Traffic Engineering

The basic idea behind traffic engineering is to optimally use the network infrastructure, including links that are underutilized, because they do not lie on the preferred path. This means that traffic engineering must provide the possibility to steer traffic through the network on paths different from the preferred path, which is the least-cost path provided by IP routing. The least-cost path is the shortest path as computed by the dynamic routing protocol. With traffic engineering implemented in the MPLS network, you could have the traffic that is destined for a particular prefix or with a particular quality of service flow from point A to point B along a path that is different from the least-cost path. The result is that the traffic can be spread more evenly over the available links in the network and make more use of underutilized links in the network. Figure 1-9 shows an example of this.

Figure 1-9 Traffic Engineering Example 1

Figure 1-9 Traffic Engineering Example 1

from A to B over the bottom path, which is not the shortest path between A and B (four hops versus three hops on the top path). As such, you can send the traffic over links that might otherwise not be used much. You can guide the traffic in this network onto the bottom path by changing the routing protocols metrics. Examine Figure 1-10.

Figure 1-10 Traffic Engineering Example 2

Figure 1-10 Traffic Engineering Example 2

History of MPLS in Cisco IOS 19

If this network is an IP-only network, you cannot have router C send the traffic along the bottom path by configuring something on router A. The router C decision to send traffic on the top or bottom path is solely its own decision. If you enable MPLS traffic engineering in this network, you can have router A send the traffic toward router B along the bottom path. The MPLS traffic engineering forces router C to forward the traffic A-B onto the bottom path. This can be done in MPLS because of the label forwarding mechanism. The head end router of a traffic-engineered path—here router A—is the router that specifies the complete path that the traffic will take through the MPLS network. Because it is the head end router that specifies the path, traffic engineering is also referred to as a form of source-based routing. The label that is attached to the packet by the head end router makes the packet flow along the path as specified by the head end router. No intermediate router forwards the packet onto another path.

An extra advantage of running MPLS traffic engineering is the possibility of Fast ReRouting (FRR). FRR allows you to reroute labeled traffic around a link or router that has become unavailable. The rerouting of traffic happens in less than 50 ms, which is fast even for standards of today.

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