Core Issue
One of the applications of Multiprotocol Label Switching (MPLS) is Traffic Engineering (TE), which is used for manipulating traffic to fit a particular network. TE is important for service providers to efficiently use their backbones and provide high resiliency.
Certain technologies at Layer 2 (L2), such as ATM, provide TE capabilities that can be used for engineering traffic flows between sources and destinations. However, this does not scale well when a full mesh connectivity is required between the various nodes. Since traditional IP routing is based only on the destination address, IP networks do not have any TE mechanism by themselves. The only option that can be used for engineering traffic is the metric associated with the Interior Gateway Protocol (IGP), which can be tuned for preferring a particular path. However, this also does not scale well in large networks. IP can be used over ATM in an overlay model to implement TE, but it leads to scalability problems.
On the other hand, MPLS TE provides an integrated approach to traffic engineering by combining the traffic engineering capabilities of ATM with the flexibility and Class of Service (CoS) differentiation of IP. The nature of MPLS TE avoids the problems associated with the overlay model. Like ATM or Frame Relay Virtual Circuits (VCs), the Label Switched Path (LSP) automatically built by MPLS TE controls the path of a traffic flow to a particular destination, rather than pure destination-based forwarding.
Resolution
MPLS TE works by learning about the topology and resources available in a network. It then maps the traffic flows to a particular path based on the resources that the traffic flow requires and the available resources. MPLS TE builds unidirectional tunnels from a source to the destination in the form of LSPs, which is then used for forwarding traffic. The point where the tunnel begins is called tunnel headend or tunnel source, and the node where the tunnel ends is called tunnel tailend or tunnel destination.
These components work together to make MPLS TE work:
- Information distribution is a link state protocol, such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS), which is necessary to discover the topology. These protocols have been enhanced to carry additional information related to TE, such as bandwidth available and other related parameters. IS-IS uses new Type Length Values (TLVs) and OSPF uses Type 10 (Opaque) Link State Advertisements (LSAs) for this purpose. Other IGPs cannot be used to implement MPLS TE.
- Path calculation is a Constraint-Based Routing (CBR) used for finding the shortest path to a particular network that meets the resource requirements of the traffic flow. The Constrained Shortest Path First (CSPF) algorithm that operates on the tunnel headend is used for this functionality.
- Path setup is a signaling protocol to reserve the resources for a traffic flow and to establish the LSP for a traffic flow. Resource Reservation Protocol (RSVP) is used for this purpose and has been enhanced with TE extensions for carrying labels and building the LSP. An alternative to RSVP for MPLS TE is constrained routing with Label Distribution Protocol (LDP), but Cisco devices do not support this protocol.
- Traffic forwarding is a component that corresponds to the forwarding plane of MPLS TE and forwards traffic through an MPLS TE tunnel, built in the form of an LSP by the path setup module, and based on the information available from other components.
For more information on MPLS TE and configuration, refer to these documents:
