Overview of MPLS TE

Purpose

Traffic engineering techniques are common for carriers operating IP/MPLS bearer networks. These techniques are used to prevent traffic congestion and uneven resource allocation.

A node on a conventional IP network selects the shortest path as an optimal route, regardless of other factors, for example, bandwidth. The shortest path may be congested with traffic, whereas other available paths are idle.

Figure 4-1 Conventional routing

Each Link on the network shown in Figure 4-1 has a bandwidth of 100 Mbit/s and the same metric value. LSRA sends LSRJ traffic at 40 Mbit/s, and LSRG sends LSRJ traffic at 80 Mbit/s. Traffic from both routers travels through the shortest path LSRA (LSRG) → LSRB → LSRC → LSRD → LSRI → LSRJ that is calculated by an Interior Gateway Protocol (IGP) protocol. As a result, the path LSRA (LSRG) → LSRB → LSRC → LSRD → LSRI → LSRJ may be congested because of overload, while the path LSRA (LSRF) → LSRB → LSRE → LSRF → LSRH → LSRI → LSRJ is idle.

Network congestion is a major cause for backbone network performance deterioration. The network congestion is resulted from insufficient resources or locally induced by incorrect resource allocation. For the former, network device expansion can prevent the problem. For the later, TE is used to allocate some traffic to idle link so that traffic allocation is improved. TE dynamically monitors network traffic and loads on network elements and adjusts the parameters for traffic management, routing, and resource constraints in real time, which prevents network congestion induced by load imbalance.

Conventional TE solutions are as follows:

• IP traffic engineering: TE controls network traffic by adjusting the metric of a path. This method eliminates congestion only on some links. Adjusting a metric is difficult on a complex network because a link change affects multiple routes.

• ATM traffic engineering: The overlay model, such as IP over Asynchronous Transfer Mode (ATM), complements IGP disadvantages. An overlay model provides a virtual topology over a physical topology for a network. This helps properly adjust traffic and implement QoS features, but has high costs and poor extensibility.

  TE directs some traffic to virtual connections (VCs) based on an overlay model. The current IGPs are topology driven and applicable to only static network connections, regardless of dynamic factors, such as bandwidth and traffic attributes.

A scalable and simple solution is required to implement TE on a large-scale network. MPLS, an overlay model, allows a virtual topology to be established over a physical topology and maps traffic to the virtual topology. MPLS can be integrated with TE. MPLS TE was introduced.

Definition

MPLS TE establishes label switched paths (LSPs) based on constraints and conducts traffic to specific LSPs so that network traffic is transmitted along the specified path. The constraints include controllable paths and sufficient link bandwidth reserved for services transmitted over the LSPs. If resources are insufficient, higher-priority LSPs preempt resources, such as bandwidth, of lower-priority LSPs so that higher-priority services' requirements can be fulfilled preferentially. In addition, if an LSP fails or a node is congested, MPLS TE protects network communication using a backup path and the fast reroute (FRR) function. Using MPLS TE allows a network administrator to deploy LSPs to properly allocate network resources, which prevents network congestion. If the number of LSPs increases, a specific offline tool can be used to analyze traffic. MPLS TE can be used on the network shown in Figure 4-1 to address congestion. MPLS TE establishes an 80 Mbit/s LSP over the path LSRG → LSRB → LSRC → LSRD → LSRI → LSRJ and a 40 Mbit/s LSP over the path LSRA → LSRB → LSRE → LSRF → LSRH → LSRI → LSRJ. MPLS TE directs traffic to the two LSPs, preventing congestion.

Figure 4-2 MPLS TE

Benefits