Mpls Label Distribution Protocol Igp Synchronization; Mpls Ldp-Igp Synchronization Compatibility With Ldp Graceful Restart; Mpls Ldp-Igp Synchronization Compatibility With Igp Nonstop Forwarding; Label Distribution Protocol Igp Auto-Configuration - Cisco NCS 6000 Series Configuration Manual
Ios xr release 6.4.x
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MPLS Label Distribution Protocol IGP Synchronization
MPLS Label Distribution Protocol IGP Synchronization
Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) Interior Gateway Protocol (IGP)
Synchronization ensures that LDP has completed label exchange before the IGP path is used for switching.
MPLS traffic loss can occur in the following two situations:
• When an IGP adjacency is established, the router begins forwarding packets using the new adjacency
• When an LDP session closes, the router continues to forward traffic using the link associated with the
This feature provides a mechanism to synchronize LDP and IS-IS to minimize MPLS packet loss. The
synchronization is accomplished by changing the link metric for a neighbor IS-IS link-state packet (LSP),
based on the state of the LDP session.
When an IS-IS adjacency is established on a link but the LDP session is lost or LDP has not yet completed
exchanging labels, IS-IS advertises the maximum metric on that link. In this instance, LDP IS-IS synchronization
is not yet achieved.
Note
In IS-IS, a link with a maximum wide metric (0xFFFFFF) is not considered for shortest path first (SPF).
Therefore, the maximum wide metric of -1 (0XFFFFFE) is used with MPLS LDP IGP synchronization.
When LDP IS-IS synchronization is achieved, IS-IS advertises a regular (configured or default) metric on
that link.
MPLS LDP-IGP Synchronization Compatibility with LDP Graceful Restart
LDP graceful restart protects traffic when an LDP session is lost. If a graceful restart-enabled LDP session
fails, MPLS LDP IS-IS synchronization is still achieved on the interface while it is protected by graceful
restart. MPLS LDP IGP synchronization is eventually lost under the following circumstances:
• LDP fails to restart before the LDP graceful restart reconnect timer expires.
• The LDP session on the protected interface fails to recover before the LDP graceful restart recovery
MPLS LDP-IGP Synchronization Compatibility with IGP Nonstop Forwarding
IS-IS nonstop forwarding (NSF) protects traffic during IS-IS process restarts and route processor (RP) failovers.
LDP IS-IS synchronization is supported with IS-IS NSF only if LDP graceful restart is also enabled over the
interface. If IS-IS NSF is not enabled, the LDP synchronization state is not retained across restarts and failovers.
Label Distribution Protocol IGP Auto-configuration
Label Distribution Protocol (LDP) Interior Gateway Protocol (IGP) auto-configuration simplifies the procedure
to enable LDP on a set of interfaces used by an IGP instance. LDP IGP auto-configuration can be used on a
large number interfaces (for example, when LDP is used for transport in the core) and on multiple IGP instances
simultaneously.
This feature supports the IPv4 address family for the default VPN routing and forwarding (VRF) instance.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
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before LDP has exchanged labels with peers on that link.
LDP peer rather than using an alternate path with an established LDP session.
timer expires.
Implementing IS-IS
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