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OSPF short cut with RSVP and Segment routing based LSP

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  • 1.  OSPF short cut with RSVP and Segment routing based LSP

    Posted 05-25-2020 15:57

    Hi everybody,

    Please consider the following cases:

    CASE A:

    MX1-------MX2-----MX3--SUBNET Y

    Above we are running OSPF , mpls family , rsvp on all  interface.

    MX1 is learning Y subnet via OSPF,  we  have statically defined RSVP LSPMX1-->MX3 

    Goal:

    MX1 must use LSP to send traffic to Y.

    I used OSPF short cut on MX1  and observed  traffic is routed over LSPMX1->MX3 

    Unfortunately, I do not have North Star controller therefore do not know  what to expect, I have following questions:

     

    1) We delete statically defined RSVP LSPMX1-->MX3, and define it again but this time, we delegate the control to North Star Controller. i.e North Star will compute the best path and ask MX1 to signal it.  Will in that case ,  using OSPF short cut on MX1 still acheive the above goal?

     

    2) We delete statically defined RSVP LSPMX1-->MX3 LSP on MX1, we then provisioned RSVP based LSP MX1-->MX3 using North Star Controller, Will in that case ,  using OSPF short cut on MX1 still acheive the above goal?

    ###########################################################################

    CASE B:

    MX1-------MX2-----MX3--SUBNET Y

    Above we are running OSPF , mpls on all interfaces, but we are using segment routing this time i.e no RSVP.

    We want to acheive the same goal  i.e  MX1 must use LSP(MX1-MX3) to send traffic to Y

    Will OSPF short cut   on MX1 still acheive above mentioned goal (MX1 must use LSP(MX1-MX3) to send traffic to Y ) if North Star is used to provisioned MX1-MX3 LSP using segment routing?

     

    Thanks and have a good day!!

     

     

     

     

     

     

     



  • 2.  RE: OSPF short cut with RSVP and Segment routing based LSP
    Best Answer

    Posted 05-25-2020 20:32

    Hello,

     

    In JUNOS, OSPF (or ISIS) shortciuts are independent of undelying RSVP LSP, and apply globally to all RSVP LSP configured on the box.

    Also, SR-TE/SPING-TE shortcuts are not supported in JUNOS.

     

    Now, the answers:

     


    @sarahr202 wrote:

     

    1) We delete statically defined RSVP LSPMX1-->MX3, and define it again but this time, we delegate the control to North Star Controller. i.e North Star will compute the best path and ask MX1 to signal it.  Will in that case ,  using OSPF short cut on MX1 still acheive the above goal?

     

     Yes

     


    @sarahr202 wrote:

     

     

    2) We delete statically defined RSVP LSPMX1-->MX3 LSP on MX1, we then provisioned RSVP based LSP MX1-->MX3 using North Star Controller, Will in that case ,  using OSPF short cut on MX1 still acheive the above goal?

    ###########################################################################

     

    AFAIK no, since this LSP is no longer present in MX1 configuration

     

     


    @sarahr202 wrote:

     

    CASE B:

    MX1-------MX2-----MX3--SUBNET Y

    Above we are running OSPF , mpls on all interfaces, but we are using segment routing this time i.e no RSVP.

    We want to acheive the same goal  i.e  MX1 must use LSP(MX1-MX3) to send traffic to Y

    Will OSPF short cut   on MX1 still acheive above mentioned goal (MX1 must use LSP(MX1-MX3) to send traffic to Y ) if North Star is used to provisioned MX1-MX3 LSP using segment routing?

     

    No, and not supported. You need to use Prefix-SIDs here as I explained to You in the other post.

     

    HTH

    Thx

    Alex



  • 3.  RE: OSPF short cut with RSVP and Segment routing based LSP

    Posted 05-26-2020 07:49

    Hi Alex,

    Thanks for your response .

    @sarahr202 wrote:

     

    CASE B:

    MX1-------MX2-----MX3--SUBNET Y

    Above we are running OSPF , mpls on all interfaces, but we are using segment routing this time i.e no RSVP.

    We want to acheive the same goal  i.e  MX1 must use LSP(MX1-MX3) to send traffic to Y

    Will OSPF short cut   on MX1 still acheive above mentioned goal (MX1 must use LSP(MX1-MX3) to send traffic to Y ) if North Star is used to provisioned MX1-MX3 LSP using segment routing?

     

    No, and not supported. You need to use Prefix-SIDs here as I explained to You in the other post.

    ###########################################################

    Let me get my thoughts across using an illustration:

     

    X-R1----R2---R3---Y

         |_____R4---R5--|

    Above,  R1 sees  "R1-R2-R3" as a shortest path to get to Y.

    North Star has provisioned "LSP R1-R5" from R1 to R5 , this LSP follows along  R1-R4-R5.

    Let's say we are using PREFIX SID for Y, by default, R1 will use shortest path ( R1-R2-R3 to reach Y which is also a labelled path, but we want R1 to use LSP-R1-R5 to reach Y. How can we acheive this goal highligthed above using PREFIX SID ?

    To me it appears to me, there is no otherway but to run IBGP between R1 and R5, this way R1 learns about Y using IBGP with next hop pointed to R5's IP, which in turns will cause R1 to use LSP-R1-R5 for Y. Any thoughts?

    Appreciated!!

     



  • 4.  RE: OSPF short cut with RSVP and Segment routing based LSP

    Posted 05-26-2020 08:03

    Hello,

     

    What You have described is a dual-homed prefix and as You correctly pointed out, by default the best path to it is a shortest-path from IGP PoV.

    If You want to traffic-engineer the path towards this prefix, then Prefix-SID is not the right tool for this job.

    Prefix-SID just allows You to assign a SR label to an arbitrary prefix, not just loopback' /32. 

     


    @sarahr202 wrote:

     

    To me it appears to me, there is no otherway but to run IBGP between R1 and R5, this way R1 learns about Y using IBGP with next hop pointed to R5's IP, which in turns will cause R1 to use LSP-R1-R5 for Y. Any thoughts?

     

     



    Correct and this is Juniper-recommended way to traffic-engineer the paths, and with colored SRTE it is much easier than before.

    HTH

    Thx

    Alex