RD in MPLS L3VPN

In the context of MPLS Layer 3 VPNs (L3VPNs), RD stands for Route Distinguisher. It plays a crucial role in distinguishing between different VPNs within the same MPLS network. Here’s a detailed look at RD and its purpose:

Purpose of Route Distinguisher (RD)

1. Unique VPN Routes:

   • Separation of Routes: The RD allows MPLS VPNs to maintain unique and non-overlapping route spaces. Without RDs, IP address conflicts could occur between different VPNs that use the same IP address space. By appending a unique identifier to the IP address, the RD ensures that routes from different VPNs are distinct and can coexist within the same MPLS network.

2. Multiprotocol Label Switching (MPLS):

   • Label Distribution: In MPLS L3VPN, RDs are used in conjunction with VPNv4 or VPNv6 address families to distribute routing information. The RD is included in the route advertisement to ensure that routes are correctly associated with their respective VPNs.

How RD Works

1. Format:

   • Structure: The RD is a 64-bit value and can be either an AS (Autonomous System) number or an IP address. It is typically formatted as either AS:nnnn (where AS is the AS number and nnnn is a 16-bit value) or IP-address:nnnn (where IP-address is an IP address and nnnn is a 16-bit value).

2. Route Encoding:

   • VPNv4 Address Family: When a route is advertised in an MPLS VPN, the RD is prepended to the IPv4 address to form a VPNv4 prefix. This makes the prefix unique within the MPLS network.
   • Example: If VPN A and VPN B both use the IP address 192.168.1.0/24, they need to be differentiated. With RD, the routes might look like 10.1.1.1:100:192.168.1.0/24 for VPN A and 10.1.1.1:200:192.168.1.0/24 for VPN B.

3. Routing and Forwarding:

   • Route Import and Export: During route distribution, the RD is used to ensure that routes from different VPNs are properly imported into and exported from the correct VPN routing tables. Each VPN maintains its own routing table, and the RD ensures that routes are placed in the appropriate table.
   • Forwarding Decision: When a packet is forwarded, the RD helps the Label Edge Routers (LERs) and Label Switching Routers (LSRs) determine which VPN the packet belongs to and thus which forwarding table to use.

Benefits of Using RD

1. Address Overlap Handling:

   • Support for Overlapping Address Spaces: RD allows the same IP address space to be used across multiple VPNs without conflicts, making it possible to use private IP address ranges in different customer VPNs.

2. VPN Isolation:

   • Traffic Segregation: By distinguishing routes with RDs, MPLS ensures that traffic from different VPNs is kept separate, even though they share the same MPLS backbone.

3. Scalability:

   • Efficient Route Management: RD supports large-scale MPLS networks by enabling efficient management and separation of routing information, allowing for the deployment of multiple VPNs over a single MPLS infrastructure.