to those destinations.
Without an EGP, each internal network along the multi‐network path between the source and the destination of a particular traffic flow would be flying blind. An EGP provides each network with an understanding of the paths available to it through external networks. With this information, each network can make an informed decision on which path to send traffic, optimising for performance, cost, additional services, or any key preference that the network operator has due to business need.
The most common EGP in use is the Border Gateway Protocol (BGP). The BGP was originally introduced to the internet in 1994 and has seen several iterations over time, resulting in the current version (Border Gateway Protocol version 4 (BGP4)), which forms the underpinning of the global IPv4 and IPv6 routing system the internet relies on to function. Although considerably older than many of the users of the internet today, BGP has proven to be a scalable protocol and a large part of the ongoing success of the modern internet.
One of the key aspects of BGP is its routing table. Although all routing protocols maintain a routing table of some type, which is a store of known destination networks and the metrics of the various paths which can be used to reach them that are known to that network endpoint, BGP is unique in that as it is the dominant EGP in use today, the size of the total routing table is orders of magnitude larger than that encountered by instances of IGPs, even compared to very large internal networks. Separate routing tables are maintained for IPv4 and IPv6, despite BGP being used for both protocols.
This is typically not an issue but has in some cases resulted in the BGP routing table exhausting the memory resources available in some routers. One technique to address this is route summarisation, where many more specific routes can be addressed as a single summary route, similar to how in a library all book titles starting with E can be used to represent all the books starting with Ea, Ex, or Es. During this section, the term subnetwork is used; this refers to a specific range of routable address space, such as books starting with Ex, to continue our library example, rather than everything titled E.
Another key concept in BGP is that of the autonomous system (AS). Each AS represents a single and unique administrative network domain that is owned and operated by a specific network operator, distinct from any other network domain. An AS is identified by a unique number which is assigned to it and which is used to differentiate one AS from another. Interoperation between networks using BGP uses these AS numbers for exactly that purpose, and people who spend a lot of time arranging internetwork connectivity will typically refer to external networks by their BGP AS numbers as well.
As BGP is used to route traffic between networks that are under the control of different operators, the protocol includes a set of capabilities to route traffic based not only on what is calculated to be the best path towards the destination in terms of performance (even though BGP’s measurements of performance are not as advanced as those of an IGP in most cases) but also based on the explicit preference of both network operators. This means that any business agreements they have created in regard to the routing of each other’s traffic can be implemented between their networks by BGP. The reasons for these arrangements are many, and they have helped spur the growth of the internet.
On each BGP router, the BGP protocol allows the configuration of the local preference and the multi exit discriminator (MED) values. Although both of these values are used to directly influence routing decisions, they operate from two different sides. As the sender of traffic, one BGP router may have a local preference value configured which instructs it to use a specific route to reach a destination, and concurrently the MED value configured on the receiver of traffic allows that router to indicate to the sender which path it would prefer to receive traffic on, which is entering its AS. Although the effect of the MED value can be overridden by any of its BGP peers, this feature allows a BGP AS to attempt to influence the paths that traffic inbound to the AS takes. BGP path selection is a detailed topic itself, but the concepts in this section will provide the required background for later sections in this book.
3.6.3 Routing Process
To contextualise all of the routing information presented so far in this chapter, let us use an end‐to‐end example of traffic being routed across multiple networks from its source to its destination (see Figure 3.1). In this example, traffic will flow from a source on the left of the diagram to a destination on the right.
In this example, traffic will need to be routed across two networks which are peering using BGP as their EGP. One network is using OSPF for its IGP, while the other will use IS‐IS. The traffic is using IPv4, though the same concepts and flow seen in this example would apply to IPv6 traffic as well:
1 Traffic is generated by a device within the first network. Its destination is a server in the second network. The majority of internet use cases follow this general structure; examples include a device accessing a video or a cloud service from a remote server across networks.Figure 3.1 Routing process example.
2 The device sends its traffic to its local gateway, which is a router. This router inspects the IP packet headers of the traffic from the device and deduces that its neighbouring router is the best route available to reach this destination. The information to inform this deduction was the result of the router querying its local routing table, which was generated by OSPF using information received from the local router and its neighbouring routers across the network.
3 The traffic is sent to that neighbouring router, which then inspects its own routing table and determines that the best route to the destination of that traffic is via an external network. This same router has established a BGP peering with that external network, and so it sends the traffic to its BGP peer. Although it is an external network, this process is identical from a routing perspective to how traffic was moved within the network, excepting the use of BGP.
4 Once the traffic has entered the second network, the router which received it repeats the process of looking up the optimal route to the destination of the traffic in its routing table. This routing table informs the router that the optimal path is through its neighbouring router within the network, and so it sends the traffic over its link to that router. Note that although this network uses IS‐IS for its internal routing protocol, once the routing table is created by the routing protocol, the traffic routing process itself is identical on a hop‐by‐hop basis.
5 Finally, the last router receives the traffic and, upon inspecting its routing table, discovers that the subnetwork containing the destination of the traffic is directly attached to one of its local interfaces. The router sends the traffic over that link to its destination, and the process of routing traffic from source to destination is complete. Often traffic will be sent responding to the traffic which was sent; in this case, the same process is performed, from right to left.
Regardless of the size or number of networks between the source and destination of traffic, this is the routing process which takes place. Although different routing protocols differ in how they create their routing tables, how they pass information between each other to create them, and the criteria they use to determine the optimal route, this does not affect the general routing process as above.
3.6.3.1 Switching
Compared to routing, switching uses only layer 2 information in order to direct network traffic to its destination. In the example of Ethernet, as described previously, the Ethernet frame header itself features a pair of MAC addresses, which are the source and destination addresses of the traffic. This section will focus primarily on Ethernet as a data link layer protocol used to perform switching, just as the previous section focused on IPv4 and IPv6 as network layer protocols used to perform routing.
In
