1. What Is MPLS?

MPLS stands for Multiprotocol Label Switching. It is a packet-forwarding technique that uses short labels to make forwarding decisions as traffic travels through an MPLS-enabled network.

In ordinary IP forwarding, a router examines the destination IP address and consults its routing information to decide where to send a packet. In MPLS, a packet can be assigned a label at the network edge. MPLS routers then use label information to forward the packet along a label-switched path.

MPLS is commonly used by service providers to deliver WAN services connecting offices, data centers, and other customer locations.

Simple Example

A company has offices in Colombo, Kandy, and Jaffna. An MPLS service provider can connect those sites through its network, allowing the company to use private IP WAN services according to the provider's service and routing design.

             Company WAN

 [Colombo Office]          [Kandy Office]
         |                       |
    [Provider PE]---[Provider P]---[Provider PE]
                            |
                       [Provider P]
                            |
                      [Provider PE]
                            |
                     [Jaffna Office]

PE = Provider Edge Router
P  = Provider Core Router
        

This is a simplified diagram. Real MPLS networks can contain many routers, paths, and redundant links.

2. Why Is MPLS Used?

MPLS can help service providers build scalable networks and deliver different services over a shared infrastructure.

• Private WAN services: Connect geographically separated company locations.
• Traffic engineering: In supported designs, influence how traffic travels through the provider network.
• Quality of Service (QoS): Carry traffic-class information so the network can apply policies to voice, video, and business applications.
• VPN services: Support separation of customer routing information using technologies such as MPLS Layer 3 VPNs.
• Scalable service delivery: Allow providers to deliver multiple services over a shared network infrastructure.

Important: MPLS does not automatically encrypt traffic. If confidentiality is required, use suitable encryption, such as IPsec, according to the network's security requirements.

3. How Does MPLS Work?

MPLS forwarding can be understood in three basic stages.

Step 1: The Packet Enters the MPLS Network

A Label Edge Router (LER) receives a packet and determines how it should be handled. In a common IP VPN deployment, the provider edge router uses the relevant routing and forwarding information to classify the packet and apply a label stack.

Step 2: The Packet Travels Through the MPLS Core

Core routers, commonly called Label Switch Routers (LSRs), use the incoming MPLS label to select the appropriate forwarding action. They may swap the label for another label as the packet moves through the network.

Step 3: The Packet Leaves the MPLS Path

At the appropriate point, the MPLS label is removed or processed, and the packet is forwarded toward its destination. The exact behavior depends on the MPLS service and configuration.

Customer IP Packet
        |
        v
+-----------------------+
| Ingress PE / LER      |
| Classify and label    |
+-----------------------+
        |
        v
+-----------------------+
| Core P Router / LSR   |
| Swap label and forward|
+-----------------------+
        |
        v
+-----------------------+
| Egress PE / LER       |
| Process/remove labels |
+-----------------------+
        |
        v
Destination Network
        

4. What Is an MPLS Label?

An MPLS label is a short identifier used by MPLS forwarding equipment to make forwarding decisions. The standard MPLS label stack entry is 32 bits long.

The label field is 20 bits, providing \(2^{20}\) possible bit patterns. Certain values are reserved or have special meanings, so not all values are available for ordinary label assignment.

An MPLS packet may carry more than one label. For example, an MPLS Layer 3 VPN often uses an outer transport label and an inner VPN label.

+------------------------------------------+
| Outer Ethernet Header                    |
+------------------------------------------+
| Outer MPLS Label: Transport Label        |
+------------------------------------------+
| Inner MPLS Label: VPN Label (if used)    |
+------------------------------------------+
| Original IP Packet                       |
+------------------------------------------+
        

This is a conceptual example of a two-label stack, not a requirement for every MPLS deployment.

5. Important MPLS Terminology

Label Edge Router (LER)

An LER operates at the edge of an MPLS domain. It classifies incoming traffic and may add or remove MPLS labels, depending on its role and the service design. In provider networks, edge routers are often called PE (Provider Edge) routers.

Label Switch Router (LSR)

An LSR forwards MPLS packets by examining their labels and applying label-forwarding actions. Core provider routers are often called P (Provider) routers.

Label-Switched Path (LSP)

An LSP is a path through an MPLS network along which packets are forwarded using labels. Depending on the deployment, LSPs can be established using mechanisms such as LDP or RSVP-TE, or other supported technologies.

Label Distribution Protocol (LDP)

LDP is a protocol used by routers to distribute labels and establish label-switched paths associated with routing information. Not all MPLS networks use LDP; other mechanisms can be used.

Provider Edge (PE) and Provider (P) Routers

• PE router: Connects customer networks to the provider network and commonly maintains customer VPN routing information.
• P router: Forwards provider-core traffic and typically does not need to maintain each customer's VPN routes in a common MPLS Layer 3 VPN architecture.

6. What Is an MPLS VPN?

An MPLS VPN is a service that uses MPLS technology to help provide logically separated connectivity between customer sites across a provider network.

In a common BGP/MPLS Layer 3 VPN architecture, Provider Edge routers maintain separate routing information for customer VPNs. BGP can distribute VPN routes between PE routers, while MPLS labels help deliver traffic through the provider network.

Customer A: Colombo ---- PE1 ===== PE2 ---- Customer A: Kandy
                           MPLS Core

Customer B: Galle ------- PE1 ===== PE2 ---- Customer B: Jaffna

===== Provider MPLS Network =====

Customer A and Customer B can use
separate VPN routing information.
        

The diagram illustrates logical separation, not separate physical networks. Both customers may share the provider's infrastructure while their routing information is kept separate by the VPN design.

Security note: VPN route separation is not the same as encryption. MPLS VPN services should not be assumed to encrypt customer data unless a separate encryption mechanism is configured.

7. MPLS vs Traditional IP Routing

MPLS does not eliminate IP routing. IP routing is still important for reachability and for decisions made at network edges. MPLS adds a label-based forwarding mechanism within the MPLS domain.

8. MPLS vs SD-WAN

MPLS and SD-WAN are often compared when organizations design WAN connectivity for multiple offices.

SD-WAN does not necessarily replace MPLS. An organization can use MPLS as one of its WAN links and manage it alongside Internet or mobile links using an SD-WAN solution.

9. Advantages of MPLS

• Provider WAN connectivity: Supports connectivity between geographically distributed locations.
• Traffic engineering options: Suitable mechanisms can provide control over traffic paths.
• QoS support: Enables traffic classes and provider policies for applications with different requirements.
• VPN services: Supports logically separated customer routing in common MPLS VPN architectures.
• Service-provider scale: Enables different services to share a provider's network infrastructure.

10. Disadvantages and Considerations of MPLS

• Cost: Some MPLS services may cost more than direct Internet connectivity, depending on the provider and service requirements.
• Provider dependency: Service availability and performance depend partly on the provider's network and service-level agreement.
• Provisioning time: Adding or changing a provider service may require coordination and lead time.
• Configuration complexity: MPLS VPNs and traffic engineering require specialist knowledge.
• Encryption is not automatic: Additional encryption may be necessary for sensitive information.
• Alternative WAN options: Internet VPNs and SD-WAN may better fit some organizations' cost, agility, and application requirements.

11. Basic MPLS Troubleshooting Approach

When troubleshooting an MPLS service, network engineers should verify connectivity from the customer edge through the provider edge and core, using the tools available on their devices.

1. Check physical interfaces, link status, and IP addressing.
2. Verify routing between customer edge and provider edge routers.
3. Check reachability between relevant provider routers.
4. Verify label distribution or the configured label-signaling mechanism.
5. Check whether the expected LSP exists and is operational.
6. For MPLS VPNs, verify VPN routing, route targets, and relevant forwarding entries.
7. Check ACLs, firewalls, MTU, QoS, and return-path routing where relevant.

On Cisco IOS/IOS XE devices, some useful commands may include the following. Exact command availability depends on the platform, software version, and configuration.

show mpls interfaces
show mpls ldp neighbor
show mpls forwarding-table
show ip route
show ip interface brief
        

For an MPLS Layer 3 VPN, additional commands may be available to inspect VRFs, VPN routes, and BGP VPN routing information. Use commands appropriate to the device and the provider's operational procedures.

12. Quick Knowledge Check

13. Key Takeaways

• MPLS stands for Multiprotocol Label Switching.
• MPLS forwards packets using labels within an MPLS domain.
• LERs handle traffic at the network edge, while LSRs perform label-based forwarding.
• An LSP is a label-switched path through the network.
• MPLS is widely associated with provider WAN connectivity, QoS, traffic engineering, and VPN services.
• MPLS can work alongside IP routing and SD-WAN.
• MPLS should not be treated as an encryption mechanism.