LDP GR
LDP Graceful Restart (GR), with the help of a helper, implements uninterrupted forwarding during an active main board (AMB)/standby main board (SMB) switchover. Without GR, during an AMB/SMB switchover, the neighboring device deletes an LSP because an LDP session is in the Down state. As a result, traffic is interrupted in a short period of time. LDP GR can be configured to remain labels to implement uninterrupted MPLS forwarding after an unexpected AMB/SMB switchover is performed.
LDP GR timers
Defined in RFC 3748, the following LDP GR timers are used:
MPLS Forwarding State Holding timer: When a GR restarter restarts the LDP protocol, it sets forwarding entries to the Down state and starts this timer. After this timer expires, a device deletes forwarding entries in the Stale state.
LDP Reconnect timer: After a GR helper finds that the LDP session established with the GR restarter goes Down, the helper retains the FEC label mapping for the LDP session, sets the mapping to the Stale state, and starts the LDP Reconnect timer. If the LDP session is not established after this timer expires, the GR helper deletes the stale FEC label mapping and forwarding entries for the LDP session.
LDP Recovery timer: After the LDP session is reestablished between the GR helper and restarter, the GR helper starts the LDP recovery timer. After this timer expires, the GR helper deletes the stale FEC label mapping and forwarding entries for the LDP session.
Figure 1 describes the usage scenario and timing sequence of using the preceding timers.
LDP GR implementation
Figure 2 illustrates the process of implementing LDP GR.
Before an AMB/SMB switchover, LDP peers negotiate GR capabilities before they establish an LDP session.
After the restarter performs an AMB/SMB switchover, it restarts the LDP protocol, starts the MPLS Forwarding State Holing timer, and sets a forwarding entry used before LDP is reset the Stale state.
After the helper detects the AMB/SMB switchover performed by the restarter, the helper sets the FEC label mapping entry sent over the LDP session established between the helper and restarter to the Stale state. The helper then starts the LDP Reconnect timer.
If the LDP is reestablished between the restarter and Helper before the LDP Reconnect timer expires, the Helper deletes the LDP Reconnect timer and starts the LDP Recovery timer.
Before the LDP Recovery timer expires, the helper and restarter exchange Label Mapping messages to restore forwarding entries.
After the MPLS Forwarding State Holding timer expires, the restarter deletes the stale forwarding entries.
After the LDP Recovery timer expires, the helper deletes the stale FEC label mapping and forwarding entries.
