MPLS Fault-Tolerance Architecture

1
Lecture 15
MPLS Fault-Tolerance Architecture
(For details, see class notes)
2
LDP Fault-Tolerance Approaches

• Three approaches have been proposed to minimize
  disruption of MPLS forwarding due to LDP restart
• Fault Tolerance for the LDP (RFC 3479)
• Checkpointing Procedures for LDP (RFC 3479)
• Graceful Restart Mechanism for LDP (RFC 3478)

3
MPLS Control Plane (BGP/LDP) Restart - Problem
Description

• LDP and BGP use TCP as a reliable transport
  mechanism for its protocol messages.
• The TCP session between two LDP/BGP peers may go
  down for several reasons such as control
  processor switchover due to HW/SW failures.
• On detection of TCP session failure, existing LDP
  and BGP control plane components would disrupt
  their forwarding state.
• To minimize disruption in MPLS forwarding plane
  due to MPLS control plane (LDP,BGP) restart, LDP
  and BGP have been enhanced.

4
LDP Restart Behavior before/after
fault-tolerance enhancement
1
Control Plane
Control Plane
Control Plane
3
3
3
Data Plane
Data Plane
Data Plane
2
2
LSPs
LSPs
1
Control Plane
Control Plane
Control Plane
Data Plane
Data Pane
Data Plane
2
2
LSPs
LSPs
5
FT Session Type Length Value (TLV)
1
0
FT Session TLV (0x0503)
Length ( 4)
FT Flags
Reserved
FT Reconnect Timeout (in milliseconds)
Recovery Time (in milliseconds)
R
L
C
A
S
Reserved
L1 means GR-LDP is selected FT Reconnect 0
means LSR is not NSF capable FT Recovery Time 0
means LSR was unable to preserve MPLS forwarding
state across restart
6
LDP Graceful Restart Operation
C
A
Active
Standby
LDP Session Setup
LDP Session Setup
LDP Address messages
LDP Address messages
LDP Label Mapping (FEC1, L1)
LDP Label Mapping (FEC1, L2)
LSP through A-B-C is ready
Switchover
Active
Standby
LDP Session re-established
LDP Session re-established
LDP Label Mapping (FEC1, L1)
LDP Label Mapping (FEC1, L2)
Normal operation
Normal operation
7
BGP/MPLS Graceful Restart

• BGP is used as a control plane mechanism for IP
  and MPLS
• BGP Graceful Restart mechanism allows a router
  to preserve its IP forwarding state across
  restart (draft-ietf-idr-restart-06.txt).
• BGP MPLS graceful mechanism is an extension to
  the above for allowing an LSR to preserve its
  MPLS forwarding state across BGP restart
  (draft-ietf-mpls-bgp-mpls-restart-02.txt)
• BGP MPLS Graceful Restart mechanism requires
  preservation of MPLS forwarding state across BGP
  restart.
• To accomplish this
• a new Graceful Restart Capability is defined
  which is carried in BGP Open message
• an BGP Update message with no reachable NLRI and
  empty withdrawn NLRI is specified as an
  End-of-RIB marker.
• AFI/SAFI pair in the Graceful Restart Cap is
  used by an LSR for indicating its ability to
  preserve MPLS forwarding state across BGP restart

8
Course Summary
9
Where do we go from here?
(For details, see class notes)