same physical underlying infrastructure can be achieved at layer 3 as well, using technologies such as virtual routing and forwarding (VRF). With VRF, a router operates with multiple instances of a routing table at the same time; these routing tables do not share routes, and so they operate in much the same way as VLANs do, with traffic being handled by each independent routing instance depending on the interface the traffic was received on, or other tagging criteria applied to that traffic to direct it to a specific table.
The ability of these and other technologies to enable a piece of physical infrastructure to support multiple users while concurrently isolating their activities from one another is, as briefly mentioned above, a key consideration; the ideal infrastructure edge computing system is itself multi‐tenant and so requires this type of isolated multi‐user operation at many levels throughout the entire system to be as attractive as possible economically to both its customers and its operator, spanning from the network infrastructure required to support it through to the distributed data centres themselves.
3.6.3.2 Network Boundaries
In the previous sections in this chapter describing the functions of layer 2 and layer 3 of the OSI model, intranetwork and internetwork network endpoint addressing were described, respectively. This leads to the question: How can we determine the boundaries of a network for the purpose of endpoint addressing? Where should switching end and routing begin to be used in a given network?
The majority of networks use a combination of both routing and switching at different locations to operate effectively. On a local network segment or subnetwork where a router connects directly or via a switch to endpoint devices such as PCs or printers, it may seem simpler to use switching and rely on layer 2 alone. However, a network architect or administrator may opt to use layer 3 across the entire network. In this case, switching is still used, as can be seen in the following example.
To describe how routing and switching are used together across the same network, we will return to the diagram we used previously for our routing process example earlier in this chapter. Segments of the network which operate only at layer 2 have been described in this section, as has the routing process, so this example will cover how layer 2 switching supports layer 3 routing operations (see Figure 3.2).
Figure 3.2 Routing and switching at a network boundary.
Returning to our description of the OSI model and its use of layers, this example shows that a single layer cannot operate in isolation and successfully pass information between endpoints across the network. This example of routing and switching being used together shows how layers 1 through 3 interoperate closely to achieve this goal and are supported in turn by other layers.
This process is repeated for each of the steps in the routing process that require traffic forwarding:
1 An endpoint (whether a router or not) determines where to forward received traffic to, based on that endpoint’s knowledge of the network topology, as from its routing table.
2 Before the traffic can be sent, the layer 2 address of the next hop destination for the traffic must be determined. Remember that in the OSI model, as we send traffic we must traverse down the stack towards layer 1. We cannot simply skip layer 2 just because we are using layer 3 addressing as well in the network. Our IP packet at layer 3 will first be encapsulated in an Ethernet frame at layer 2, which needs a pair of source and destination MAC addresses. The source MAC address is that of the interface about to send the traffic on to its destination; but the destination MAC address may not be known yet.
3 The endpoint checks its switching table, also referred to as its Address Resolution Protocol (ARP) table in the case of IPv4. This table contains a list of MAC addresses that are matched with IP addresses. If an entry exists that matches the destination IP address of the traffic to a MAC address, that MAC address is then used as the destination MAC address of the Ethernet frame being created. If it is not known, the ARP protocol (or an equivalent process based on the specific layer 3 protocol in use) is invoked to discover that destination MAC address.
4 With the IP packet encapsulated in an Ethernet frame with both source and destination MAC addresses, the Ethernet frame is then ready to be transmitted to its destination. Where layer 2 switching occurs, only the layer 2 information contained in the frame is required; and when a device uses layer 3 information to perform routing, it decapsulates the Ethernet frame and then acts upon the layer 3 information of the IP packet within. When traffic must then be sent towards its next hop destination, this encapsulation process is repeated.
In this way, layer 2 and layer 3 technologies function together closely to transport traffic from its source to its destination across a network that may span from one end of the same building, or it may span the globe in the case of an application delivered across the internet. Regardless, the same basic processes are repeated to move traffic over the network irrespective of any physical distance.
The examples in this section are a key foundation for topics in later chapters, which will use network virtualisation and inter‐layer interoperation to provide the flexibility and performance required to support next‐generation infrastructure and applications. Although many of the characteristics of and use cases enabled by infrastructure edge computing are new, these same basic processes apply from a network infrastructure and operation perspective just as they do to the current networks of today.
3.7 LAN, MAN, and WAN
Modern networks exist at drastically varying sizes, and it is useful to categorise them into three main classes according to their scale. The three most commonly used terms to describe the scope or scale of a particular network are the local area network (LAN), metropolitan area network (MAN), and wide area network (WAN), in order of their increasing geographical size. Although there are no real hard and fast standards that dictate the size a specific network must be to qualify as a specific scale denoted by one of these terms, it is usually not difficult to come to an agreement on terms to use.
In many cases, there is an hierarchical relationship between these three grades of network scale. One LAN may be combined with many others in a metropolitan area and can be interconnected to form a MAN; and one or more MANs across multiple metropolitan areas may be interconnected to form a WAN. On the other hand, networks at any of these three scales could be created as single networks. This choice is driven by a combination of business and technical factors depending on the individual parties and technologies involved in a specific area and is not prescribed by the terms themselves.
To show each of these network types visually, consider the diagram in Figure 3.3, which shows two cities:
Figure 3.3 LAN, MAN, and WAN networks.
The diagram shows multiple LANs within a city, a single MAN covering all of that city, and a WAN that is connecting the two cities to each other despite them being 100 miles apart. Of course, a city may have hundreds or thousands of LANs, and there may be multiple MANs and also multiple WANS in that area or between areas; but this example serves to show the difference in scale between typical networks in each of these categories and how one may appear to nest within another from above.
There is
