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How RSVP establishes a signaled LSP

The traffic-engineered path calculated by CSPF consists of a sequential list of physical interface addresses, corresponding to a path from the ingress LER to the egress LER. Using this traffic-engineered path, RSVP establishes the forwarding state and resource reservations on each LSR in the path.

As with OSPF, special extensions for traffic engineering are defined for RSVP. These extensions include the EXPLICIT_ROUTE, LABEL_REQUEST, LABEL, and RECORD_ROUTE objects. These extensions are described in RFC 3209.

The following diagram illustrates how RSVP establishes a signaled LSP.

Figure 1. How RSVP establishes a signaled LSP

RSVP signaling for LSPs works as described below.

  1. The ingress LER sends an RSVP Path message towards the egress LER.

    The Path message also describes the traffic for which resources are being requested and specifies the bandwidth that needs to be reserved to accommodate this traffic. In addition, the Path message includes a LABEL_REQUEST object, which requests that labels be allocated on LSRs and tells the egress LER to place a LABEL object in the Resv message that it sends back to the ingress LER.

    Before sending the Path message, the ingress LSR performs admission control on the outbound interface, ensuring that enough bandwidth can be reserved on the interface to meet the LSPs requirements. Admission control examines the LSPs configured setup priority and mean-rate settings. For the LSP to pass admission control, the outbound interface must have reservable bandwidth at the LSPs setup priority level that is greater than the amount of bandwidth specified by the LSPs meanrate setting.

  2. The Path message requests resource reservations on the LSRs along the path specified in the ERO.

    When the LSP passes admission control, the ingress LER sends a Path message to the address at the top of the ERO list. This is the address of a physical interface on the next LSR in the path. As the ingress LER did, this LSR performs admission control to make sure the outbound interface has enough reservable bandwidth to accommodate the LSP.

    When the LSP passes admission control, the LSR then removes its address from the top of the ERO list and sends the Path message to the address now at the top of the ERO list. This process repeats until the Path message reaches the last node in the ERO list, which is the egress LER.

  3. The egress LER receives the Path message and sends a Resv message towards the ingress LER.

    Resv messages flow upstream from the receiver of the Path message to the sender (that is, from the egress LER to the ingress LER), taking the exact reverse of the path specified in the ERO. In response to the LABEL_REQUEST object in the Path message, the Resv message from the egress LER includes a LABEL object. The LABEL object is used to associate labels with interfaces on the LSRs that make up the LSP.

  4. As the Resv messages travel upstream, resources are reserved on each LSR.

    When an LSR receives a Resv message, it again performs admission control on the interface where the Resv message was received (that is, the interface that is the outbound interface for packets traveling through the LSP). When the LSP still passes admission control, bandwidth is allocated to the LSP. The LSR allocates the amount of bandwidth specified by the LSPs meanrate setting, using the bandwidth available to its hold priority level. This may cause lower priority LSPs active on the device to be preempted.

    Once bandwidth has been allocated to the LSP, the LABEL object in the Resv message is used to associate labels with interfaces in the LSRs MPLS forwarding table. The following diagram shows an example of how this works.

    Figure 2. How the RSVP LABEL object associates a label with an interface in the MPLS forwarding table

    In the example above, the LSR receives a Resv message on interface 3/1 from the downstream LSR in the ERO. The Resv message has a LABEL object containing label 456. After performing admission control and bandwidth allocation, the LSR adds an entry to its MPLS forwarding table for this LSP, associating label 456 with outbound interface 3/1.

    The LSR then takes a label from its range of available labels (for example, 123) and places it in the LABEL object in the Resv message that it sends to the upstream LSR. In this example, the LSR sends the Resv message out interface 2/1 to the upstream LSR in the ERO. In its MPLS forwarding table for this LSP, the LSR associates label 123 with inbound interface 2/1.

    This process repeats at each LSR until the Resv message reaches the ingress LER.
    Note: To enable penultimate hop popping for the LSP, the LABEL object sent by the egress LER to the penultimate LSR contains a value of three (3) (Implicit Null Label). This is an IETF-reserved label value that indicates to the penultimate LSR that it must pop the label of MPLS-encoded packets that belong to this LSP.
  5. Once the Resv message reaches the ingress LER, and the process described in Step 4 takes place, the LSP is activated. At this point, each LSR in the LSP has reserved resources, allocated labels, and associated labels with interfaces. The LSP is activated, and the ingress LER can assign packets to the LSP.