A routing domain is a term used to identify a lower level of the hierarchy of a computer network with respect to routing network traffic. All computers and routers within the domain must be managed by a single source, such as a company or organization, and adhere to a single routing protocol. Additional subnets can exist within a given domain to detail a network topology, as long as the subnets follow the same routing protocol. Also, a given domain can exist as part of a larger network.
All routers in a routing domain must be managed by a single source.
How a routing domain fits into the overall hierarchy depends to some extent on the construction of the network and the routing protocols used. It is typically a subnet of what is known as an administrative domain, which can have any number of routing domains within it. In this way, two different routing domains can operate under different routing protocols in a single administrative domain, but still be managed by a single source. Two or more administrative domains can also be connected in case a third routing protocol needs to be implemented, but it will be kept separate from the other two.
Moving up the hierarchy, an administrative domain may exist in what is known as an autonomous system. Basically, an autonomous system can be viewed as any collection of routing domains that have an established route to the Internet. In most cases, a single administrative domain and its routing domain will be an autonomous system, also sometimes called a congruent domain. This is because for network traffic to reach another administrative domain, it typically has to traverse the Internet to reach the second administrative domain.
The way a routing domain works is by using an explicitly stated routing protocol. Inside are a large number of computers, called end systems (ES). Connecting them in groups are usually routers or other network devices, which are called intermediate systems (IS). These groupings, or subnets, are called End System to Middle System (ES-IS) protocols. The protocols that group the intermediate systems that share the common routing protocol are called the intra-domain intermediate system-to-intermediate system (IS-IS) protocol.
Although the rules for routing domains specify that a single routing protocol be used throughout the domain, there are occasional exceptions. For example, a single ES may have a direct route to an IS. Technically, this can be seen as another routing protocol, since the route is established between the ES and the IS, although it does not interfere with the intra-primary domain IS-IS protocol. In general, an intra-domain IS-IS protocol is often referred to as an Interior Gateway Protocol (IGP).
Since it is possible to have multiple routing domains in a single administrative domain, there are methods for interconnecting instances where multiple administrative domains need to be connected. In this case, a different protocol than the one used by the routing domain can be used to connect the two administrative domains. This is known as a cross-domain IS-IS protocol. Although there may be two administrative domains managed by a single administrative source, for security reasons they are usually connected through what is known as Border Gateway Protocol (BGP).
