A routing domain is a set of routers and addresses, under a single technical administration. Experience shows that additional constraints, which should be applied, are:
- A single interior routing protocol should be used, although static routes and default routes can be part of the domain. For communications to external Autonomous Systems, the Border Gateway Protocol (BGP) can accept and advertise external routes.
- Within the routing protocol, there should be a single set of routing cost and routing metric definitions, as well as mechanisms for policy constraints.
It is possible that an Autonomous System (AS) will contain only one routing domain, but it certainly can have multiple routing domains, with different administrators and different technical policies. Remember that an AS is concerned with connectivity to the public Internet or a privately administered set of routing systems with different policies; a routing domain is at a finer level of granularity more commonly associated with a single organization.
For example, a given organization might use Open Shortest Path First (OSPF) as its main interior routing protocol, but have one or more older domains that use the Routing Information Protocol (RIP). In this case, the organization would have at least two routing domains, OSPF and RIP, or OSPF and multiple RIP domains. These still can form a single AS if all the domains, via BGP, present a common routing policy to the Internet.