Multiprotocol Label Switching - MPLS and IP

MPLS and IP

MPLS works in conjunction with IP and its routing protocols, such as the Interior Gateway Protocol (IGP). MPLS LSPs provide dynamic, transparent virtual networks with support for traffic engineering, the ability to transport layer-3 (IP) VPNs with overlapping address spaces, and support for layer-2 pseudowires using Pseudowire Emulation Edge-to-Edge (PWE3) that are capable of transporting a variety of transport payloads (IPv4, IPv6, ATM, Frame Relay, etc.). MPLS-capable devices are referred to as LSRs. The paths an LSR knows can be defined using explicit hop-by-hop configuration, or are dynamically routed by the constrained shortest path first (CSPF) algorithm, or are configured as a loose route that avoids a particular IP address or that is partly explicit and partly dynamic.

In a pure IP network, the shortest path to a destination is chosen even when the path becomes congested. Meanwhile, in an IP network with MPLS Traffic Engineering CSPF routing, constraints such as the RSVP bandwidth of the traversed links can also be considered, such that the shortest path with available bandwidth will be chosen. MPLS Traffic Engineering relies upon the use of TE extensions to Open Shortest Path First (OSPF) or Intermediate System To Intermediate System (IS-IS) and RSVP. In addition to the constraint of RSVP bandwidth, users can also define their own constraints by specifying link attributes and special requirements for tunnels to route (or not to route) over links with certain attributes.

For end-users the use of MPLS is not visible directly, but can be assumed when doing a traceroute: only nodes that do full ip routing are shown as hops in the path, thus not the MPLS nodes used in between, therefore when you see that a packet hops between two very distant nodes and hardly any other 'hop' is seen in that providers network (or AS) it is very likely that that network uses MPLS.