R1

R1#sh running-config 
Building configuration...

Current configuration : 1641 bytes
!

hostname R1
!


!
!
interface Loopback1
 ip address 192.168.1.0 255.255.255.255
!         
interface Loopback2
 ip address 192.168.2.0 255.255.255.255
!
interface Loopback3
 ip address 192.168.3.0 255.255.255.255
!
interface Loopback4
 ip address 192.168.4.0 255.255.255.255
!

interface Serial1/0
 ip address 10.0.0.0 255.255.255.254
 serial restart-delay 0
!
interface Serial1/1
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/2
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router bgp 100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 redistribute connected
 neighbor 10.0.0.1 remote-as 200
!         

end

R1#

R2

R2#sh running-config 
Building configuration...

Current configuration : 1572 bytes
!
version 15.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R2
!
interface Serial1/0
 ip address 10.0.0.1 255.255.255.254
 serial restart-delay 0
!
interface Serial1/1
 ip address 20.0.0.0 255.255.255.254
 serial restart-delay 0
!

router bgp 200
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 redistribute connected
 neighbor 10.0.0.0 remote-as 100
 neighbor 20.0.0.1 remote-as 300
!
ip forward-protocol nd
!

end

R2#

R3

R3#sh running-config 
Building configuration...

Current configuration : 1393 bytes

hostname R3
!

interface Ethernet0/3
 no ip address
!
interface Serial1/0
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/1
 ip address 20.0.0.1 255.255.255.254
 serial restart-delay 0
!
interface Serial1/2
 no ip address
 shutdown
 serial restart-delay 0
!         
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router bgp 300
 bgp router-id 3.3.3.3
 bgp log-neighbor-changes
 redistribute connected
 neighbor 20.0.0.0 remote-as 200
!

!

end

R3#


Youtube link https://youtu.be/OwdaDUVZvLE?si=aOQK7t7Ae6zq0z67