Why do we need RT when there’s already RD

Imagine a SP has 1000 CUs, each CU is a VRF so 1000 VRFs. And then there is a service VRF that provides common services to those 1000 CUs. So the service VRF has to import all 1000 CU’s routes to allow mutual communication (CUs needs to import service VRF and service VRF needs to import CUs)

Each CU has a unique RD. So if there was only RD, no concept of RT, then the service VRF needs to import 1000 RD’s.

But luckily, there is RT. So each CU can export a common RT, in addition to its unique RT (because a VRF can import/export multiple RTs, but only one RD). Then the service RT can simply import this common RT so all 1000 CU’s routes are imported.