Flex LSP also known as Associated Bidirectional LSPs is the combination of static bidirectional MPLS-TP and dynamic MPLS-TE. Flex LSP provides bidirectional label switched paths (LSPs) set up dynamically through Resource Reservation Protocol–Traffic Engineering (RSVP-TE). It does not support non-co routed LSPs.

Flex Label Switched Paths are LSP instances where the forward and the reverse direction paths are setup, monitored and protected independently and associated together during signaling. You use a RSVP Association object to bind the two forward and reverse LSPs together to form a co-routed associated bidirectional TE tunnel.

You can associate a protecting MPLS-TE tunnel with either a working MPLS-TE LSP, protecting MPLS-TE LSP, or both. The working LSP is the primary LSP backed up by the protecting LSP. When a working LSP goes down, the protecting LSP is automatically activated. You can configure a MPLS-TE tunnel to operate without protection as well.

This section provides an overview of the association signaling methods for the bidirectional LSPs. Two unidirectional LSPs can be bound to form an associated bidirectional LSP in the following scenarios:

• No unidirectional LSP exists, and both must be established.
• Both unidirectional LSPs exist, but the association must be established.
• One unidirectional LSP exists, but the reverse associated LSP must be established.

This section provides an overview of associated bidirectional co-routed LSPs. Establishment of MPLS TE-LSP involves computation of a path between a head-end node to a tail-end node, signaling along the path, and modification of intermediate nodes along the path. The signaling process ensures bandwidth reservation (if signaled bandwidth is lesser than 0 and programming of forwarding entries).

Path computation is performed by the head-end nodes of both the participating LSPs using Constrained Shortest Path First (CSPF). CSPF is the 'shortest path (measured in terms of cost) that satisfies all relevant LSP TE constraints or attributes, such as required bandwidth, priority and so on.

Associated Bidirectional Co-routed LSPs: A co-routed bidirectional TE LSP denotes a bidirectional tunnel where the forward direction LSP and reverse direction LSP must follow the same path, for example, the same nodes and paths. Here is an illustration.

• Exp-null over Flex-LSP is not supported.

• 50 msec convergence is not guaranteed without WRAP protection. WRAP protection is mandatory to achieve 50 msec convergence for remote failures.

• TE NSR and IGP NSR are mandatory for RSP switchover.

• VPLS over Flex-LSP is not supported.

• Non-co routed Flex LSP is not supported.

• Sub interface shut will not guarantee 50 msec convergence.

• MPLS forwarding table stats is not supported.

• 1000 tunnels are supported with wrap protection and path protection.

Following list outlines key features supported:

• Protection -

  Following Protection features are supported:

  • Lockout: Using Lockout feature, user can perform lockout protection on a selected LSP and can switch traffic to protecting LSP, if the selected LSP carries the traffic. LSP on locked out interface remains up but no traffic flows on the locked out path. To configure lockout under an MPLS-TE enabled interface:

    
    RP/0/RP0:hostname# configure
    RP/0/RP0:hostname(config)# mpls traffic-eng
    RP/0/RP0:hostname(config-mpls-te)# interface tenGigE0/1/0/1
    RP/0/RP0:hostname(config-mpls-te-if)# fault-oam lockout

  • Wrap Protection: Using Wrap Protection, each LSP signals unique wrap label for head-end to identify lookback traffic and sends it over protect LSP. To configure Wrap Protection:

    
    RP/0/RP0:hostname# configure
    RP/0/RP0:hostname(config)# interface tunnel-te1
    RP/0/RP0:hostname(config-if)# ipv4 unnumbered Loopback0
    RP/0/RP0:hostname(config-if)# destination 49.49.49.2
    RP/0/RP0:hostname(config-if)# path-option 10 explicit name PATH1-2-3 
    RP/0/RP0:hostname(config-if)# bidirectional association id 100 source-address 49.49.49.2
    RP/0/RP0:hostname(config-if)# bidirectional association association type co-routed
    RP/0/RP0:hostname(config-if)# wrap-protection
    RP/0/RP0:hostname(config-if)# fault-oam 
    

• MPLS-OAM: MPLS-OAM supports single segment pseudowire going over the associated bidirectional TE tunnels. This support includes pseudowires signaled dynamically, statically, or using a mix of both modes. To configure MPLS-OAM, use the following command:

  
  RP/0/RP0:hostname# configure
  RP/0/RP0:hostname(config)# mpls oam
  RP/0/RP0:hostname(config-oam)# echo reply-mode control-channel allow-reverse-lsp
  

  Following features are supported:

  • LSP Ping: Using LSP ping, use can enable on demand ping. It supports IP encapsulation for both request and reply messages. It also performs reverse path verification.

    Following is the example of LSP Ping configuration:

    
    RP/0/RP0:hostname# ping mpls traffic-eng tunnel-te 1 reply mode control-channel
    
    Tue May 21 11:04:12.211 EDT
    Sending 5, 100-byte MPLS Echos to tunnel-te1,
          timeout is 2 seconds, send interval is 0 msec:
    
    Codes: '!' - success, 'Q' - request not sent, '.' - timeout,
      'L' - labeled output interface, 'B' - unlabeled output interface,
      'D' - DS Map mismatch, 'F' - no FEC mapping, 'f' - FEC mismatch,
      'M' - malformed request, 'm' - unsupported tlvs, 'N' - no rx label,
      'P' - no rx intf label prot, 'p' - premature termination of LSP,
      'R' - transit router, 'I' - unknown upstream index,
      'X' - unknown return code, 'x' - return code 0
    
    Type escape sequence to abort.
    
    !!!!!
    Success rate is 100 percent (5/5), round-trip min/avg/max = 3/4/5 ms

  • Traceroute: Traceroute supports IP-encapsulation but not for echo reply. Following is the example of Traceroute configuration:

    
    RP/0/RP0:hostname# traceroute mpls traffic-eng tunnel-te 1
    
    Tue May 21 11:06:16.056 EDT
    Tracing MPLS TE Label Switched Path on tunnel-te1, timeout is 2 seconds
    
    Codes: '!' - success, 'Q' - request not sent, '.' - timeout,
      'L' - labeled output interface, 'B' - unlabeled output interface,
      'D' - DS Map mismatch, 'F' - no FEC mapping, 'f' - FEC mismatch,
      'M' - malformed request, 'm' - unsupported tlvs, 'N' - no rx label,
      'P' - no rx intf label prot, 'p' - premature termination of LSP,
      'R' - transit router, 'I' - unknown upstream index,
      'X' - unknown return code, 'x' - return code 0
    
    Type escape sequence to abort.
    
      0 10.10.10.1 MRU 1500 [Labels: 16005 Exp: 0]
    L 1 10.10.10.2 MRU 1500 [Labels: implicit-null Exp: 0] 11 ms
    ! 2 13.13.13.4 4 ms
    

• Protecting LSP reoptimization: Changes to the protecting LSP are performed using a tear and resetup mechanism. When protecting LSP changes are in progress there will be a window during which the tunnel is unprotected and working LSP failures will result in traffic loss.

  Note	This feature is applicable only for bidirectional co-routed tunnels.

• SRLG-Aware Path Protection : This feature specifies that a protecting LSP should be SRLG-diverse from the primary LSP. The user can also specify node-diversity.

  
  RP/0/RP0:hostname# configure
  RP/0/RP0:hostname(config)#interface tunnel-te 100
  RP/0/RP0:hostname(config-if)#path-protection srlg-diverse    
  RP/0/RP0:hostname(config-if)#

• Sticky Paths: This feature allows tunnels to maintain a constant path through the network using dynamic path-options. It applies to both working and protected LSP's and it is an extension of the lockdown property. Sticky paths persist across a tunnel-down event and a sticky LSP that goes down will not come back up until the original path is available.

  Following example shows how a configuration switch is added to the path-option configuration to enable sticky paths:

  
  RP/0/RP0:hostname# configure 
  RP/0/RP0:hostname(config)# interface tunnel-te 1
  RP/0/RP0:hostname(config-if)# path-option 1
  RP/0/RP0:hostname(config-if)# path-option 1 dynamic lockdown sticky

  Following example shows how to verify the sticky path configuration:

  
  RP/0/RP0:hostname# show mpls traffic-eng tunnel
  
  
  Name: tunnel-te1  Destination: 192.168.0.2  Ifhandle:0xd0 
  Signalled-Name: XR_A_t1
  Status:
      Admin:    up Oper:   up (Uptime 2d19h)
  
      path option 10, (LOCKDOWN STICKY) type dynamic  (Basis for Setup, path weight 9 (reverse 9))
        Accumulative metrics: TE 9 (reverse 9) IGP 9 (reverse 9) Delay 300000 (reverse 300000) 
        Protected-by PO index: 20
        Bandwidth:    30000 kbps (CT0) Priority:  7  7 Affinity: 0x0/0xffff
        Metric Type: TE (global)
        Path Selection:
          Tiebreaker: Min-fill (default)
        Hop-limit: disabled
        Cost-limit: disabled
        Delay-limit: disabled
        Path-invalidation timeout: 10000 msec (default), Action: Tear (default)
      path option 20, (LOCKDOWN STICKY) type dynamic  (Basis for Standby, path weight 10 (reverse 10))
        Accumulative metrics: TE 10 (reverse 10) IGP 10 (reverse 10) Delay 300000 (reverse 300000) 
        Bandwidth:    30000 kbps (CT0) Priority:  7  7 Affinity: 0x0/0xffff
        Metric Type: TE (global)
        Path Selection:
          Tiebreaker: Min-fill (default)
        Hop-limit: disabled
        Cost-limit: disabled
        Delay-limit: disabled
        Path-invalidation timeout: 10000 msec (default), Action: Tear (default)
      G-PID: 0x0800 (derived from egress interface properties)
      Bandwidth Requested: 30000 kbps  CT0
      Creation Time: Mon Feb 26 15:35:17 2018 (2d19h ago)
    Config Parameters:
      Bandwidth:    30000 kbps (CT0) Priority:  7  7 Affinity: 0x0/0xffff
      Metric Type: TE (global)
      Path Selection:
        Tiebreaker: Min-fill (default)
      Hop-limit: disabled
      Cost-limit: disabled
      Delay-limit: disabled
      Path-invalidation timeout: 10000 msec (default), Action: Tear (default)
      AutoRoute:  enabled  LockDown: disabled   Policy class: not set
      Forward class: 0 (not enabled)
      Forwarding-Adjacency: disabled
      Autoroute Destinations: 0
      Loadshare:          0 equal loadshares
      Auto-bw: disabled
  Auto-Capacity: Disabled:
      Fast Reroute: Disabled, Protection Desired: None
      Path Protection: Enabled
        Diversity: node, SRLG
        Non-revertive
      Association Type: Single Sided Bidirectional LSPs, Co-routed: YES
      Association ID: 1, Source: 192.168.0.1, Global ID: 1
      Reverse Bandwidth: 30000 kbps (CT0), Standby: 30000 kbps (CT0)
      LSP Wrap Protection: Not Enabled
      BFD Fast Detection: Disabled
      Reoptimization after affinity failure: Enabled
      Soft Preemption: Disabled
    History:
      Tunnel has been up for: 2d19h (since Mon Feb 26 15:36:20 EST 2018)
      Current LSP:
        Uptime: 2d18h (since Mon Feb 26 16:35:15 EST 2018)
      Reopt. LSP:
        Last Failure:
          LSP not signalled, identical to the [CURRENT] LSP
          Date/Time: Thu Mar 01 02:35:15 EST 2018 [08:30:59 ago]
      Standby Reopt LSP:
        Last Failure:
          LSP not signalled, identical to the [STANDBY] LSP
          Date/Time: Thu Mar 01 02:35:15 EST 2018 [08:30:59 ago]
          First Destination Failed: 192.168.0.2
      Prior LSP:
        ID: 2 Path Option: 20
        Removal Trigger: reoptimization completed
      Standby LSP:
        Uptime: 00:01:01 (since Thu Mar 01 11:05:13 EST 2018)
  
    Path info (OSPF 100 area 0):
    Node hop count: 1
    Hop0: 10.10.10.2
    Hop1: 192.168.0.2
  
    Standby LSP Path info (OSPF 100 area 0), Oper State: Up :
    Node hop count: 1
    Hop0: 11.11.11.2
    Hop1: 192.168.0.2
  
    Sticky working path:
      Path-option 10
      Node hop count: 1
      Hop0: 10.10.10.2
  Hop1: 192.168.0.2
  
    Sticky protecting path:
      Path-option 20
      Node hop count: 1
      Hop0: 11.11.11.2
      Hop1: 192.168.0.2
  s
  
  

  Follwing example shows how to clear sticky paths and trigger reoptimization of a specified tunnel. :

  RP/0/RP0:hostname#mpls traffic-eng reroute name t1

• Non-revertive (NRV) Behavior: In case of failure, no new working LSP is established and traffic remains on the protecting LSP. Recovery occurs only in response to user intervention or in event of failure of the protecting path.

  Following example shows how to add a configuration switch to the path-protection configuration and hence enable NRV behavior:

  
    RP/0/RP0:hostname # configure
    RP/0/RP0:hostname(config)# interface tunnel-te 1
    RP/0/RP0:hostname(config-if)# path-protection
    RP/0/RP0:hostname(config-if)# protection-mode non-revertive

  Following example shows how the existing mpls traffic-eng switchover command is extended for NRV recovery:

  
  RP/0/RP0:hostname# mpls traffic-eng path-protection switchover non-revertive tunnel-name t1

• Path Protection Switch Over (PPSO) Recovery:

  PPSO is the XR path-protection recovery mechanism. It is now compliant with the standards and can interop with XE.

• Interoperability: A compatibility configuration switch is added to enable interoperability with XE platform. This switch exists on a per-tunnel basis and is not backward compatible with XR. Enabling or disabling this switch on a tunnel results in tunnel flap. Following is the example of compatibility switch configuration:

  
  RP/0/RP0:hostname# config
  RP/0/RP0:hostname(config)# interface tunnel-te1
  RP/0/RP0:hostname(config-if)# bidirectional                            
  RP/0/RP0:hostname(config-if-bidir)# association type co-routed 
  RP/0/RP0:hostname(config-if-bidir-co-routed)# signaling protection-object  disable

  
  RP/0/RP0:hostname# show mpls traffic-eng tunnels
  
  Association:
         Association Type: Single Sided Bidirectional LSPs (Tie breaking slave)
         Association ID: 999, Source: 192.168.0.4
       Extended Association:
         Global source: 0
         Extended ID:
           0xc0a80003 0x2 0x0 0x0 
         Decoded Extended ID:
           Master source address: 192.168.0.3
           Master LSP-ID: 2
           Master protecting LSP-ID: 0
           Flags: 0x0 (BFD FALSE, NRV FALSE, restore-lsp FALSE)
  

Scale details for Flex LSP:

A co-routed bidirectional packet LSP is a combination of two LSPs (one in the forward direction and the other in reverse direction) sharing the same path between a pair of ingress and egress nodes. It is established using the extensions to RSVP-TE. This type of LSP can be used to carry any of the standard types of MPLS-based traffic, including Layer 2 VPNs.