Flat vs. Hierarchical

In a flat routing infrastructure, each network ID is represented individually in the routing table. The network IDs have no network/subnet structure and cannot be summarized. RIP-based IPX internetworks use flat network addressing and have a flat routing infrastructure.

In a hierarchical routing infrastructure, groups of network IDs can be represented as a single routing table entry through route summarization. The network IDs in a hierarchical internetwork have a network/subnet/sub-subnet structure. A routing table entry for the highest level (the network) is also the route used for the subnets and sub-subnets of the network. Hierarchical routing infrastructures simplify routing tables and lower the amount of routing information that is exchanged, but they require more planning. IP implements hierarchical network addressing, and IP internetworks can have a hierarchical routing structure.

In hierarchical routing infrastructures, the internetwork can be divided into routing domains (also known as regions or areas). A routing domain is a collection of contiguous networks connected by routers that share the routing information for the routes within the domain. Routing domains are connected by a common routing domain called the backbone. Intra-domain routing is performed by the routers within the domain. Inter-domain routing is performed by domain routers connected to the backbone.

© 1985-2000 Microsoft Corporation. All rights reserved.