Use below mentioned configurations for four routers i hope issue will be solved and after doing this use
clear ip bgp *
R1
router bgp 333
no synchronization
bgp log-neighbor-changes
network 131.108.1.0 mask 255.255.255.0
neighbor ibgp peer-group
neighbor ibgp remote-as 333
neighbor ibgp update-source Loopback0
neighbor 131.108.254.2 peer-group ibgp
neighbor 131.108.254.3 peer-group ibgp
neighbor 131.108.254.4 remote-as 333
neighbor 131.108.254.6 peer-group ibgp
distance bgp 20 50 50
no auto-summary
!
R2
router bgp 333
no synchronization
bgp log-neighbor-changes
neighbor ibgp peer-group
neighbor ibgp remote-as 333
neighbor ibgp update-source Loopback0
neighbor 131.108.254.1 peer-group ibgp
neighbor 131.108.254.3 peer-group ibgp
neighbor 131.108.254.4 peer-group ibgp
no auto-summary
R3
router bgp 333
synchronization
bgp log-neighbor-changes
network 131.108.3.0 mask 255.255.255.0
neighbor ibgp peer-group
neighbor ibgp remote-as 333
neighbor ibgp update-source Loopback0
neighbor 131.108.254.1 peer-group ibgp
neighbor 131.108.254.2 peer-group ibgp
neighbor 131.108.254.4 peer-group ibgp
neighbor 131.108.254.6 peer-group ibgp
distance bgp 20 50 50
no auto-summary
R4
router bgp 333
no synchronization
bgp log-neighbor-changes
neighbor ibgp peer-group
neighbor ibgp remote-as 333
neighbor ibgp update-source Loopback0
neighbor 131.108.254.1 peer-group ibgp
neighbor 131.108.254.3 peer-group ibgp
neighbor 131.108.254.6 peer-group ibgp
distance bgp 20 50 50
no auto-summary
R6
router bgp 333
no synchronization
bgp log-neighbor-changes
neighbor ibgp peer-group
neighbor ibgp remote-as 333
neighbor ibgp update-source Loopback0
neighbor 131.108.254.3 peer-group ibgp
no auto-summary
