A route distinguisher is an address qualifier used only within a single internet service provider's Multiprotocol Label Switching (MPLS) network.
It is used to distinguish the distinct virtual private network (VPN) routes of separate customers who connect to the provider.
The route distinguisher is an 8-octet field prefixed to the customer's Internet Protocol address (IPv4).
The route distinguisher has only one purpose: to make IPv4 prefixes globally unique.
For example, for a PE router to be able to distinguish between the IP address 10.0.0.0 of one customer from the 10.0.0.0 of another customer, the network administrator must configure the PE to add a unique route distinguisher to each packet arriving from the CEs.