<P> Multicast was for the most part an after - thought in MPLS design . It was introduced by point - to - multipoint RSVP - TE . It was driven by service provider requirements to transport broadband video over MPLS . Since the inception of RFC 4875 there has been tremendous surge in interest and deployment of MPLS multicast and this has led to several new developments both in the IETF and in shipping products . </P> <P> The hub&spoke multipoint LSP is also introduced by IETF, short as HSMP LSP . HSMP LSP is mainly used for multicast, time synchronization and other purpose . </P> <P> MPLS works in conjunction with the Internet Protocol (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 . </P> <P> 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 . </P>

How does traffic flow through an mpls network