R1 is in AS1, R5 in AS5.
R2, R3 and R4 are in AS234.
R1 has an eBGP session with R2.
R5 has an eBGP session with R4.
EIGRP is configured inside AS234.
To allow R1 Loopback0 to reach R5 Loopback0, we must establish an iBGP session between R2 ans R4.
We will use a GRE Tunnel between R2 and R4:
R1:
!
hostname r1
!
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface Serial1/0
ip address 10.150.12.1 255.255.255.0
!
router bgp 1
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255
neighbor 10.150.12.2 remote-as 234
no auto-summary
!
ip route 0.0.0.0 0.0.0.0 10.150.12.2
!
On R5:
!
hostname r5
!
interface Loopback0
ip address 5.5.5.5 255.255.255.255
!
interface Serial1/0
ip address 10.150.45.5 255.255.255.0
!
router bgp 5
no synchronization
bgp router-id 5.5.5.5
bgp log-neighbor-changes
network 5.5.5.5 mask 255.255.255.255
neighbor 10.150.45.4 remote-as 234
no auto-summary
!
ip route 0.0.0.0 0.0.0.0 10.150.45.4
!
On R3:
!
hostname r3
!
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Serial1/0
ip address 10.150.23.3 255.255.255.0
!
interface Serial1/1
ip address 10.150.34.3 255.255.255.0
!
router eigrp 1
network 3.3.3.3 0.0.0.0
network 10.150.23.3 0.0.0.0
network 10.150.34.3 0.0.0.0
no auto-summary
eigrp router-id 3.3.3.3
!
On R2:
!
hostname r2
!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Tunnel0
ip address 10.150.24.2 255.255.255.0
tunnel source 2.2.2.2
tunnel destination 4.4.4.4
!
interface Serial1/0
ip address 10.150.12.2 255.255.255.0
!
interface Serial1/1
ip address 10.150.23.2 255.255.255.0
!
router eigrp 1
network 2.2.2.2 0.0.0.0
network 10.150.23.2 0.0.0.0
no auto-summary
eigrp router-id 2.2.2.2
!
router bgp 234
no synchronization
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor 10.150.12.1 remote-as 1
neighbor 10.150.24.4 remote-as 234
neighbor 10.150.24.4 update-source Tunnel0
neighbor 10.150.24.4 next-hop-self
no auto-summary
!
On R4:
!
hostname r4
!
interface Loopback0
ip address 4.4.4.4 255.255.255.255
!
interface Tunnel0
ip address 10.150.24.4 255.255.255.0
tunnel source 4.4.4.4
tunnel destination 2.2.2.2
!
interface Serial1/0
ip address 10.150.34.4 255.255.255.0
!
interface Serial1/1
ip address 10.150.45.4 255.255.255.0
!
router eigrp 1
network 4.4.4.4 0.0.0.0
network 10.150.34.4 0.0.0.0
no auto-summary
eigrp router-id 4.4.4.4
!
router bgp 234
no synchronization
bgp router-id 4.4.4.4
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 234
neighbor 2.2.2.2 shutdown
neighbor 2.2.2.2 update-source Loopback0
neighbor 10.150.24.2 remote-as 234
neighbor 10.150.24.2 update-source Tunnel0
neighbor 10.150.24.2 next-hop-self
neighbor 10.150.45.5 remote-as 5
no auto-summary
!
We have the same result by establishing a session between R2 and R4 Loopback0...
