to-point.

     

    Nice job! Kudos for you.

     

    🙂