What functions do GR timers provide in the MPLS LDP view

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After GR Restarter performs an active/standby switchover, the MPLS Forwarding State Holding Timer is started, which is the Neighbor-liveness timer in the MPLS LDP view. Before the Neighbor-liveness timer expires, all forwarding entries before the active/standby switchover are retained. After the Neighbor-liveness timer expires, the GR Helper deletes the forwarding entries that are not restored.

When detecting that the GR Restarter performs an active/standby switchover or the LDP protocol restarts, the GR Helper starts the GR Reconnect timer. Before the GR Reconnect timer expires, the GR Helper retains all forwarding entries related to the GR Restarter. After the GR Reconnect timer expires, the GR Helper deletes the forwarding entries that are not restored. If a session is re-established between the GR Restarter and Helper before the GR Reconnect timer expires, the GR Helper deletes the GR Reconnect timer and starts the GR Recovery timer. Before the GR Recovery timer expires, the GR Helper helps the GR Restarter restore forwarding entries. After the GR Recovery timer times out, the GR Helper deletes all the GR Restarter-related forwarding entries that are not restored.

Other related questions:
What are the default intervals of the GR timers in the MPLS LDP view
The default interval of the MPLS Forwarding State Holding timer (Neighbor-liveness timer) is 10 minutes. The default intervals of the GR Reconnect timer and GR Recovery timer are both 5 minutes.

Why is the LDP GR duration in the log longer than 10 minutes, which is the interval of the MPLS Forwarding State Holding timer
After the GR restarter performs an active/standby switchover, LDP GR starts only after route GR is complete. The MPLS Forwarding State Holding timer is started immediately after the active/standby switchover but restarted when route GR is complete. In the log, the GR begin time is the time when the MPLS Forwarding State Holding timer is first started; the GR end time is the time when the MPLS Forwarding State Holding timer expires. Therefore, the LDP GR duration recorded in the log is the sum of the route GR duration and MPLS Forwarding State Holding timer interval (10 minutes).

What functions does MPLS ping provide
In MPLS, the control panel used for establishing an LSP cannot detect the failure in data forwarding of the LSP. This makes network maintenance difficult. The MPLS ping mechanism detects LSP errors and locates faulty nodes. Similar to IP ping, MPLS ping checks LSP connectivity through MPLS Echo Request and MPLS Echo Reply messages. These messages are transmitted in User Datagram Protocol (UDP) packets with the port number being 3503. An MPLS Echo Request message carries FEC information to be detected, and is sent along the same LSP as other packets with the same FEC. In this manner, LSP connectivity is checked. In MPLS ping, an MPLS Echo Request message reaches the egress node of the LSP, and the control panel of the egress node checks whether the local LSR is the egress of the FEC. To prevent the egress node from forwarding the received Echo Request message to other nodes, the destination address in the IP header of the Echo Request message is set to 127.0.0.1/8 (the local loopback address), and the TTL value contained in the IP header is set to 1.

Functions and default values of IS-IS timers on S series switches
The functions and default values of IS-IS timers are as follows: CSNP Timer 10s csnp-interval: specifies the interval for sending CSNPs on a broadcast network. HELLO Timer 10s (3s for the DIS) Specifies the interval for sending Hello messages. lsp-retransmit Timer 5s Specifies the interval for re-sending LSP packets, that is, the interval between marking LSP packets with SRM on a P2P link and sending LSP packets. lsp-throttle Timer 50 ms Specifies the minimum interval for sending LSP packets. lsp-generation Timer 2s Specifies the maximum interval for generating LSP packets. lsp-max-age Timer 1200s Specifies the maximum Keepalive time of an LSP generated by the current IS-IS process, which is configured using the timer lsp-max-age command. lsp-refresh Timer 900s Specifies the LSP refresh interval, which is configured using the timer lsp-refresh command. spf 10s Specifies the maximum delay in route calculation. PSNP Timer 5s Specifies the interval between marking PSNPs with SSN and sending PSNPs. Specifies the minimum interval for sending packets on a broadcast network. 50 ms Specifies the interval between marking LSP packets with SRM and sending LSP packets.

What are the reasons for failure in establishing MPLS LDP sessions
An MPLS LDP session fails to be established due to the following possible causes: The route to the peer is not generated, or MPLS LDP is disabled on the outbound interface of the route to the peer. LDP protocol packets have been limited by CPCAR on the device, and LDP protocol packets cannot be properly processed.

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