RSVP Hello
The RSVP Hello extension mechanism can rapidly monitor the reachability of RSVP nodes. If an RSVP node becomes unreachable, TE FRR protection is triggered. In addition, RSVP Hello extension can detect whether the neighboring node is in the restart state to help the neighboring node implement RSVP GR.
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.
However, using Path and Resv messages to monitor neighbor reachability takes a long time. If a fault occurs on the primary path, traffic cannot be switched to the backup path in a timely manner. RSVP Hello extension can address this problem.
Related Concepts
- RSVP Refresh messages: After an MPLS TE tunnel is established, RSVP still sends standard Path and Resv messages to refresh the RSVP status on each node. These Path and Resv messages are called RSVP Refresh messages.
- RSVP GR: ensures uninterrupted transmission on the forwarding plane while an active/standby 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
RSVP Hello is implemented as follows:
Hello handshake mechanism
As shown in Figure 1, LSRA and LSRB are directly connected.
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 its neighbor 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 LSRB lost, triggers TE FRR switching, 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 and starts to allow B to restore the RSVP state. After receiving the Hello ACK message from LSRA, 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.
Usage Scenario
The RSVP Hello extension applies to networks enabled with both RSVP GR and TE FRR.