RSVP Hello

RSVP Hello mechanism is used to rapidly detect reachability between RSVP nodes and trigger path protection provided by TE FRR. In addition, a node can use the RSVP Hello mechanism to detect whether a neighboring node is in Restart state so it can help the neighboring node in implementing RSVP GR.

Background

RSVP Refresh messages can synchronize PSB and RSB between nodes, monitor reachability between RSVP neighbors, and maintain RSVP neighbor relationships.

This soft state mechanism detects neighbor relationships using Path and Resv messages. The detection speed is low and a link failure cannot promptly trigger a service traffic switchover. RSVP Hello is introduced to solve this problem.

Implementation

RSVP Hello is implemented as follows:

1. Hello handshake

   Figure 1 Hello handshake mechanism
   

   As shown in Figure 1, LSRA and LSRB are directly connected.

   • When RSVP Hello is enabled on the interface of LSRA, LSRA sends a Hello Request message to LSRB.

   • If LSRB is enabled with RSVP Hello, LSRB replies to LSRA with a Hello ACK message after receiving the Hello Request message.

   • After LSRA receives the Hello ACK message from LSRB, LSRA determines that the neighbor LSRB is reachable.

2. Neighbor loss detection

   After a successful Hello handshake, LSRA and LSRB exchange Hello messages. If LSRA receives no Hello ACK message from LSRB after sending three consecutive Hello Request messages to LSRB, LSRA considers the neighbor LSRB lost. TE FRR is triggered and LSRA restarts an RSVP Hello handshake.

3. Neighbor restart detection

   After LSRA detects the loss of the neighbor LSRB (they are both RSVP GR capable), LSRA waits for the Hello Request message carrying a GR extension from LSRB. After receiving this message, LSRA helps LSRB restore RSVP state information and sends a Hello ACK message to LSRB. LSRB receives the Hello ACK message from LSRA and knows that LSRA is helping it implement GR. LSRA and LSRB exchange Hello messages to maintain the restored GR status.

When LSRA and LSRB are located on the same CR-LSP:

• If GR is disabled but TE FRR is enabled on LSRA, LSRA switches traffic to the bypass CR-LSP to ensure uninterrupted traffic transmission when detecting loss of the neighbor LSRB.

• If GR is enabled on LSRA, LSRA preferentially uses RSVP GR to ensure uninterrupted traffic transmission on the forwarding plane upon a control plane failure.

Usage Scenario

RSVP Hello applies to scenarios with TE FRR or RSVP GR enabled.