vMX

Expand all | Collapse all

Symmetric LSP questions

Jump to Best Answer
  • 1.  Symmetric LSP questions

    Posted 09-12-2020 17:58

    Hi everyone,

    Let say we have two LSP1 and LSP2 between location A and B, we configured these LSPs as symmetric on North Star controller.

    1) Because of topology change, these LSP can no longer remain symmetric, what would happen next? Will North Star allow these LSPs to be routed over different  common link/nodes or North star will not provision them because being symmetric is not possible for these LSP?

    Thanks and have a good weekend!!

     

     

     



  • 2.  RE: Symmetric LSP questions

    Posted 09-12-2020 18:32

    https://www.juniper.net/documentation/en_US/northstar3.2.0/topics/concept/northstar-web-ui-appl-prov-div-lsp.html

     

     

    NOTE

    If NorthStar Controller is not able to achieve the diversity level you request, it still creates the diverse tunnel pair, using a diversity level as close as possible to the level you requested.



  • 3.  RE: Symmetric LSP questions
    Best Answer

    Posted 09-13-2020 01:58

    Hello,

     


    @sarahr202 wrote:

     

    1) Because of topology change, these LSP can no longer remain symmetric, what would happen next? Will North Star allow these LSPs to be routed over different  common link/nodes or North star will not provision them because being symmetric is not possible for these LSP?

     


     

    I think the confusion lies in a definition what is a "symmetric" LSP.

    According to Northstar User Guide, "symmetric" is the same as "corouted bidirectional" LSP (which have been supported in JUNOS for years). As such, it is not possible to break LSP "symmetry" by simply changing the topology.

    Say, You have LSP-A->Z which goes via link1->link2->link9->link66.

    Then to be "symmetric", LSP-Z->A will have to go link66->link9->link2->link1. 

    Say, the alternate path to avoid link9 is to use link2->link19->link29.

    Hence, if You break link 9, the LSPs can still remain "symmetric" by using a longer path link1<->link2<->link19<->link29<->link66 (except for a short time while LSPs resignal).

     

    It is possible to break LSP "symmetry" by introducing unidirectional links (not supported in JUNOS), or adding bandwidth constraints to the LSPs and limiting available BW in one direction only (by playing with "protocols rsvp" stanza, making subscription different on adjacent nodes). Both of these tricks are good for lab but very harmful for production.

     

    Hope this makes sense.

    HTH

    Thx

    Alex

     

     



  • 4.  RE: Symmetric LSP questions

    Posted 09-13-2020 16:27

    Thanks Alex,

    Say, You have LSP-A->Z which goes via link1->link2->link9->link66.

    Then to be "symmetric", LSP-Z->A will have to go link66->link9->link2->link1. 

    Say, the alternate path to avoid link9 is to use link2->link19->link29.

    Hence, if You break link 9, the LSPs can still remain "symmetric" by using a longer path link1<->link2<->link19<->link29<->link66 (except for a short time while LSPs resignal).

     

    Let say we extend this example as below:

     

    Let say LSP-Z->A needs 10M, since we are creating symmetric pair, another LSP from Z->A is also created with BW 10M

     

    Z --link1-link2--link9-----------link66-----A

                      |-----link19---link29--|

     

    1) Both LSP are using top path ( link1,link2,link9,link66)

    2) Link 19 is on transit LSR with many other LSP going though it, total BW available from link19->link29 is 20M, but only 5M is available from link29-link19.

    3) Link 9, goes down, LSP-Z->A is routed over link1,link2,link19,link29,link66, since LSP Z->A is configured for symmetric pair,return LSP A->Z needs to go over exactly same links ( link66,link29,link19,link2,link1) but there is no enough BW available in this direction for this LSP ( 10M is needed but only 5M is available from link29->link19)

    In this case, symmetry  is not possible, will north star controller establish single LSP Z->A ( link1,link2,link19,link29,link66) because we have required BW along this path ( 10M) , without establishing return lsp A->Z which can not be established because required 10M is not available ?

     

    Much appreciated!!

     

     

     

     

     

     

     

     

     

     

     

     

     

     



  • 5.  RE: Symmetric LSP questions

    Posted 09-13-2020 23:30

    Hello,

     


    @sarahr202 wrote:

    there is no enough BW available in this direction for this LSP ( 10M is needed but only 5M is available from link29->link19)

    In this case, symmetry  is not possible, will north star controller establish single LSP Z->A ( link1,link2,link19,link29,link66) because we have required BW along this path ( 10M) , without establishing return lsp A->Z which can not be established because required 10M is not available ?

     

    AFAIK, You can avoid this problem by configuring all LSP as "adaptive" https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/mpls-configuring-adaptive-lsps.html

    HTH

    Thx

    Alex