Overview of CR-LSP Setup

CR-LSP Setup Modes

A CR-LSP can be statically or dynamically set up.

A static CR-LSP is set up depending on manual configuration. This section describes how dynamic CR-LSPs are set up through RSVP-TE.

Overview of RSVP-TE

The Resource Reservation Protocol (RSVP) is designed for the integrated services model, and reserves resources for nodes along a path. This bandwidth reservation capability makes RSVP-TE a suitable signaling protocol for establishing MPLS TE paths.

RSVP-TE provides the following extensions based on RSVP to support MPLS TE implementation:

RSVP-TE adds Label Request objects to Path messages to request labels and adds Label objects to Resv messages to allocate labels.
An extended RSVP message can carry path constraints in addition to label binding information.
The extended objects carry MPLS TE bandwidth constraints to implement resource reservation.

RSVP Message Types

RSVP defines the following types of messages:

Path message: is sent downstream by the sender and saves path information on the nodes it passes through.
Resv message: is sent upstream by the receiver to respond to the Path message and to request resource reservation.
PathErr message: is sent by an RSVP node to its upstream node if an error occurs while this node is processing a Path message.
ResvErr message: is sent by an RSVP node to its downstream node if an error occurs while this node is processing a Resv message.
PathTear message: is sent to delete path information and functions in the opposite way to a Path message.
ResvTear message: is sent to delete the resource reservation state and functions in the opposite way to a Resv message.
ResvConf message: is sent downstream from the sender hop by hop to confirm a resource reservation request. This message is sent only when the Resv message contains the RESV_CONFIRM object.
Srefresh message: is used to update the RSVP state.

RSVP-TE Implementation

Table 1 describes RSVP-TE implementation.

**Table 1** RSVP-TE implementation
Function	Description
Setup of Dynamic CR-LSPs	A CR-LSP is set up according to the CSPF calculation result or an explicit path. CR-LSP setup is triggered on the ingress node.
Maintenance of Dynamic CR-LSPs	Path Status Maintenance After a CR-LSP is set up, RSVP-TE still sends RSVP messages to maintain the path state on each node. Error Signaling RSVP nodes send error messages to notify upstream and downstream nodes that faults have occurred during path establishment or maintenance. Path Teardown A CR-LSP is torn down, and labels and bandwidth on each node are released. The ingress node initiates teardown requests.