configure BGP on the router

R5 (ISP)
conf t
router bgp 6500
neighbor 10.1.45.1 remote-as 5500

R4
conf t
router bgp 5500
neighbor 10.1.45.1 remote-as 6500

 IBGP configuration

R1
 conf t
int loopback 0
ip add 1.1.1.1 255.255.255.255
!
router bgp 5500
neighbor 4.4.4.4 update-source loopback 0
!

``` R4 conf t int loopback 0 ip add 4.4.4.4 255.255.255.255 ! router bgp 5500 neighbor 1.1.1.1 update-source loopback 0

eBGP configuration

neighbor 5.5.5.5 ebgp-multihop 3

advertise networks in BGP

R5 (ISP) conf t router bgp 6500 network 50.1.1.0 mask 255.255.255.0

R4 BGP

router bgp 5500 bgp log-neighbor-changes neighbor 1.1.1.1 remote-as 5500 neighbor 1.1.1.1 update-source Loopback0 neighbor 5.5.5.5 remote-as 6500 neighbor 5.5.5.5 ebgp-multihop 3 neighbor 5.5.5.5 update-source Loopback0

# R5 BGP router bgp 6500 bgp log-neighbor-changes network 50.1.1.0 mask 255.255.255.0 neighbor 4.4.4.4 remote-as 5500 neighbor 4.4.4.4 ebgp-multihop 3 neighbor 4.4.4.4 update-source Loopback0

# transform eBgp routes to iBgp R4 conf t router bgp 5500 neighbor 1.1.1.1 next-hop-self