A routing domain is a lower level of a computer network hierarchy that must adhere to a single routing protocol. It can exist as part of an administrative domain, which can have multiple routing domains within it. An autonomous system can contain multiple administrative domains. The use of an explicitly established routing protocol connects end systems and intermediate systems within the domain. Multiple routing domains within a single administrative domain can be interconnected using inter-domain IS-IS protocol or Border Gateway Protocol (BGP) for security reasons.
A routing domain is a term used to identify a lower level of a computer network hierarchy with respect to the routing of network traffic. All computers and routers in the domain must be administered by a single source, such as a business 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 particular domain can exist as part of a larger network.
How a routing domain fits into the overall hierarchy depends somewhat on the construction of the network and the routing protocols used. It is often 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 with different routing protocols within a single administrative domain, but still be administered 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 can exist in what is known as an autonomous system. An autonomous system can essentially be viewed as any set of routing domains that have an established path to the Internet. In most cases, a single administrative domain and its routing domain will be a self-contained system, sometimes also called a congruent domain. This is because, for network traffic to reach another administrative domain, it usually has to traverse the Internet to reach the second administrative domain.
The way a routing domain works is through the use of an explicitly established routing protocol. Inside, there are any number of computers, referred to as end systems (ES). Connecting them in groups are usually routers or other network devices, called intermediate systems (IS). These groupings, or subnetworks, are referred to as End System to Intermediate System (ES-IS) protocols. Protocols that group intermediate systems that share the common routing protocol are referred to as intra-domain intermediate system to intermediate system (IS-IS) protocol.
Although the rules for routing domains specify that a single routing protocol should be used throughout the domain, there are occasional exceptions. For example, a single ES may have a direct path to an IS. Technically, this can be seen as another routing protocol, as the route is established between the ES and the IS, although it does not interfere with the primary domain’s internal IS-IS protocol. In general, an IS-IS protocol within the domain is often referred to as an internal gateway protocol (IGP).
Because it is possible to have multiple routing domains within a single administrative domain, there are methods for interconnecting those instances where multiple administrative domains need to connect. 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 the inter-domain IS-IS protocol. Although there may be two administrative domains managed by a single administrative source, for security reasons, they are usually connected via the so-called Border Gateway Protocol (BGP).
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