- Customer reported that bgp peers went down after upgrade from 23.4R2-S3.9  to 24.2R1-S1.9
- Customer reported that only IPV4 peers went down while IPV6 peers remain in established state
- Customer has firewall policy filter applied.
- When downgraded to 23.4R3-S3.9 with the exact same config, the peers gets into established state automatically.
- Can ping to the peer( ping is woking fine)
run show bgp summary 
Threading mode: BGP I/O
Default eBGP mode: advertise - accept, receive - accept
Groups: 2 Peers: 4 Down peers: 2
Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
inet.0               
                       0          0          0          0          0          0
inet6.0              
                       2          1          0          0          0          0
Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
96.109.0.25             777          0          0       0       0     2:14:20 Active
96.109.0.29             777          0          0       0       0     2:14:20 Active
2001:558:96:125::1         777        298        298       0       0     2:13:21 Establ
  inet6.0: 1/1/1/0
2001:558:96:129::1         777        296        298       0       0     2:13:19 Establ
  inet6.0: 0/1/1/0

Trigger is upgrading from 23.4R2-S3.9 to 24.2R1-S1.9

Starting 24.2 release "set chassis loopback-firewall-optimization" CLI is supported via RLI 56582

* Behaviour reported BGP down is seen only when CLI knob is enabled in 24.2 release.

* BGP packets are dropped because of deny-all term in the lo0 filter.

* Upon capturing the dropped BGP packets, they are reaching with ttl as 1.

* For ttl-1 BGP packets to get accepted when "loopback-firewall-optimization" is enabled, we need to add extra ttl-1 term.

* Later on, BGP is coming up as expected and no issue is seen.

In issue state when we delete the "set chassis loopback-firewall-optimization" CLI, issue is NOT seen. BGP is up in 24.2 release.

TTL-1 BGP are dropped due to "Loopback-firewall-optimization" enabled without ttl-1 term. To accept TTL-1 BGP packets when loopback-firewall-optimization is enabled, it is necessary to add an additional term for TTL-1 in the lo0 filter.