FPC4 was showing Major Errors:

> show system alarms

1 alarms currently active

Alarm time Class Description

2024-07-05 21:56:13 CDT Major FPC 4 Major Errors

Apart from this BGP sessions flapped for five peers though they was no config for interfaces associated with FPC4

Platform Details:

Model: mx960

Junos: 21.4R3.15

Routing Engine 0 REV 07  XXXXX  XXXXX    RE-S-1800x4

Routing Engine 1 REV 07  XXXXX  XXXXX    RE-S-1800x4

FPC 4      REV 45  XXXXX  XXXXX    MPC5E 3D 24XGE+6XLGE

• Validated that there was CMError seen on FPC due to which Major Error was raised and pfe 0 got disabled.

Jul 5 21:56:06 : %PFE-5: fpc4 CMError: /fpc/4/pfe/0/cm/0/XMCHIP(0)/0/XMCHIP_CMERROR_PT_MEM_PROTECT_FSET_REG_DETECTED_PCT_FL_MEM (0x702c6), scope: pfe, category: functional, severity: major, module: XMCHIP(0), type: PT_PROTECT: Detected: Parity error for PCT freelist

!

Jul 5 21:56:13 : %PFE-5: fpc4 Performing action disable-pfe for error /fpc/4/pfe/0/cm/0/XMCHIP(0)/0/XMCHIP_CMERROR_PT_MEM_PROTECT_FSET_REG_DETECTED_PCT_FL_MEM (0x702c6) in module: XMCHIP(0) with scope: pfe category: functional level: major

It is a known error and the cause is due to an internal transient issue detected in the XMCHIP Packet Table processing. A single occurrence of this message indicates just a one-time condition.

Reference KB:
https://supportportal.juniper.net/s/article/SRXMX-FPC-Major-ErrorsXMCHIP-CMERROR-PT-MEM-PROTECT-FSET-REG-DETECTED-PCT-FL-MEM

• The BGP flapped due to BFD with following logs:

Jul 5 21:56:20 hname rpd[15575]: %DAEMON-4-RPD_BGP_NEIGHBOR_STATE_CHANGED: BGP peer x.x.x.x (Internal AS ##) changed state from Established to Idle (event RecvNotify) (instance master)                     
Jul 5 21:56:20 hname rpd[15575]: %DAEMON-4: bgp_handle_notify:4587: NOTIFICATION received from x.x.x.x (Internal AS ##): code 6 (Cease) subcode 9 (Hard Reset) [code 6 (Cease) subcode 10 (BFD Down)]

• For BGP session flapped observed at the same time was due to BFD session flap for the respective peers.
• BFD session were stuck in init state for 5 bgp sessions which flapped.

uname@hname> show bfd session | except UP 
Jul 05 23:53:13
                         Detect  Transmit
Address         State   Interface   Time   Interval Multiplier
a.a.a.a       Init           7.500   2.500    3  
b.b.b.b     Init           7.500   2.500    3  
x.x.x.x      Init           7.500   2.500    3  
c.c.c.c      Init           7.500   2.500    3  
d.d.d.d       Init           7.500   2.500    3  
e.e.e.e       Init           7.500   2.500    3

!

• Validated that BFD session is distributed multihop and destination fpc /anchor fpc is 4 for these bfd sessions.

uname@hname> show bfd session address x.x.x.x extensive 
Jul 05 23:58:40
                         Detect  Transmit
Address         State   Interface   Time   Interval Multiplier
x.x.x.x     Init           7.500   2.500    3  
 Client BGP, TX interval 2.500, RX interval 2.500
Session down time 02:02:21, previous up time 31w5d 18:23:15
Local diagnostic NbrSignal, remote diagnostic CtlExpire
Remote state Down, version 1
Replicated 
 Session type: Multi hop BFD
Min async interval 2.500, min slow interval 2.500
Adaptive async TX interval 2.500, RX interval 2.500
Local min TX interval 2.500, minimum RX interval 2.500, multiplier 3
Remote min TX interval 2.500, min RX interval 2.500, multiplier 3
Threshold for detection time 8.000
Local discriminator 245, remote discriminator 380
Echo TX interval 0.000, echo detection interval 0.000
Echo mode disabled/inactiveno-absorb, no-refresh
Remote is control-plane independent
Multi-hop route table 0, local-address z.z.z.z
 Session ID: 0

 1 sessions, 1 clients
Cumulative transmit rate 0.4 pps, cumulative receive rate 0.4 pps

uname@hname> show ppm adjacencies protocol bfd detail remote    

!!!!! 
Protocol: BFD, Hold time: 7500, IFL-index: 0
No-absorb, No-refresh, Do-not-age, Distributed: TRUE
Replicated
In Rule thread:
BFD discriminator: 245, BFD routing table index: 0
Redirection Type: DYNAMIC_FILTER, Rule Term Src: x.x.x.x, Rule Term Port: 4784, Rule Term Action: 1226
Num Packets: 0, Absorbed Packets: 0
Distribution handle: 281, Distribution address: fpc4  

!!!!

Restarting FPC4 removed the anchor fpc4 on the affected bfd session and bgp was restored and also FPC card came up after reset.