Prerequisites for NSR LDP Support
The Label Distribution Protocol (LDP) must be up and running on the standby Route Processor (RP) for NSR LDP Support to work.
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The NSR LDP Support feature allows the Label Distribution Protocol (LDP) to continue to operate across a Router Processor (RP) failure in redundant systems, without losing peer sessions. Before the introduction of nonstop routing (NSR), LDP sessions with peers reset if an RP failover (in a redundant system) or a Cisco In-Service Software Upgrade (ISSU) occurred. When peers reset, traffic is lost while the session is down. Protocol reconvergence occurs after the session is reestablished.
When NSR is enabled, RP failover and Cisco ISSU events are not visible to the peer device, and the LDP sessions that were established prior to failover do not flap. The protocol state learned from the peers persists across an RP failover or Cisco ISSU event and does not need to be relearned.
The Label Distribution Protocol (LDP) must be up and running on the standby Route Processor (RP) for NSR LDP Support to work.
Information About NSR LDP Support
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:
In the initial state, the active Label Distribution Protocol (LDP) process sets up the standby LDP to be ready to support nonstop routing (NSR). The active LDP performs the following tasks:
Replicates all TCP sessions used by LDP with the standby LDP
Synchronizes all existing session-state information with the standby LDP
Synchronizes the LDP database with the standby LDP
LDP could be in the initial state because of one of these conditions:
NSR is enabled
NSR was enabled and the standby Route Processor (RP) starts up (asymmetric startup)
System boots up and NSR is configured (symmetric startup)
In the steady state, the active and standby Label Distribution Protocol (LDP) databases are synchronized. The active and standby LDP process the same LDP messages and update their states independently. The standby LDP is ready to take over the active LDP role in a switchover event.
On the active Route Processor (RP), the active LDP performs the following tasks:
Continues to manage all existing sessions and checkpoints any significant session event to the standby LDP (such as adjacency delete, session shutdown, timers)
Notifies the standby LDP of new adjacencies and neighbors
On the standby RP, the standby LDP performs these tasks:
Processes all received messages but does not send any messages to its neighbor
Processes checkpoint information from the active LDP
Manages session keepalive timers but does not bring down the session if a keepalive timer times out
In the post switchover state, the standby Label Distribution Protocol (LDP) process takes over the active LDP role while the active Route Processor (RP) is reloading.
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).
How to Configure NSR LDP Support
Command or Action | Purpose | |
---|---|---|
Step 1 |
enable Example:
|
Enables privileged EXEC mode.
|
Step 2 |
configure terminal Example:
|
Enters global configuration mode. |
Step 3 |
mpls ldp nsr Example:
|
Enables nonstop routing (NSR) for all Label Distribution Protocol (LDP) sessions for both link and targeted. |
Step 4 |
exit Example:
|
Returns to privileged EXEC mode. |
Step 5 |
show mpls ldp nsr Example:
|
Displays whether NSR is enabled. |
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.
Configuration Examples for NSR LDP Support
Router# show mpls ldp nsr
LDP Non-Stop Routing is enabled
LDP Non-Stop Routing Sessions:
VRF default:
Peer LDP Ident: 3.3.3.3:0 State: Ready
Router# show mpls ldp nsr statistics
Peer: 3.3.3.3:0
In label Request Records created: 0, freed: 0
In label Withdraw Records created: 0, freed: 0
Local Address Withdraw Set: 0, Cleared: 0
Transmit contexts enqueued: 0, dequeued: 0
Total In label Request Records created: 0, freed: 0
Total In label Withdraw Records created: 0, freed: 0
Total Local Address Withdraw Records created: 0, freed: 0
Label Request Acks:
Number of chkpt msg sent: 0
Number of chkpt msg in queue: 0
Number of chkpt msg in state none: 0
Number of chkpt msg in state send: 0
Number of chkpt msg in state wait: 0
Label Withdraw Acks:
Number of chkpt msg sent: 0
Number of chkpt msg in queue: 0
Number of chkpt msg in state none: 0
Number of chkpt msg in state send: 0
Number of chkpt msg in state wait: 0
Address Withdraw Acks:
Number of chkpt msg sent: 0
Number of chkpt msg in queue: 0
Number of chkpt msg in state none: 0
Number of chkpt msg in state send: 0
Number of chkpt msg in state wait: 0
Session Sync:
Number of session-sync msg sent: 3
Number of address records created: 1
Number of address records freed: 1
Number of dup-address records created: 1
Number of dup-address records freed: 1
Number of remote binding records created: 1
Number of remote binding records freed: 1
Number of capability records created: 1
Number of capability records freed: 1
Number of addr msg in state none: 0
Number of dup-addr msg in state none: 0
Number of remote binding msg in state none: 0
Number of capability msg in state none: 0
Number of addr msg in state send: 0
Number of dup-addr msg in state send: 0
Number of remote binding msg in state send: 0
Number of capability msg in state send: 0
Number of addr msg in state wait: 0
Number of dup-addr msg in state wait: 0
Number of remote binding msg in state wait: 0
Number of capability msg in state wait: 0
Number of sync-done msg sent: 1
Router# show mpls ldp neighbor
Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 2.2.2.2:0
TCP connection: 3.3.3.3.646 - 5.5.5.5.13395
State: Oper; Msgs sent/rcvd: 222/219; Downstream
Up time: 02:44:11
LDP discovery sources:
Port-channel1, Src IP addr: 10.5.1.1
TenGigabitEthernet0/4/1, Src IP addr: 10.3.1.1
TenGigabitEthernet0/0/1, Src IP addr: 10.4.1.1
Addresses bound to peer LDP Ident:
3.3.3.3 10.5.1.1 10.7.1.1 10.6.1.1
10.8.1.1 10.3.1.1 10.4.1.1
Router# show mpls ldp nsr
LDP Non-Stop Routing is disabled
Related Topic | Document Title |
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Cisco IOS commands |
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MPLS commands |
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LDP configuration tasks |
MPLS Label Distribution Protocol Configuration Guide |
Description | Link |
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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
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Feature Name |
Release |
Feature Information |
---|---|---|
NSR LDP Support |
IOS XE 3.5 |
This feature was introduced on the Cisco RSP1 Module in this release. |
NSR LDP Support |
IOS XE 3.13 |
This feature was introduced on the Cisco RSP2 Module in this release. |
NSR LDP Support |
IOS XE 3.16 |
This feature was introduced on the Cisco RSP3 Module in this release. |
Feature Name |
Release |
Feature Information |
NSR LDP Support |
IOS XE 3.18SP |
This feature was introduced on the Cisco NCS 4200 Series. |