This article describes the meaning of syslog message RPD_MPLS_PATH_BFD_DOWN MPLS BFD session for path PATH_NAME

The following BFD logs are observed when there is a change in the MPLS LSP path from primary to secondary with OAM-based BFD enabled for said LSP:
May 24 00:15:12.024 MX960_RE0 rpd[80942]: RPD_MPLS_LSP_BANDWIDTH_CHANGE: MPLS LSP MX_TO_PTX:LSP-1 bandwidth changed, lsp bandwidth 23381542 bps
May 24 00:15:12.026 MX960_RE0 bfdd[79981]: BFDD_STATE_UP_TO_DOWN: BFD Session 127.0.0.1 (IFL 331) state Up -> Down LD/RD(5016688/10472) Up time:00:23:57 Local diag: AdminDown Remote diag: None Reason: Received Upstream Destroy Session.
May 24 00:15:12.026 MX960_RE0 bfdd[79981]: BFDD_TRAP_STATE_DOWN: local discriminator: 5016688, new state: down
May 24 00:15:12.027 MX960_RE0 bfdd[79981]: BFDD_STATE_UP_TO_DOWN: BFD Session 127.0.0.1 (IFL 331) state Up -> Down LD/RD(5014681/1795) Up time:00:40:33 Local diag: AdminDown Remote diag: None Reason: Received Upstream Destroy Session.
May 24 00:15:12.027 MX960_RE0 bfdd[79981]: BFDD_TRAP_STATE_DOWN: local discriminator: 5014681, new state: down
May 24 00:15:39.742 MX960_RE0 root[67483]: invoke-commands: Executed /tmp/evt_cmd_XlHDGo, output to /tmp/evt_op_ICPnwX in text format
May 24 00:15:48.188 MX960_RE0 bfdd[79981]: BFDD_STATE_UP_TO_DOWN: BFD Session 127.0.0.1 (IFL 331) state Up -> Down LD/RD(5018721/13475) Up time:00:04:45 Local diag: CtlExpire Remote diag: None Reason: Detect Timer Expiry.
May 24 00:15:48.188 MX960_RE0 bfdd[79981]: BFDD_TRAP_STATE_DOWN: local discriminator: 5018721, new state: down
May 24 00:15:48.255 MX960_RE0 rpd[80942]: RPD_MPLS_PATH_BFD_DOWN: BFD session for MPLS path went down on LSP PTX_TO_MX-1 ----> Log is observed when a BFD session for a path fails [in our case for secondary path]
May 24 00:15:49.635  MX960_RE0 rpd[80942]: RPD_IFL_NOTIFICATION: EVENT [Bandwidth] ae1.106 index 329 [Up Broadcast Multicast] address #0 fe.96.43.45.a5.14
May 24 00:15:49.652  MX960_RE0 lfmd[80012]: Action Syslog on et-0/0/0 [link-down]: :Adjacency Lost
May 24 00:15:49.637  MX960_RE0 lfmd[80012]: LFMD_3AH_LINKDOWN: (et-0/0/0): 802.3ah link-fault status changed to fault with reason [Local link fault]
May 24 00:15:49.638  MX960_RE0 rpd[80942]: RPD_IFL_NOTIFICATION: EVENT [UpDown] et-0/0/0.106 index 384 [Broadcast Multicast] address #0 fe.96.43.45.a5.14
May 24 00:15:49.639  MX960_RE0 rpd[80942]: RPD_IFL_NOTIFICATION: EVENT [Bandwidth] ae1.106 index 329 [Up Broadcast Multicast] address #0 fe.96.43.45.a5.14
May 24 00:15:49.705  MX960_RE0 mib2d[81336]: SNMP_TRAP_LINK_DOWN: ifIndex 582, ifAdminStatus up(1), ifOperStatus down(2), ifName et-0/0/0
May 24 00:15:51.421  MX960_RE0 bfdd[79981]: BFDD_STATE_UP_TO_DOWN: BFD Session 127.0.0.1 (IFL 331) state Up -> Down LD/RD(5018726/13495) Up time:00:04:53 Local diag: CtlExpire Remote diag: None Reason: Detect Timer Expiry.
May 24 00:15:51.422  MX960_RE0 bfdd[79981]: BFDD_TRAP_STATE_DOWN: local discriminator: 5018726, new state: down 

A snippet of the configuration where OAM-based BFD is enabled for MPLS LSP:
interfaces {
    et-0/0/0 {
        gigether-options {
            802.3ad ae1;
        }
    }
    ae1 {
        aggregated-ether-options {
            minimum-links 1;
            lacp {
                active;
            }
        }
        unit 106 {
            vlan-id 106;
            family inet {
                address 10.10.10.1/30;
            }
            family iso;
            family inet6 {
                dad-disable;
            }
            family mpls {
                maximum-labels 5;
            }
 
  mpls {
         admin-groups {
            CONVERGE 20; 
        }       
        advertisement-hold-time 10;
        rsvp-error-hold-time 180;
        optimize-aggressive;
        no-propagate-ttl;
        explicit-null;
        oam {
            bfd-port {
                import UDP-4784;
            }
            lsp-ping-interval 3600;
            lsp-ping-multiplier 3;
        }
        label-switched-path LSP-TEMPLATE {
            template;
            ldp-tunneling;
            admin-group include-any CONVERGE;
            priority 3 3;
            oam {
                bfd-liveness-detection {
                    minimum-interval 20000;
                    minimum-receive-interval 20000;
                    multiplier 3;
                    no-router-alert-option;
                    use-ip-ttl-1;
                }
            }
            optimize-timer 1800;
            node-link-protection;
            adaptive;
            auto-bandwidth {
                adjust-interval 2000;
                adjust-threshold 10;
                minimum-bandwidth 1m;
                maximum-bandwidth 40g;
                adjust-threshold-overflow-limit 2;
            } 
    rsvp {
        no-enhanced-frr-bypass;
        interface lo0.0;
        interface ae1.106 {
            link-protection {
                admin-group include-any [ CONVERGE ];
            }
        }

We can enable BFD for RSVP for unicast IPv4 LSPs. When BFD is configured for an RSVP LSP on the ingress router, it is enabled on the primary path and on all standby secondary paths for that LSP. The source IP address for outgoing BFD packets from the egress side of an MPLS BFD session is based on the outgoing interface IP address. 

Whenever a BFD session for a path fails an error is logged for it. The following example shows how BFD for RSVP LSP log messages might appear:
RPD_MPLS_PATH_BFD_UP: MPLS BFD session for path path1 up on LSP R0_to_R3
RPD_MPLS_PATH_BFD_DOWN: MPLS BFD session for path path1 down on LSP R0_to_R3

To diagnose the root-cause, we need to verify why there was change in the LSP path from primary to secondary. We can use below show commands or traceoptions [mpls|rsvp] for analysis:
show mpls lsp extensive
show mpls lsp name <LSP_NAME> extensive   

Enable traceoptions for MPLS & RSVP via:
set protocols mpls traceoptions flag all
set protocols mpls traceoptions file MPLS size 100m files 10 world-readable

set protocols rsvp traceoptions flag all 
set protocols mpls traceoptions file RSVP size 100m files 10 world-readable

Refer to the below document for more details:
https://www.juniper.net/documentation/us/en/software/junos/mpls/topics/topic-map/bfd-for-mpls.html#id-19110__d248e76

Contact JTAC if you need assistance with issue isolation.