Different Types of TE Design

There are two basic types of TE network design—tactical (deploying TE tunnels as needed) and strategic (deploying a full mesh of TE tunnels in some part of your network). They're really two points along a spectrum, which means that depending on your design goals and network policies, you could do one, the other, or something in between.

Figure 9-4 shows the network design spectrum.

The four points along the spectrum can be described as follows:

• IGP— This is what you have today. All paths to destinations are determined by your routing protocol. Changing traffic flows requires changing link metrics, which might influence traffic you don't want to affect. This kind of network runs either LDP or IP forwarding, or perhaps uses TE for traffic measurement (see Chapter 10).

• Tactical— Your traffic is predominantly forwarded along the IGP-calculated path, but you build the occasional TE LSP to carry traffic along a path the IGP didn't select. You might do this to steer traffic away from congestion without having to change link metrics, or you might do this to take advantage of TE's unequal-cost forwarding.

• Strategic online— You build a full mesh of TE LSPs between the routers at the edge of your MPLS TE cloud. This might be all the routers at the access layer in your network, it might be all the routers in your core, or it might be some other boundary. TE LSPs used for strategic TE typically are configured to reserve bandwidth in accordance with the amount of bandwidth that will actually go down the TE tunnel; this leads to more-efficient use of your network's capacity. A headend runs CSPF (as described in Chapter 4, "Path Calculation and Setup") to calculate the path that each of its LSPs will take.

• Strategic offline— You build a full mesh of TE LSPs, just as in the strategic online case, but the paths that the LSPs take are calculated by an offline path calculation tool. This is the most efficient way to use your network resources. It relies on an offline path calculator to monitor your topology, your traffic patterns, and your LSPs.

It should be clear by now that these are points on a spectrum. For example, there's no reason why you couldn't do a full mesh between most of your routers, but not between pairs of nodes that don't have much traffic between them.

Part of what TE model you pick has to do with what your needs are. And part has to do with your experience and preferences. People coming from an ATM PVC or TDM world might prefer the strategic offline model; people coming from an IP world might feel more comfortable with a tactical or strategic online model. The models are flexible enough that it's easy to move from one point to another; the choice is up to you.

The following sections cover all three models. There's no way to cover all possible features and knobs used in every architecture, but hopefully this chapter gives you plenty to think about.