This article explains the behavior of a Resource Reservation Protocol (RSVP) label-switched path (LSP) when the no-decrement-ttl option is enabled or disabled.

Topology

R1 ------- R2

R1's loopback address in this case is 150.222.105.91 and R2's loopback address is 3.3.3.1.

When the no-decrement-ttl option is enabled under the RSVP LSP configuration, then the LSP is expected to flap. This is behavior as designed, which is demonstrated in the following example.

On the ingress of the LSP, if you include the no-decrement-ttl statement, the ingress router negotiates with all downstream routers by using a proprietary RSVP object to ensure that all routers are in agreement. If the negotiation succeeds, the entire LSP will be re-signaled and established.

After applying the no-decrement-ttl option under the LSP "LSP-R1-to-R2" on R1, a clear call is generated for this LSP and this is re-signaled as seen in the outputs below.

labroot@R1# set protocols mpls label-switched-path LSP-R1-to-R2 no-decrement-ttl 

labroot@R1> show mpls lsp extensive name LSP-R1-to-R2    
Ingress LSP: 1077 sessions

3.3.3.1
  From: 150.222.105.91, State: Up, ActiveRoute: 0, LSPname: LSP-R1-to-R2
  ActivePath:  (primary)
  LSPtype: Static Configured, Penultimate hop popping
  LoadBalance: Random
  Metric: 200
  Autobandwidth 
  MinBW: 10Mbps, MaxBW: 1000Gbps
  AdjustTimer: 3600 secs AdjustThreshold: 15% Adjustment Threshold Absolute Bandwidth: 20Mbps
  Max AvgBW util: 0bps, Bandwidth Adjustment in 2219 second(s).
  Overflow limit: 1, Overflow sample count: 0
  Underflow limit: 2, Underflow sample count: 0, Underflow Max AvgBW: 0bps
  Encoding type: Packet, Switching type: Packet, GPID: IPv4
  LSP Self-ping Status : Enabled
 *Primary                    State: Up, No-decrement-ttl
    Priorities: 5 5
    Bandwidth: 10Mbps
    OptimizeTimer: 300
    SmartOptimizeTimer: 100
          Exclude: ALL-AVOID
    Flap Count: 1
    MBB Count: 0
    Reoptimization in 247 second(s).
    Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 16777214)
 10.10.10.34 S 
    Received RRO (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt 20=Node-ID):
          10.10.10.34(Label=0)
   17 Dec  5 21:21:51.876 Self-ping ended successfully
   16 Dec  5 21:21:50.010 Up
   15 Dec  5 21:21:50.010 Self-ping started
   14 Dec  5 21:21:50.010 Self-ping enqueued
   13 Dec  5 21:21:50.010 Record Route:  10.10.10.34(Label=0)
   12 Dec  5 21:21:49.807 Originate Call
   11 Dec  5 21:21:49.806 CSPF: computation result accepted  10.10.10.34
   10 Dec  5 21:21:49.804 Clear Call
    9 Dec  5 21:19:40.855 CSPF: computation result ignored, new path no benefit[2 times, first Dec  5 21:14:41.733]
  Created: Sun Dec  5 21:04:06 2021
  Retrytimer: 75

Dec  5 21:21:49.805  R1 rpd[23642]: RPD_MPLS_PATH_DOWN: MPLS path  down on LSP LSP-R1-to-R2
Dec  5 21:21:50.010  R1 rpd[23642]: RPD_MPLS_PATH_UP: MPLS path  up on LSP LSP-R1-to-R2 path bandwidth 10000000 bps

Similarly, the same behavior is observed if the option is deleted from the same LSP on R1 as shown below.

[edit protocols mpls label-switched-path LSP-R1-to-R2]
-    no-decrement-ttl;

labroot@R1> show mpls lsp extensive name LSP-R1-to-R2    
Ingress LSP: 1077 sessions

3.3.3.1
  From: 150.222.105.91, State: Up, ActiveRoute: 0, LSPname: LSP-R1-to-R2
  ActivePath:  (primary)
  LSPtype: Static Configured, Penultimate hop popping
  LoadBalance: Random
  Metric: 200
  Autobandwidth 
  MinBW: 10Mbps, MaxBW: 1000Gbps
  AdjustTimer: 3600 secs AdjustThreshold: 15% Adjustment Threshold Absolute Bandwidth: 20Mbps
  Max AvgBW util: 0bps, Bandwidth Adjustment in 1873 second(s).
  Overflow limit: 1, Overflow sample count: 0
  Underflow limit: 2, Underflow sample count: 0, Underflow Max AvgBW: 0bps
  Encoding type: Packet, Switching type: Packet, GPID: IPv4
  LSP Self-ping Status : Enabled
 *Primary                    State: Up
    Priorities: 5 5
    Bandwidth: 10Mbps
    OptimizeTimer: 300
    SmartOptimizeTimer: 100
          Exclude: ALL-AVOID
    Flap Count: 2
    MBB Count: 0
    Reoptimization in 285 second(s).
    Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 16777214)
 10.10.10.34 S 
    Received RRO (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt 20=Node-ID):
          10.10.10.34(Label=0)
   26 Dec  5 21:28:12.198 Self-ping ended successfully
   25 Dec  5 21:28:08.512 Up
   24 Dec  5 21:28:08.511 Self-ping started
   23 Dec  5 21:28:08.511 Self-ping enqueued
   22 Dec  5 21:28:08.511 Record Route:  10.10.10.34(Label=0)
   21 Dec  5 21:28:08.409 Originate Call
   20 Dec  5 21:28:08.408 CSPF: computation result accepted  10.10.10.34
   19 Dec  5 21:28:08.406 Clear Call
   18 Dec  5 21:26:30.596 CSPF: computation result ignored, new path no benefit
   17 Dec  5 21:21:51.876 Self-ping ended successfully
   16 Dec  5 21:21:50.010 Up
   15 Dec  5 21:21:50.010 Self-ping started
   14 Dec  5 21:21:50.010 Self-ping enqueued
   13 Dec  5 21:21:50.010 Record Route:  10.10.10.34(Label=0)
   12 Dec  5 21:21:49.807 Originate Call
   11 Dec  5 21:21:49.806 CSPF: computation result accepted  10.10.10.34
   10 Dec  5 21:21:49.804 Clear Call
  Created: Sun Dec  5 21:04:06 2021
  Retrytimer: 75
Total 1 displayed, Up 1, Down 0

Dec  5 21:28:08.407  cayenne-re0 rpd[23642]: RPD_MPLS_PATH_DOWN: MPLS path  down on LSP LSP-R1-to-R2
Dec  5 21:28:08.512  cayenne-re0 rpd[23642]: RPD_MPLS_PATH_UP: MPLS path  up on LSP LSP-R1-to-R2 path bandwidth 10000000 bps

• mpls-disabling-normal-ttl-decrementing