Setup of Dynamic CR-LSPs

To establish a dynamic CR-LSP from an ingress node to an egress node, the ingress node sends Path messages to the egress node and the egress node sends Resv messages back to the ingress node. Path messages are sent to create Resource Reservation Protocol (RSVP) sessions and associate the path status. Every node that receives a path message creates a path state block (PSB). A Resv message carries resource reservation information. Every node that receives a Resv message creates a reservation state block (RSB) and allocates a label.

Figure 1 shows how RSVP-TE sets up a CR-LSP.

Figure 1 CR-SLP setup through RSVP-TE

1. PE1 uses CSPF to calculate a path from PE1 to PE2, on which the IP address of every hop is specified. PE1 generates an explicit route object (ERO) with the IP address of each hop and adds the ERO in a Path message. PE1 then creates a PSB and sends the Path message to P1 according to information in the ERO. Table 1 describes objects carried in the Path message.

   Table 1 Path message on PE1

   Object	Value
   SESSION	Source: PE1-if1; Destination: PE2-if0
   RSVP_HOP	PE1-if1
   EXPLICIT_ROUTE	P1-if0; P2-if0; PE2-if0
   LABEL	LABEL_REQUEST

2. After P1 receives the Path message, it parses the message and creates a PSB according to information in the message. Then P1 updates the message and sends it to P2 according to the ERO. Table 2 describes objects in the Path message.

   • The RSVP_HOP object specifies the IP address of the outbound interface through which a Path message is sent. Therefore, PE1 sets the RSVP_HOP object to the IP address of the outbound interface toward P1, and P1 sets the RSVP_HOP field to the IP address of the outbound interface toward P2.

   • P1 deletes the local LSR ID and IP addresses of the inbound and outbound interfaces from the ERO field in the Path message.

   Table 2 Path message on P1

   Object	Value
   SESSION	Source: PE1-if1; Destination: PE2-if0
   RSVP_HOP	P1-if1
   EXPLICIT_ROUTE	P2-if0; PE2-if0
   LABEL	LABEL_REQUEST

3. After receiving the Path message, P2 creates a PSB according to information in the message, updates the message, and then sends it to PE2 according to the ERO field. Table 3 describes objects in the Path message.

   Table 3 Path message on P2

   Object	Value
   SESSION	Source: PE1-if1; Destination: PE2-if0
   RSVP_HOP	P2-if1
   EXPLICIT_ROUTE	PE2-if0
   LABEL	LABEL_REQUEST

4. After PE2 receives the Path message, PE2 knows that it is the egress of the CR-LSP to be set up according to the Destination field in the Session object. PE2 then allocates a label and reserves bandwidth, and generates a Resv message based on the local PSB. The Resv message carries the label allocated by PE2 and is sent to P2.

   PE2 uses the address carried in the RSVP_HOP field of the received Path message as the destination IP address of the Resv message. The Resv message does not carry the ERO field because it is forwarded along the reverse path. Table 4 describes objects in the Resv message.

   

   If a Resv message carries the RESV_CONFIRM object, the receiver needs to send a ResvConf message to the sender to confirm the resource reservation request.

   Table 4 Resv message on PE2

   Object	Value
   SESSION	Source: PE2-if0; Destination: PE1-if1
   RSVP_HOP	PE2-if0
   LABEL	3
   RECORD_ROUTE	PE2-if0

5. When P2 receives the Resv message, P2 creates an RSB according to information in the message, allocates a new label, updates the message, and then sends it to P1. Table 5 describes objects in the Resv message.

   Table 5 Resv message on P2

   Object	Value
   SESSION	Source: PE2-if0; Destination: PE1-if1
   RSVP_HOP	P2-if0
   LABEL	17
   RECORD_ROUTE	P2-if0; PE2-if0

6. After receiving the Resv message, P1 creates an RSB according to information in the message, updates the message, and then sends it to PE1. Table 6 describes objects in the Resv message.

   PE1 obtains the label allocated by P1 from the received Resv message. Resources are successfully reserved and a CR-LSP is set up.

   Table 6 Resv message on P1

   Object	Value
   SESSION	Source: PE2-if0; Destination: PE1-if1
   RSVP_HOP	P1-if0
   LABEL	18
   RECORD_ROUTE	P1-if0; P2-if0; PE2-if0