In the previous chapter we considered two classes of routing-update algorithms: distance-vector and link-state. Each of these approaches requires that participating routers have agreed first to a common protocol, and then to a common understanding of how link costs are to be assigned. We will address this further below in 10.6 Border Gateway Protocol, BGP, but a basic problem is that if one site prefers the hop-count approach, assigning every link a cost of 1, while another site prefers to assign link costs in proportion to their bandwidth, then meaningful path cost comparisons between the two sites simply cannot be done.
The term routing domain is used to refer to a set of routers under common administration, using a common link-cost assignment. Another term for this is autonomous system. While use of a common routing-update protocol within the routing domain is not an absolute requirement – for example, some subnets may internally use distance-vector while the site’s “backbone” routers use link-state – we can assume that all routers have a uniform view of the site’s topology and cost metrics.
One of the things included in the term “large-scale” IP routing is the coordination of routing between multiple routing domains. Even in the earliest Internet there were multiple routing domains, if for no other reason than that how to measure link costs was (and still is) too unsettled to set in stone. However, another component of large-scale routing is support for hierarchical routing, above the level of subnets; we turn to this next.