- It’s given that customers of a Service Provider will have overlapping IP addressing in their VPNs, e.g. you will have more than two customers who use the 10.0.0.0/8 network. The RD is how you tell them apart. If you have customer “A” with RD “A” and customer “B” with RD “B” the routes “A:10.0.0.0/8” and “B:10.0.0.0/8” become unique. This is all the RD does.
What is an RD? An RD is a 64 bit value that is attached to the customer’s IPv4 address, to make it a Unique 96 bit address called VPNv4. These addresses are ONLY exchanged between the PE routers.
Once the PE router attaches the RD to the CE routes, it then sends the VPNv4 address/es to the other PE router/s. The receiving PE router strips the RD from the VPNv4 prefix, and it s left with an IPv4 address.
- The Route Target tells you which VRF table the route belongs to. You have to separate the two attributes because sometimes you want the same route to belong to multiple VRF tables. This is common in what’s known as “Central Services VPNs”. For example if the Service Provider hosts email for customers, that route to the mail server would have to be in the routing table of multiple customers. This doesn’t break the rule of the route having to be unique though, which is what the RD does.
NOW How does the receiving PE know which VRF does the IP address belong to? The answer is Route-Target .
The Route-target is a BGP extended community that indicates which routes should be exported from a given VRF or imported into a given VRF.