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[Junos] LSP with "no-cspf" enabled remains down after link flap

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Article ID: KB37414 KB Last Updated: 30 Aug 2021Version: 1.0
Summary:

On devices that are running Junos OS versions prior to the fix recommended in PR1339916, when label-switched paths (LSPs) are configured with "no-cspf" and without any strict Explicit Route Object (ERO), the LSP remains down after link flap. 

Note: This behavior applies to devices running Junos OS versions prior to Junos OS Release 17.3R3, 17.4R2, 18.1R2 , 18.2R1, 18.3R1.

This article provides a few workarounds that can be implemented in the event that the fix recommended in PR1339916 has not been implemented. 

Symptoms:

The behavior can easily be reproduced in the lab, for example:

  1. LSP "to-2" is using "no-cspf" and strict ERO is not configured.

@mx# show logical-systems 1 protocols mpls 
label-switched-path to-2 {
    to 2.2.2.2;
    no-cspf;
}

@mx> show rsvp session logical-system 1 
Ingress RSVP: 1 sessions
To              From            State   Rt Style Labelin Labelout LSPname 
2.2.2.2         1.1.1.1         Up       0  1 FF       -        3 to-2
Total 1 displayed, Up 1, Down 0

@mx> show rsvp session logical-system 1 detail 
Ingress RSVP: 1 sessions

2.2.2.2
  From: 1.1.1.1, LSPstate: Up, ActiveRoute: 0
  LSPname: to-2, LSPpath: Primary
..
  PATH sentto: 100.0.12.2 (lt-5/0/10.1) 1 pkts
  RESV rcvfrom: 100.0.12.2 (lt-5/0/10.1) 1 pkts, Entropy label: Yes
  Record route: <self> 100.0.12.2  
Total 1 displayed, Up 1, Down 0#2 
  1. After lt-5/0/10.1 is flapped, LSP "to-2" remains down as shown below.

@mx> start shell user root                            
Password:
root@jtac-mx480-r2040-re0:/var/home/labroot # ifconfig lt-5/0/10.1 down
root@jtac-mx480-r2040-re0:/var/home/labroot # ifconfig lt-5/0/10.1 up

@mx> show mpls lsp logical-system 1 extensive 
Ingress LSP: 1 sessions

2.2.2.2
  From: 0.0.0.0, State: Dn, ActiveRoute: 0, LSPname: to-2
  ActivePath: (none)
  LSPtype: Static Configured, Penultimate hop popping
  LoadBalance: Random
  Follow destination IGP metric
  Encoding type: Packet, Switching type: Packet, GPID: IPv4
  LSP Self-ping Status : Enabled
  Primary                    State: Dn
    Priorities: 7 0
    SmartOptimizeTimer: 180
    Flap Count: 1
    MBB Count: 0
   17 Aug 18 11:19:41.380 No Route toward dest
   16 Aug 18 11:19:41.379 100.0.12.1: No Route toward dest
   15 Aug 18 11:19:36.123 Deselected as active
   14 Aug 18 11:19:36.122 100.0.12.1: No Route toward dest
   13 Aug 18 11:19:36.122 100.0.12.1: Down
   12 Aug 18 01:33:00.445 Self-ping ended successfully
   11 Aug 18 01:32:59.428 Selected as active path
   10 Aug 18 01:32:59.427 Up
    9 Aug 18 01:32:59.427 Self-ping started
    8 Aug 18 01:32:59.427 Self-ping enqueued
    7 Aug 18 01:32:59.427 Record Route:  100.0.12.2(Label=3)
    6 Aug 18 01:31:29.396 No Route toward dest[5 times, first Aug 18 01:28:07.393]
    5 Aug 18 01:28:07.392 Originate Call
    4 Aug 18 01:27:38.943 Bad Source Addr
    3 Aug 18 01:27:38.943 Originate Call
    2 Aug 18 01:27:38.943 Bad Source Addr
    1 Aug 18 01:27:38.943 Originate Call
  Created: Wed Aug 18 01:27:38 2021
Total 1 displayed, Up 0, Down 1

Egress LSP: 0 sessions
Total 0 displayed, Up 0, Down 0

Transit LSP: 0 sessions
Total 0 displayed, Up 0, Down 0
  1. LSP "to-2" cannot come up until we manually clear the LSP (clear mpls lsp).

@mx> clear mpls lsp name to-2 logical-system 1 

@mx> show mpls lsp logical-system 1 extensive     
Ingress LSP: 1 sessions

2.2.2.2
  From: 1.1.1.1, State: Up, ActiveRoute: 0, LSPname: to-2
  ActivePath:  (primary)
  LSPtype: Static Configured, Penultimate hop popping
  LoadBalance: Random
  Follow destination IGP metric
  Encoding type: Packet, Switching type: Packet, GPID: IPv4
  LSP Self-ping Status : Enabled
 *Primary                    State: Up
    Priorities: 7 0
    SmartOptimizeTimer: 180
    Flap Count: 2
    MBB Count: 0
    Received RRO (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt 20=Node-ID):
          100.0.12.2(Label=3)
   33 Aug 18 11:22:14.682 Self-ping ended successfully
   32 Aug 18 11:22:14.372 Selected as active path
   31 Aug 18 11:22:14.371 Up
   30 Aug 18 11:22:14.370 Self-ping started
   29 Aug 18 11:22:14.370 Self-ping enqueued
   28 Aug 18 11:22:14.370 Record Route:  100.0.12.2(Label=3)
   27 Aug 18 11:22:14.368 Originate Call
   26 Aug 18 11:22:14.367 Clear Call
   25 Aug 18 11:22:14.367 Deselected as active
   24 Aug 18 11:20:32.307 Self-ping ended successfully
   23 Aug 18 11:20:31.893 Selected as active path
   22 Aug 18 11:20:31.892 Up
   21 Aug 18 11:20:31.892 Self-ping started
   20 Aug 18 11:20:31.892 Self-ping enqueued
   19 Aug 18 11:20:31.892 Record Route:  100.0.12.2(Label=3)
   18 Aug 18 11:20:31.892 Stats related identifier changed
   17 Aug 18 11:19:41.380 No Route toward dest
   16 Aug 18 11:19:41.379 100.0.12.1: No Route toward dest
   15 Aug 18 11:19:36.123 Deselected as active
   14 Aug 18 11:19:36.122 100.0.12.1: No Route toward dest
   13 Aug 18 11:19:36.122 100.0.12.1: Down
   12 Aug 18 01:33:00.445 Self-ping ended successfully
   11 Aug 18 01:32:59.428 Selected as active path
   10 Aug 18 01:32:59.427 Up
    9 Aug 18 01:32:59.427 Self-ping started
    8 Aug 18 01:32:59.427 Self-ping enqueued
    7 Aug 18 01:32:59.427 Record Route:  100.0.12.2(Label=3)
    6 Aug 18 01:31:29.396 No Route toward dest[5 times, first Aug 18 01:28:07.393]
    5 Aug 18 01:28:07.392 Originate Call
    4 Aug 18 01:27:38.943 Bad Source Addr
    3 Aug 18 01:27:38.943 Originate Call
    2 Aug 18 01:27:38.943 Bad Source Addr
    1 Aug 18 01:27:38.943 Originate Call
  Created: Wed Aug 18 01:27:38 2021
Total 1 displayed, Up 1, Down 0

Egress LSP: 0 sessions
Total 0 displayed, Up 0, Down 0

Transit LSP: 0 sessions
Total 0 displayed, Up 0, Down 0
Cause:

The behavior is fixed in PR1339916.

Solution:

In devices that have not implemented the fix recommended in PR1339916, you can use any one of the following workarounds to avoid the problem. 

  • To use "cspf" rather than "no-cspf" for LSPs, consider the following:

label-switched-path to-2 {
    to 2.2.2.2;        << cspf is enabled by default. If the "no-cspf" keyword does not exist, "cspf" is used.
 }
  • To use strict ERO for LSPs, use the following:

mpls {
        label-switched-path to-2 {
            to 2.2.2.2;
            no-cspf;
            primary main;
        }
        path main {
            100.0.12.2;
        }
}
  • On devices running Junos OS 16.1 and later, use the following knob:

set protocols rsvp local-reversion 

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