Complete MPLS L3VPN Multi-AS Configuration Guide
This complete MPLS VPN tutorial explains how to configure a full MPLS Layer 3 VPN infrastructure using OSPF, IS-IS, MPLS LDP, MP-BGP VPNv4 Route Reflectors, VRFs, and PE-CE BGP routing.
This guide is designed for CCNP, CCIE Service Provider, Enterprise MPLS engineers, and network architects who want deep understanding of MPLS VPN architecture.
🎯 Topics Covered
- OSPF Underlay Configuration
- IS-IS Underlay Configuration
- MPLS LDP Configuration
- VPNv4 Route Reflectors
- MP-BGP Configuration
- VRF Configuration
- PE-CE BGP Configuration
- MPLS VPN Mathematics
- Verification Commands
- Troubleshooting
Table of Contents
- 1. MPLS VPN Introduction
- 2. OSPF Underlay Configuration
- 3. MPLS LDP in AS100
- 4. IS-IS Underlay Configuration
- 5. MPLS LDP in AS200
- 6. VPNv4 Route Reflector AS100
- 7. VRF Configuration AS100
- 8. PE-CE BGP AS100
- 9. VPNv4 Route Reflector AS200
- 10. VRF Configuration AS200
- 11. PE-CE BGP AS200
- 12. Verification Commands
- 13. Troubleshooting
- 14. Related Articles
1. MPLS VPN Introduction
MPLS VPN allows multiple customers to use the same service provider backbone while keeping their routing information isolated.
The MPLS backbone uses labels instead of traditional IP routing lookups.
MPLS Label Switching Formula
$$ Forwarding = Label\\ Lookup + LFIB\\ Decision $$Where:
- LFIB = Label Forwarding Information Base
- Labels improve forwarding efficiency
- MPLS reduces complex routing lookups
2. Configure OSPF Underlay in AS100
OSPF is used as the IGP underlay routing protocol between routers R1, R2, R3 and R4.
All internal links belong to Area 0.
OSPF Cost Formula
$$ Cost = \frac{Reference\\ Bandwidth}{Interface\\ Bandwidth} $$Default Cisco reference bandwidth:
$$ 100Mbps $$R1 OSPF Configuration
R1 router ospf 1 router-id 1.1.1.1 network 1.1.1.1 0.0.0.0 area 0 network 192.1.13.0 0.0.0.255 area 0 network 192.1.100.0 0.0.0.255 area 0
R2 OSPF Configuration
R2 router ospf 1 router-id 2.2.2.2 network 2.2.2.2 0.0.0.0 area 0 network 192.1.23.0 0.0.0.255 area 0 network 192.1.100.0 0.0.0.255 area 0
R3 OSPF Configuration
R3 router ospf 1 router-id 3.3.3.3 network 3.3.3.3 0.0.0.0 area 0 network 192.1.13.0 0.0.0.255 area 0 network 192.1.24.0 0.0.0.255 area 0 network 192.1.34.0 0.0.0.255 area 0
R4 OSPF Configuration
R4 router ospf 1 router-id 4.4.4.4 network 4.4.4.4 0.0.0.0 area 0 network 192.1.34.0 0.0.0.255 area 0 network 192.1.100.0 0.0.0.255 area 0
Why OSPF as MPLS Underlay?
OSPF provides:
- Fast convergence
- Loop-free topology
- Efficient SPF calculations
- Scalable link-state architecture
3. Configure MPLS LDP in AS100
LDP distributes MPLS labels between routers.
MPLS Label Distribution Logic
$$ FEC \rightarrow Label\\ Mapping $$Where:
- FEC = Forwarding Equivalence Class
- Every route receives a unique label
R1 MPLS Configuration
R1 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
R2 MPLS Configuration
R2 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
R3 MPLS Configuration
R3 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
R4 MPLS Configuration
R4 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip interface e0/2 mpls ip
How MPLS LDP Works
LDP establishes neighbor relationships and exchanges labels.
The label-switched path is dynamically created.
4. Configure IS-IS Underlay in AS200
IS-IS is another link-state protocol heavily used in service provider networks.
Shortest Path First Formula
$$ SPF = Minimum\\ Path\\ Cost $$R5 IS-IS Configuration
R5 router isis net 49.0000.5555.5555.5555.00 is-type level-2 metric-style wide interface e0/0 ip router isis interface e0/1 ip router isis interface loopback0 ip router isis
R6 IS-IS Configuration
R6 router isis net 49.0000.6666.6666.6666.00 is-type level-2 metric-style wide interface e0/0 ip router isis interface e0/1 ip router isis interface loopback0 ip router isis
R7 IS-IS Configuration
R7 router isis net 49.0000.7777.7777.7777.00 is-type level-2 metric-style wide interface e0/0 ip router isis interface e0/1 ip router isis interface e0/2 ip router isis interface loopback0 ip router isis
R8 IS-IS Configuration
R8 router isis net 49.0000.8888.8888.8888.00 is-type level-2 metric-style wide interface e0/0 ip router isis interface e0/1 ip router isis interface loopback0 ip router isis
Why IS-IS in Service Provider Networks?
- Scales better in large networks
- Runs directly over Layer 2
- Efficient SPF calculations
- Widely used in ISP cores
5. Configure MPLS LDP in AS200
R5 MPLS LDP
R5 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
R6 MPLS LDP
R6 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
R7 MPLS LDP
R7 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip interface e0/2 mpls ip
R8 MPLS LDP
R8 mpls ldp router-id Loopback0 interface e0/0 mpls ip interface e0/1 mpls ip
6. Configure VPNv4 Route Reflector in AS100
R3 acts as VPNv4 Route Reflector.
BGP Scalability Formula
$$ Total\\ Sessions = \frac{n(n-1)}{2} $$Route Reflectors reduce IBGP full mesh complexity.
R1 VPNv4 BGP
R1 router bgp 100 neighbor 3.3.3.3 remote-as 100 neighbor 3.3.3.3 update-source loopback0 address-family vpnv4 neighbor 3.3.3.3 activate
R3 Route Reflector
R3 router bgp 100 neighbor IBGP peer-group neighbor IBGP remote-as 100 neighbor IBGP update-source loopback0 address-family vpnv4 neighbor IBGP route-reflector-client neighbor 1.1.1.1 activate neighbor 2.2.2.2 activate neighbor 4.4.4.4 activate
Route Reflector Logic
Without Route Reflectors:
$$ IBGP = Full\\ Mesh $$With Route Reflectors:
$$ IBGP = Centralized\\ Reflection $$7. Configure VRF Cust-A in AS100
VPNv4 Route Formula
$$ VPNv4 = RD + IPv4\\ Prefix $$R1 VRF Configuration
R1 vrf definition Cust-A rd 100:1 address-family ipv4 route-target both 100:1 interface e0/2 vrf forwarding Cust-A ip address 192.1.10.1 255.255.255.0 no shutdown
R2 VRF Configuration
R2 vrf definition Cust-A rd 100:1 address-family ipv4 route-target both 100:1 interface e0/2 vrf forwarding Cust-A ip address 192.1.20.2 255.255.255.0 no shutdown
8. Configure PE-CE BGP in AS100
R1 PE-CE BGP
R1 router bgp 100 address-family ipv4 vrf Cust-A neighbor 192.1.10.9 remote-as 65009
R9 CE Configuration
R9 router bgp 65009 network 10.9.9.0 mask 255.255.255.0 neighbor 192.1.10.1 remote-as 100
9. Configure VPNv4 Route Reflector in AS200
R7 Route Reflector
R7 router bgp 200 neighbor IBGP peer-group neighbor IBGP remote-as 200 neighbor IBGP update-source loopback0 address-family vpnv4 neighbor IBGP route-reflector-client neighbor 5.5.5.5 activate neighbor 6.6.6.6 activate neighbor 8.8.8.8 activate
10. Configure VRF Cust-A in AS200
R5 VRF Configuration
R5 vrf definition Cust-A rd 200:1 address-family ipv4 route-target both 200:1 interface e0/2 vrf forwarding Cust-A ip address 192.1.50.5 255.255.255.0 no shutdown
11. Configure PE-CE BGP in AS200
R5 PE-CE BGP
R5 router bgp 200 address-family ipv4 vrf Cust-A neighbor 192.1.50.11 remote-as 65011
R11 CE BGP
R11 router bgp 65011 network 10.11.11.0 mask 255.255.255.0 neighbor 192.1.50.5 remote-as 200
12. Verification Commands
Verify OSPF
show ip ospf neighbor show ip route ospf
Verify IS-IS
show isis neighbors show clns neighbors
Verify MPLS
show mpls ldp neighbor show mpls forwarding-table
Verify VPNv4
show bgp vpnv4 unicast all
Verify VRFs
show vrf show ip route vrf Cust-A
13. MPLS Troubleshooting
| Problem | Possible Cause | Solution |
|---|---|---|
| No LDP Neighbor | mpls ip missing | Enable MPLS on interfaces |
| BGP VPNv4 down | Loopback unreachable | Check IGP reachability |
| VRF routes missing | RT mismatch | Verify route-targets |
| OSPF neighbor stuck | Area mismatch | Verify area IDs |
| IS-IS adjacency failure | NET mismatch | Verify system-id and area |
💡 Key Takeaways
- OSPF and IS-IS act as MPLS underlay routing protocols
- LDP distributes MPLS labels
- MP-BGP distributes VPNv4 routes
- VRFs isolate customer traffic
- Route Reflectors improve BGP scalability
- Route Targets control VPN route import/export
14. Related Networking Articles
- Cisco Nexus FEX and vPC Configuration
- Cisco Nexus vPC Peer Gateway and VDC
- Cisco Nexus VRRP Configuration Guide
- Complete Cisco Nexus VXLAN EVPN
- Complete Cisco Nexus OSPF
- Complete Cisco Nexus BGP Authentication
- Complete Cisco Nexus Static Routing Lab
Final Conclusion
This MPLS L3VPN tutorial demonstrated a complete provider backbone architecture using OSPF, IS-IS, MPLS LDP, VPNv4 MP-BGP Route Reflectors, VRFs, and PE-CE BGP routing.
By understanding:
- Underlay routing
- MPLS label distribution
- VPNv4 route exchange
- VRF isolation
- MP-BGP route reflection
you can build scalable enterprise and service provider MPLS VPN networks.
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