Route distinguisher

Last updated February 19, 2019

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.

An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. Internet service providers may be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privately owned.

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on short path labels rather than long network addresses, thus avoiding complex lookups in a routing table and speeding traffic flows. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols, hence the "multiprotocol" reference on its name. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

A virtual private network (VPN) extends a private network across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across a VPN may therefore benefit from the functionality, security, and management of the private network.

The route distinguisher is an 8-octet field prefixed to the customer's Internet Protocol address (IPv4). The resulting 12-octet field is a unique "VPN-IPv4" address. There is a more detailed description in RFC 4364.^[1] At the edge of an MPLS provider's network, a router which connects to a customer's network is called a Provider Edge (PE) router. Similarly, the customer's edge router at the other end of the connection is called a Customer Edge (CE) router. Within an MPLS network, a PE router needs to be configured to associate each route distinguisher with routes which lead to a particular CE router. The PE router may be configured to associate all routes leading to the same CE router with the same route distinguisher, or it may be configured to associate different routes with different route distinguishers, even if they lead to the same CE router.

An Internet Protocol address is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing.

Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet, and was the first version deployed for production in the ARPANET in 1983. It still routes most Internet traffic today, despite the ongoing deployment of a successor protocol, IPv6. IPv4 is described in IETF publication RFC 791, replacing an earlier definition.

The route distinguisher has only one purpose: to make IPv4 prefixes globally unique. It is not used for IP forwarding by the provider's core (non-edge) routers (within the MPLS cloud), but it is used by the edge routers to identify which VPN a packet belongs to. 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.

The route distinguisher (RD) is an 8-octet value consisting of two major fields, the Type Field (2 octets) and Value Field (6 octets). The type field determines how the value field should be interpreted. The three Type values, as defined in the Internet draft, are:

Type 0:

Octet	1	2	3	4	5	6	7	8
Field	Type (2 octets)		Administrator subfield AS Number (2 octets)		Assigned number subfield Selected by Service Provider (4 octets)

The administrator field must contain an AS number (using private AS numbers is discouraged). The Assigned field contains a number assigned by the service provider.

Type 1:

Octet	1	2	3	4	5	6	7	8
Field	Type (2 octets)		Administrator subfield IPv4 address (4 octets)				Assigned Number subfield Assigned by Service Provider (2 octets)

The administrator field must contain an IP address (using private IP address space is discouraged). The Assigned field contains a number assigned by the service provider.

Type 2:

Octet	1	2	3	4	5	6	7	8
Field	Type (2 octets)		Administrator subfield 4-octet AS Number (4 octets)				Assigned Number subfield Assigned by Service Provider (2 octets)

The administrator field must contain a four-octet AS number (using private AS numbers is discouraged). The Assigned field contains a number assigned by the service provider.

Normally the Border Gateway Protocol (BGP) used by the provider's routers only looks at the four-octet IP address, but the BGP Multiprotocol Extensions allow BGP to view the entire 12-octet VPN-IPv4 address, and carry routes from multiple "address families". If the route distinguisher Administrator subfield and the Assigned Number subfield of a VPN-IPv4 address are both set to all zeroes, the VPN-IPv4 address is considered to have exactly the same meaning as the corresponding globally unique IPv4 address. In particular, this VPN-IPv4 address and the corresponding globally unique IPv4 address will be considered comparable by BGP. In all other cases, a VPN-IPv4 address and its corresponding globally unique IPv4 address will be considered noncomparable by BGP. A given per-site forwarding table will only have one VPN-IPv4 route for any given IPv4 address prefix. When a packet's destination address is matched against a VPN-IPv4 route, only the IPv4 part is actually matched.

Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. The protocol is classified as a path vector protocol. The Border Gateway Protocol makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator and is involved in making core routing decisions.

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References

↑ RFC 4364, BGP/MPLS IP Virtual Private Networks (VPNs), E. Rosen and Y. Rekhter, The Internet Society (February 2006)

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[rfc4364-1] RFC 4364, BGP/MPLS IP Virtual Private Networks (VPNs), E. Rosen and Y. Rekhter, The Internet Society (February 2006)