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The Label Distribution Protocol (LDP) must be up and running on the standby Route Processor (RP) for NSR LDP Support to work.

For the NSR LDP Support feature to work, the Label Distribution Protocol (LDP) must be up and running on the standby Route Processor (RP). The LDP component running on the active RP is called the active LDP, and the LDP component running on the standby RP is called the standby LDP.

When nonstop routing (NSR) is enabled, the standby LDP runs independently from the active LDP, but with the assistance of some software components. The standby LDP maintains LDP session states and database information, ready to take over for the active LDP if the failover occurs.

Standby LDP maintains its local database by querying or receiving notifications of interface status change, configuration changes from the CLI, and checkpoints from the active LDP for other information that is not directly available on the standby RP.

To keep the protocol and session-state information synchronized with the active LDP, the standby LDP depends on TCP to replicate all LDP messages on the active RP to the standby RP. The standby LDP processes all received messages, updates its state, but does not send any responses to its neighbors.

The standby LDP performs the following tasks:

• Processes LDP configuration on startup and during steady state

• Processes active LDP checkpoints of state and session information such as LDP adjacencies, remote addresses, remote bindings, and so forth

• Builds its database of local interfaces

• Processes interface change events

• Receives and processes all LDP messages replicated by TCP

• Updates remote address and label databases

After a switchover and notification that the RP has become active, the standby LDP takes over the role of the active LDP and performs the following tasks:

• Sends hello messages immediately to prevent neighbors from reaching the discovery timeout and bringing down the session

• Retransmits any protocol-level response that has not been sent by the previous active LDP

• Readvertises label bindings

• Refreshes all forwarding entries

• Processes and responds to any LDP message from its neighbor

When the NSR LDP Support feature is disabled, the active LDP performs the following tasks:

• Stops checkpointing to the standby LDP

• Continues to manage all existing sessions

The standby LDP performs the following tasks:

• Cleans up all session-state information

• Reverses to the behavior before NSR is enabled

When the NSR LDP Support feature is enabled, the Label Distribution Protocol (LDP) operates in the following states:

The NSR LDP Support feature is supported under the following scenarios:

• Route Processor (RP) failover or node failure

  The Label Distribution Protocol (LDP) keeps the session up during an RP or node failover because the LDP adjacency and session-state information between LDP on the active and standby RPs are synchronized. As sessions are created on the active RP, new adjacencies are synchronized to the standby RP. If a standby RP is brought online after sessions are already up (asymmetric startup), LDP synchronizes the existing session-state information from the active to the standby RP.

• Cisco In-Service Software Upgrade (ISSU)

  LDP supports Cisco ISSU negotiation between RPs when a standby RP comes online for the MPLS LDP IGP Synchronization feature. Current Cisco ISSU negotiation is not impacted by NSR. For NSR, LDP negotiates messages specific to NSR, which are checkpointed during initial synchronization (adjacency and session-state information).

Use the debug mpls ldp nsr command to enable the display of Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) nonstop routing (NSR) debugging events for all NSR sessions or for the specified peer.