4-2 MPLS VPNs Explained

Key Concepts

• MPLS (Multiprotocol Label Switching)
• VPN (Virtual Private Network)
• PE (Provider Edge) Routers
• CE (Customer Edge) Routers
• VRF (Virtual Routing and Forwarding)
• LDP (Label Distribution Protocol)
• BGP (Border Gateway Protocol)

MPLS (Multiprotocol Label Switching)

MPLS is a data-carrying mechanism that operates at a layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer) and Layer 3 (Network Layer). MPLS forwards packets based on short path labels rather than long network addresses, thus speeding up network traffic flow and making routing more efficient.

VPN (Virtual Private Network)

A VPN is a private network that uses a public network (usually the Internet) to connect remote sites or users together. Instead of using a dedicated, real-world connection such as leased line, a VPN uses "virtual" connections routed through the Internet from the company's private network to the remote site or employee.

PE (Provider Edge) Routers

PE routers are the edge routers of the service provider's network that interface with the customer's CE routers. They are responsible for maintaining VPN-specific routing information and forwarding packets between different VPNs.

CE (Customer Edge) Routers

CE routers are the edge routers of the customer's network that interface with the service provider's PE routers. They are responsible for routing traffic within the customer's network and exchanging routing information with the PE routers.

VRF (Virtual Routing and Forwarding)

VRF is a technology that allows multiple instances of a routing table to exist in a router and work simultaneously. Each VRF instance can have its own routing table, independent of other VRF instances. This allows a single PE router to support multiple VPNs, each with its own routing and forwarding information.

LDP (Label Distribution Protocol)

LDP is a protocol used by MPLS-enabled routers to exchange label information. It allows routers to establish label-switched paths (LSPs) through the network. LDP is essential for the operation of MPLS VPNs, as it ensures that packets are correctly labeled and forwarded across the service provider's network.

BGP (Border Gateway Protocol)

BGP is an exterior gateway protocol used to exchange routing and reachability information among autonomous systems (AS) on the Internet. In the context of MPLS VPNs, BGP is used to distribute VPN routes between PE routers, ensuring that each VPN has the correct routing information to reach its destinations.

Examples and Analogies

Consider a large corporation with multiple branch offices. Each branch office has its own network (CE routers) and needs to communicate securely with the headquarters. The corporation hires a service provider to connect all branch offices using MPLS VPNs. The service provider's network (PE routers) acts as a secure tunnel, ensuring that data is transmitted efficiently and securely between the branch offices and the headquarters.

In this scenario, MPLS is like a high-speed expressway that allows data packets to travel quickly between different locations. VPNs are like secure tunnels built on this expressway, ensuring that data is protected from unauthorized access. PE routers are like toll booths at the entrance and exit of the expressway, managing the flow of traffic and ensuring that each branch office's data reaches the correct destination.

VRF is like having multiple lanes on the expressway, each dedicated to a specific branch office. This ensures that traffic from one branch office does not interfere with traffic from another. LDP is like the traffic signals that guide vehicles onto the correct lanes, while BGP is like the navigation system that ensures each vehicle reaches its final destination.