Cisco CRS Configuration Guide

This figure illustrates a control plane failure and shows the process and results of a control plane failure leading
to loss of connectivity.
Figure 4: Control Plane Failure
1
The R4 LSR control plane restarts.
2
LIB is lost when the control plane restarts.
3
The forwarding states installed by the R4 LDP control plane are immediately deleted.
4
Any in-transit packets flowing from R3 to R4 (still labeled with L4) arrive at R4.
5
The MPLS forwarding plane at R4 performs a lookup on local label L4 which fails. Because of this failure,
the packet is dropped and NSF is not met.
6
The R3 LDP peer detects the failure of the control plane channel and deletes its label bindings from R4.
7
The R3 control plane stops using outgoing labels from R4 and deletes the corresponding forwarding state
(rewrites), which in turn causes forwarding disruption.
8
The established LSPs connected to R4 are terminated at R3, resulting in broken end-to-end LSPs from R1
to R4.
9
The established LSPs connected to R4 are terminated at R3, resulting in broken LSPs end-to-end from R2
to R4.
Phases in Graceful Restart
The graceful restart mechanism is divided into different phases:
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
12
Implementing MPLS Label Distribution Protocol
LDP Graceful Restart

Cisco CRS Specifications

General

Operating System	Cisco IOS XR
Product Type	Core Router
Form Factor	Modular Chassis
Dimensions	Varies by chassis type
Weight	Varies by chassis type
Architecture	Distributed
Switch Fabric	Multi-stage
Interface Support	Ethernet, SONET/SDH, OTN
Interfaces/Ports	Varies by line card
Port Density	Varies by line card
Routing Protocol	BGP, IS-IS, OSPF, MPLS
Management	CLI, SNMP
Power Supply	Redundant, hot-swappable
Cooling	Redundant, Hot-swappable Fan Trays
Supported Protocols	IPv4, IPv6, MPLS
Management Interfaces	Ethernet, console
Security Features	ACLs