Ships in the Night

One of the requirements for multiservice switching networks is independence between different services. In other words, services do not interfere with each other. An upgrade of the multiprotocol label switching (MPLS) software that controls the switches should not affect the digital subscriber line (DSL) customers who use ATM SVCs from network access or voice customers who are supported with VoATM using AAL2. A first draft of a multiservice switching network might look like Figure 1-1.

Figure 1-1. Example of a Multiservice Switching Network Providing Three Services

A number of things are worth mentioning about this figure. First, note that the only physical network is the big dark cloud. The smaller clouds within the physical network are logical partitions of the physical network by service.

This multiservice switching network provides unified access to multiple services or virtual networksMPLS, Private Network-to-Network Interface (PNNI), and voice in this example. The links from the multiservice switch into each logical network are then logical links, created from logically partitioning resources in the physical link.

But the most important point to note is that this example has no overlays. All the services are running natively on the multiservice switches. For example, with the MPLS logical network, if you have an underlying ATM network, the multiservice switches run an IGP and LDP for setting up LVCs and not MPLS over PVCs. In other words, every switch in the multiservice network is aware of the services it provides.

NOTE

Resource partitioning is a key element in the architecture. It is revisited several times in this chapter.

A diagram of a network as a cloud is an oversimplification. Let's explore this a bit further. Figure 1-2 represents the physical topology of the ATM network.

Figure 1-2. Underlying Physical ATM Networks

Resources in the network can be partitioned for the different services to create the logical networks over a common infrastructure. Links and even entire nodes can be made invisible to some control planes in the process of building these virtual networks, as shown in Figure 1-3.

Figure 1-3. Virtual Networks over the Common Infrastructures

From the total of seven nodes and ten links that exist in the physical network, the MPLS network is visible to only five nodes and six links, and the PNNI network "believes" that there are only three nodes and two links. Moreover, from the MPLS network's perspective, the PNNI network does not exist. Two nodes and one link are shared by the two logical networks, but the MPLS control plane in those common nodes or any other node is unaware of the existence of the PNNI network with which it shares resources, and vice versa. This behavior is called ships in the night. It means that the different control planes act like the brain of the multiservice switches, operating independently on the same network fabric simultaneously.