In this chapter, you review the following topics:
- MPLS TE Introduction
- Basic Operation of MPLS TE
- DiffServ-Aware Traffic Engineering
- Fast Reroute
This chapter presents a review of Multiprotocol Label Switching Traffic Engineering (MPLS TE) technology. MPLS TE can play an important role in the implementation of network services with quality of service (QoS) guarantees. The initial sections describe the basic operation of the technology. This description includes the details of TE information distribution, path computation, and the signaling of TE LSPs. The subsequent sections present how Differentiated Services (DiffServ)-Aware traffic engineering (DS-TE) helps integrate the implementation of DiffServ and MPLS TE. This chapter closes with a review of the fast reroute (FRR) capabilities in MPLS TE. Chapter 4,"Cisco MPLS Traffic Engineering," covers in depth the Cisco implementation of MPLS TE in Cisco IOS and Cisco IOS XR. In addition, Chapter 5,"Backbone Infrastructure," discusses the different network designs that can combine QoS with MPLS TE.
MPLS TE Introduction
MPLS networks can use native TE mechanisms to minimize network congestion and improve network performance. TE modifies routing patterns to provide efficient mapping of traffic streams to network resources. This efficient mapping can reduce the occurrence of congestion and improves service quality in terms of the latency, jitter, and loss that packets experience. Historically, IP networks relied on the optimization of underlying network infrastructure or Interior Gateway Protocol (IGP) tuning for TE. Instead, MPLS extends existing IP protocols and makes use of MPLS forwarding capabilities to provide native TE. In addition, MPLS TE can reduce the impact of network failures and increase service availability. RFC 2702 discusses the requirements for TE in MPLS networks.
MPLS TE brings explicit routing capabilities to MPLS networks. An originating label switching route (LSR) (or headend) can set up a TE label switched path (LSP) to a terminating LSR (or tail end) through an explicitly defined path containing a list of intermediate LSRs (or midpoints). IP uses destination-based routing and does not provide a general and scalable method for explicitly routing traffic. In contrast, MPLS networks can support destination-based and explicit routing simultaneously. MPLS TE uses extensions to RSVP and the MPLS forwarding paradigm to provide explicit routing. These enhancements provide a level of routing control that makes MPLS suitable for TE.
Figure 2-1 shows a sample MPLS network using TE. This network has multiple paths from nodes A and E toward nodes D and H. In this figure, traffic from A and E toward D follows explicitly routed LSPs through nodes B and C. Traffic from A and E toward H follows explicitly routed LSPs through nodes F and G. Without TE, the IGP would compute the shortest path using only a single metric or cost. You could tune that metric, but that would provide you limited capabilities to allocate network resources when compared with MPLS TE (specially, when you consider larger more complex network topologies). This chapter describes those routing and signaling enhancements that make MPLS TE possible.
Figure 2-1 Sample MPLS Network Using TE
MPLS TE also extends the MPLS routing capabilities with support for constraint-based routing. As mentioned earlier, IGPs typically compute routing information using a single metric. Instead of that simple approach, constraint-based routing can take into account more detailed information about network constraints, and policy resources. MPLS TE extends current link-state protocols (IS-IS and OSPF) to distribute such information.
Constraint-based routing and explicit routing allow an originating LSR to compute a path that meets some requirements (constraints) to a terminating LSR and then set up a TE LSP through that path. Constraint-based routing is optional within MPLS TE. An offline tool can perform path computation and leave TE LSP signaling to the LSRs.
MPLS TE supports preemption between TE LSPs of different priorities. Each TE LSP has a setup and a holding priority, which can range from zero (best priority) through seven (worst priority). When a node signals a new TE LSP, other nodes throughout the path compare the setup priority of the new TE LSPs with the holding priority of existing TE LSPs to make a preemption decision. A better setup priority can preempt worse-holding priorities a TE LSP can use hard or soft preemption. A node implementing hard preemption tears down the existing TE LSP to accommodate the new TE LSP. In contrast, a node implementing soft preemption signals back the pending preemption to the headend of the existing TE LSP. The headend can then reroute the TE LSP without impacting the traffic flow. RFC 3209 and draft-ietf-mpls-soft-preemption-07. define TE LSP preemption.