CE-PE: BGP

Starting with the remote sites.

R17
interface Loopback17
 ip address 17.17.17.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 172.16.37.2 255.255.255.0
 speed 100
 full-duplex
!
router bgp 65000
 no synchronization
 bgp log-neighbor-changes
 network 17.17.17.0 mask 255.255.255.0
 neighbor 172.16.37.1 remote-as 100
 neighbor 172.16.37.1 allowas-in
 no auto-summary
!

R19
interface Loopback19
 ip address 19.19.19.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 172.16.198.2 255.255.255.0
 speed 100
 full-duplex
!
router bgp 65000
 no synchronization
 bgp log-neighbor-changes
 network 19.19.19.0 mask 255.255.255.0
 neighbor 172.16.198.1 remote-as 100
 neighbor 172.16.198.1 allowas-in
 no auto-summary
!

Moving to the PEs facing the CEs.

R8
router bgp 100
 address-family ipv4 vrf CUSTC100
 neighbor 172.16.198.2 remote-as 65000
 neighbor 172.16.198.2 activate
 no synchronization
 network 172.16.198.0 mask 255.255.255.0
 exit-address-family
!

R13
router bgp 100
 address-family ipv4 vrf CUSTC222
 neighbor 172.16.37.2 remote-as 65000
 neighbor 172.16.37.2 activate
 no synchronization
 network 172.16.37.0 mask 255.255.255.0
 exit-address-family
!

Verifying that BGP is up and established between the PEs-PEs:

Last step is configuring ISP to ISP PEs.

R5:
router bgp 100
 address-family ipv4 vrf CUSTC100
  neighbor 172.16.153.1 remote-as 500
  neighbor 172.16.153.1 activate
  neighbor 172.16.153.1 as-override
  neighbor 172.16.153.1 soft-reconfiguration inbound
  no synchronization
exit-address-family
!

R1:
router bgp 500
 address-family ipv4 vrf CUSTC500
  neighbor 172.16.153.2 remote-as 100
  neighbor 172.16.153.2 activate
  neighbor 172.16.153.2 as-override
  neighbor 172.16.153.2 soft-reconfiguration inbound
  no synchronization
 exit-address-family
!

R4:
router bgp 500
 address-family ipv4 vrf CUSTC500 
  neighbor 172.16.143.2 remote-as 100
  neighbor 172.16.143.2 activate
  neighbor 172.16.143.2 as-override
  neighbor 172.16.143.2 soft-reconfiguration inbound
no synchronization
exit-address-family
!

R10:
router bgp 100
 address-family ipv4 vrf CUSTC222
  neighbor 172.16.143.1 remote-as 500
  neighbor 172.16.143.1 activate
  neighbor 172.16.143.1 soft-reconfiguration inbound
  no synchronization
 exit-address-family
!

Confirming that both remote routers are aware of each other networks and site to site connectivity is present across the MPLS colud: