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[PTX] Understanding basic MPLS DS-TE



Article ID: KB33614 KB Last Updated: 08 May 2019Version: 1.0

This article describes MPLS DS-TE technology and discuss its related features with configuration examples. MPLS Diffserv TE makes MPLS TE as QoS aware, allowing resource reservation with Qos granularity. It allows more granular classification, and treatment of sensitive traffic such as voice. MPLS TE does not take into account class of service/IP DSCP.


With MPLS DS-TE, it allows mapping traffic from a given DiffServ class-of-service on a separate LSP. It allows this traffic to use resources available to the given class on paths that meet constraints specific to the given class.

  • ​RFC3564 introduces concepts of class-types. DS_TE keeps track of the available bandwidth for each class type, aka classes-of traffic. For that purpose, class-types are defined. There are no rules that governs which traffic maps to which class-type. A given LSP in DiffServ domain belongs to the same CT on all links. There are a total of 8 CTs defined; CT0 through CT7. A DS-TE LSP can only carry traffic from one CT. The example below shows class-type statements with bandwidth constraints defined:

    [edit protocols mpls]

    label-switched-path LSP1 {
        bandwidth ct1 200m;    <-- Indicates this LSP belongs to class-type CT1 with CT1  bandwidth constraint of 200 Mbps. 
        priority 2 2;          <-- Indicates setup and hold  priority configured as 2. 

    label-switched-path LSP2 {
        bandwidth ct0 100m;   <-- Indicates this LSP belongs to class-type CT0 with CT0 bandwidth constraint of 100Mbps.
        priority 7 7;         <-- Indicates setup and hold priority configured as 7. 

  • CSPF is used for computation of a bath based on user-defined constraints such as bandwidth and link priorities. Such as set-up priority and hold priority.  There are 8 setup and holding priorities. 0- most preferred and 7- least preferred.

  • Setup Priority is used to define preference for preempting less preferred tunnels. In other words, most preferred tunnels can push less preferred tunnels out of the way.

  • Hold-Priority is used to define a priority maintaining the current established LSP/tunnel. In other words, you can have a tunnel that once established, you never want it to go down. In that case, you can set setup priority - 7 and holding priority - 0, that way the tunnel will never get pre-empted once established. 

  • You must include the diffserv-te statement in the configuration on all routers participating in the Differentiated Services domain. However, configuring traffic-class matrix is optional. Junos provides default mapping of TE classes and Class types CTs in terms of default forwarding classes defined in the CoS configuration. If you want to override default mapping in Junos, for each traffic engineering class, you configure a class type (or queue) from 0 through 3. For each class type, you configure a priority from 0 through 7. For example:

    [edit protocols mpls]
    diffserv-te {
        te-class-matrix {
            te0 traffic-class ct0 priority 7;  <-- configured traffic-engineering class TE0 with Class-type CT0 and a priority of 7.
            te1 traffic-class ct1 priority 2;  <-- configured traffic-engineering class TE1 with Class-type CT1 and a priority of 2.

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