RSVP Hello
The RSVP Hello extension can rapidly monitor the reachability of RSVP nodes. If an RSVP node becomes unreachable, TE FRR protection is triggered. The RSVP Hello extension can also monitor whether an RSVP GR neighboring node is in the restart process.
Background
RSVP Refresh messages are used to synchronize path state block (PSB) and reservation state block (RSB) information between nodes. They can also be used to monitor the reachability between RSVP neighbors and maintain RSVP neighbor relationships.
Using Path and Resv messages to monitor neighbor reachability delays a traffic switchover if a link fault occurs and therefore is slow. The RSVP Hello extension can address this problem.
Related Concepts
- RSVP Refresh messages: Although an MPLS TE tunnel is established using Path and Resv messages, RSVP nodes still send Path and Resv messages over the established tunnel to update the RSVP status. These Path and Resv messages are called RSVP Refresh messages.
- RSVP GR: ensures uninterrupted transmission on the forwarding plane while an AMB/SMB switchover is performed on the control plane. A GR helper assists a GR restarter in rapidly restoring the RSVP status.
- TE FRR: a local protection mechanism for MPLS TE tunnels. If a fault occurs on a tunnel, TE FRR rapidly switches traffic to a bypass tunnel.
Implementation
The principles of the RSVP Hello extension are as follows:
Hello handshake mechanism
LSRA and LSRB are directly connected on the network shown in Figure 4-10.
If RSVP Hello is enabled on LSRA, LSRA sends a Hello Request message to LSRB.
After LSRB receives the Hello Request message and is also enabled with RSVP Hello, LSRB sends a Hello ACK message to LSRA.
After receiving the Hello ACK message, LSRA considers LSRB reachable.
Detecting neighbor loss
After a successful Hello handshake is implemented, LSRA and LSRB exchange Hello messages. If LSRB does not respond to three consecutive Hello Request messages sent by LSRA, LSRA considers router B lost and re-initializes the RSVP Hello process.
Detecting neighbor restart
If LSRA and LSRB are enabled with RSVP GR, and the Hello extension detects that LSRB is lost, LSRA waits for LSRB to send a Hello Request message carrying a GR extension. After receiving the message, LSRA starts the GR process on LSRB and sends a Hello ACK message to LSRB. After receiving the Hello ACK message, LSRB performs the GR process and restores the RSVP soft state. LSRA and LSRB exchange Hello messages to maintain the restored RSVP soft state.
There are two scenarios if a CR-LSP is set up between LSRs:
If GR is disabled and FRR is enabled, FRR switches traffic to a bypass CR-LSP after the Hello extension detects that the RSVP neighbor relationship is lost to ensure proper traffic transmission.
If GR is enabled, the GR process is performed.
Deployment Scenarios
The RSVP Hello extension applies to networks enabled with both RSVP GR and TE FRR.
