Bug 1948422

BGP advertised route prefixes are sometimes (~20% of the time) removed on peer BGP routers when the local BGP router is restarted even when BGP graceful restart is enabled. This is known as route flapping causing recalculation of the topology by all participating routers.

Version-Release number of selected component (if applicable):
- frr-7.0-10.el8.x86_64
- frr-7.5-4.el8.x86_64

How reproducible:
~20%

Steps to Reproduce:
1. Create two routers: router-1 (10.20.30.43) and router-2 (10.20.30.44) with the following FRR configuration:

	frr version 7.5
	frr defaults traditional
	hostname router-2.localdomain
	log file /var/log/frr/frr.log
	no ip forwarding
	no ipv6 forwarding
	service integrated-vtysh-config
	!
	debug bgp keepalives
	debug bgp neighbor-events
	debug bgp updates in
	debug bgp updates out
	debug bgp zebra
	!
	router bgp 64999
	 bgp log-neighbor-changes
	 no bgp suppress-duplicates
	 bgp graceful-shutdown
	 bgp graceful-restart
	 bgp graceful-restart preserve-fw-state
	 neighbor 10.20.30.43 remote-as 64999
	 !
	 address-family ipv4 unicast
	  redistribute connected
	 exit-address-family
	!
	line vty

2. On router-1, add dummy route:
	$ sudo ip a a 10.20.50.98/32 dev lo

3. Verify router-2 received route prefix and installed it in the kernel routing table:
	$ sudo ip r | grep 10.20.50.98
	10.20.50.98 nhid 16 via 10.20.30.43 dev eth1 proto bgp metric 20

4. Restart FRR on router-1:
	$ sudo systemctl restart frr

5. Check /var/frr/frr.log in router-2 and note that route 10.20.50.98/32 was deleted as soon as FRR@router-1 was stopped ("Tx route delete VRF 0 10.20.50.98/32"):

	BGP: 10.20.30.41 [Event] BGP connection closed fd 23
	BGP: %NOTIFICATION: received from neighbor 10.20.30.41 6/3 (Cease/Peer Unconfigured) 0 bytes
	BGP: 10.20.30.41 [FSM] Receive_NOTIFICATION_message (Established->Clearing), fd 23
	BGP: %ADJCHANGE: neighbor 10.20.30.41(router-1.localdomain) in vrf default Down BGP Notification received
	BGP: 10.20.30.41 graceful restart stalepath timer stopped
	BGP: bgp_fsm_change_status : vrf default(0), Status: Clearing established_peers 0
	BGP: RID change : vrf VRF default(0), RTR ID 192.168.121.66
	BGP: 10.20.30.41 went from Established to Clearing
	BGP: 10.20.30.41 [FSM] Clearing_Completed (Clearing->Idle), fd -1
	BGP: bgp_fsm_change_status : vrf default(0), Status: Idle established_peers 0
	BGP: 10.20.30.41 went from Clearing to Idle
	BGP: Tx route delete VRF 0 10.20.50.99/32
	BGP: [Event] BGP connection from host 10.20.30.41 fd 23
	BGP: bgp_fsm_change_status : vrf default(0), Status: Active established_peers 0
	BGP: 10.20.30.41 went from Idle to Active
	BGP: 10.20.30.41 [FSM] TCP_connection_open (Active->OpenSent), fd 23
	BGP: 10.20.30.41 passive open
	BGP: 10.20.30.41 Sending hostname cap with hn = router-2.localdomain, dn = (null)
	BGP: 10.20.30.41 sending OPEN, version 4, my as 64999, holdtime 180, id 192.168.121.66
	BGP: bgp_fsm_change_status : vrf default(0), Status: OpenSent established_peers 0
	BGP: 10.20.30.41 went from Active to OpenSent