Problem with MPLS LDP labels

[mathureshruti]

Hello,

I am implementing MPLS LDP using FRR with Virtual Box Internal Network.
My topology look like this:

When I am running the command sh mpls ldp ipv4 binding and trying to see the label for 192.168.4.0/24 which is Router 4, the Router 1 give me label but Router 2 gives me explicit null at the outgoing due to which Router 3 incoming and outgoing labels both have explicit-null.
The Loopback have proper labels but how can i make my IP interface with correct labels?I am attaching my results for all Routers below.
Router 1:

yfrr version 7.5.1
frr defaults traditional
hostname shruti-VirtualBox
log syslog informational
service integrated-vtysh-config
!
interface enp0s8
ip ospf network point-to-point
!
interface loop1
ip ospf network non-broadcast
!
router ospf
ospf router-id 20.0.0.1
network 20.0.0.1/32 area 0
network 192.168.1.0/24 area 0
network 192.168.2.0/24 area 0
!
mpls ldp
router-id 20.0.0.1
dual-stack transport-connection prefer ipv4
!
address-family ipv4
discovery transport-address 20.0.0.1
label local advertise explicit-null
!
interface enp0s8
!
interface loop1
!
exit-address-family
!
!
access-list LDP seq 10 permit 192.168.1.0/24
!
mpls label global-block 16000 1048575
!
line vty
!

Router 2:

frr version 7.5.1
frr defaults traditional
hostname shruti-VirtualBox
log syslog informational
service integrated-vtysh-config
!
interface enp0s8
ip ospf network point-to-point
!
interface enp0s9
ip ospf network point-to-point
!
interface loop1
ip ospf network non-broadcast
!
router ospf
ospf router-id 11.0.0.1
network 11.0.0.1/32 area 0
network 192.168.1.0/24 area 0
network 192.168.2.0/24 area 0
network 192.168.4.0/24 area 0
!
mpls ldp
router-id 11.0.0.1
dual-stack transport-connection prefer ipv4
!
address-family ipv4
discovery transport-address 11.0.0.1
label local advertise explicit-null
!
interface enp0s8
!
interface enp0s9
!
interface loop1
!
exit-address-family
!
!
access-list LDP seq 5 permit 192.168.1.0/24
access-list LDP seq 10 permit 192.168.2.0/24
!
mpls label global-block 16000 1048575
!
line vty
!

Router 3:

frr version 7.5.1
frr defaults traditional
hostname shruti-VirtualBox
log syslog informational
service integrated-vtysh-config
!
interface enp0s8
ip ospf network point-to-point
!
interface enp0s9
ip ospf network point-to-point
!
interface loop1
ip ospf network non-broadcast
!
router ospf
ospf router-id 12.0.0.1
network 12.0.0.1/32 area 0
network 192.168.2.0/24 area 0
network 192.168.4.0/24 area 0
!
mpls ldp
router-id 12.0.0.1
dual-stack transport-connection prefer ipv4
!
address-family ipv4
discovery transport-address 12.0.0.1
label local advertise explicit-null
!
interface enp0s8
!
interface enp0s9
!
interface loop1
!
exit-address-family
!
!
access-list LDP seq 10 permit 192.168.2.0/24
access-list LDP seq 5 permit 192.168.4.0/24
!
mpls label global-block 16000 1048575
!
line vty
!

Router 4:

frr version 7.5.1
frr defaults traditional
hostname shruti-VirtualBox
log syslog informational
service integrated-vtysh-config
!
interface enp0s8
ip ospf network point-to-point
!
interface loop1
ip ospf network non-broadcast
!
router ospf
ospf router-id 13.0.0.1
network 13.0.0.1/32 area 0
network 192.168.4.0/24 area 0
network 192.168.2.0/24 area 0
!
mpls ldp
router-id 13.0.0.1
dual-stack transport-connection prefer ipv4
!
address-family ipv4
discovery transport-address 13.0.0.1
label local advertise explicit-null
!
interface enp0s8
!
interface loop1
!
exit-address-family
!
!
mpls label global-block 16000 1048575
!
line vty
!

Can someone help me with this?