1. Underlay IGP

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Objective: ensure every PE and P node learns the 32-bit loopback of every other PE/P.

! PE-A
router ospf 1
 router-id 10.0.0.1
 network 10.0.0.1 0.0.0.0 area 0
 network 100.1.1.0 0.0.0.3 area 0

! P
router ospf 1
 router-id 10.0.0.3
 network 10.0.0.3 0.0.0.0 area 0
 network 100.1.1.0 0.0.0.3 area 0
 network 100.1.2.0 0.0.0.3 area 0

! PE-B
router ospf 1
 router-id 10.0.0.2
 network 10.0.0.2 0.0.0.0 area 0
 network 100.1.2.0 0.0.0.3 area 0

Verify adjacencies:

PE-A# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.3          1   FULL/DR         00:00:37    100.1.1.2       Gi0/0

2. Enable MPLS & LDP

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Global:

mpls label protocol ldp
mpls ldp router-id Loopback0 force

On every core-facing interface:

interface GigabitEthernet0/0
 mpls ip

Check LDP sessions:

PE-A# show mpls ldp neighbor
    Peer LDP Ident: 10.0.0.3:0; Local LDP Ident 10.0.0.1:0
        TCP connection: 10.0.0.3.646 - 10.0.0.1.11052
        State: Oper; Msgs sent/rcvd: 42/42

3. Create VRFs

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Design RD/RT to keep VPN-A and VPN-B separate while alloiwng sites of the same VPN too talk.

! PE-A
ip vrf VPN-A
 rd 65000:1001
 route-target export 65000:1001
 route-target import 65000:1001
!
ip vrf VPN-B
 rd 65000:1002
 route-target export 65000:1002
 route-target import 65000:1002

! PE-B (mirror image)
ip vrf VPN-A
 rd 65000:1001
 route-target export 65000:1001
 route-target import 65000:1001
!
ip vrf VPN-B
 rd 65000:1002
 route-target export 65000:1002
 route-target import 65000:1002

4. Attach Customer Links

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! PE-A
interface Gi0/1
 ip vrf forwarding VPN-A
 ip address 192.168.11.1 255.255.255.252
!
interface Gi0/2
 ip vrf forwarding VPN-B
 ip address 192.168.12.1 255.255.255.252

! PE-B
interface Gi0/1
 ip vrf forwarding VPN-A
 ip address 192.168.21.1 255.255.255.252
!
interface Gi0/2
 ip vrf forwarding VPN-B
 ip address 192.168.22.1 255.255.255.252

5. PE-CE Routing

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Use OSPF multi-instance; each VRF runs its own process.

! PE-A
router ospf 101 vrf VPN-A
 network 192.168.11.0 0.0.0.3 area 0
!
router ospf 102 vrf VPN-B
 network 192.168.12.0 0.0.0.3 area 0

! CE-A1 (VPN-A)
router ospf 101
 network 192.168.11.0 0.0.0.3 area 0
 network 10.1.1.0 0.0.0.255 area 0

Repeat symmetrically for PE-B and its CEs.

6. MP-BGP Between PEs

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! PE-A
router bgp 65000
 neighbor 10.0.0.2 remote-as 65000
 neighbor 10.0.0.2 update-source Loopback0
 !
 address-family vpnv4
  neighbor 10.0.0.2 activate
  neighbor 10.0.0.2 send-community both
 exit-address-family

! PE-B (mirror)
router bgp 65000
 neighbor 10.0.0.1 remote-as 65000
 neighbor 10.0.0.1 update-source Loopback0
 !
 address-family vpnv4
  neighbor 10.0.0.1 activate
  neighbor 10.0.0.1 send-community both
 exit-address-family

Confirm:

PE-A# show bgp vpnv4 unicast all summary
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.0.2        4 65000     105     108      152    0    0 01:33:20        4

7. Route Redistribution

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! PE-A
router bgp 65000
 address-family ipv4 vrf VPN-A
  redistribute ospf 101 match internal external
 !
 address-family ipv4 vrf VPN-B
  redistribute ospf 102 match internal external
!
router ospf 101 vrf VPN-A
 redistribute bgp 65000 subnets
!
router ospf 102 vrf VPN-B
 redistribute bgp 65000 subnets

Mirror the same on PE-B.

8. Verification

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On CE-A1 check routes:

CE-A1# show ip route | include 10.2.1.0
O E2    10.2.1.0/24 [110/1] via 192.168.11.1, 00:05:21, GigabitEthernet0/0

Reachability test:

CE-A1# ping 10.2.1.100 source 10.1.1.100
Type escape sequence to abort.
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/4/6 ms

Repeat for VPN-B to confirm isolation and correct forawrding.