OSPF Routing Hierarchy

One of most important features within the OSPF protocol is its capability to use a hierarchical routing structure. Remember the following characteristics when considering how OSPF operates within this type of hierarchical structure:

• Structure must exist or be created in order for OSPF to operate properly.
• Explicit topology has precedence over addressing.

An AS is a group of areas sharing a common routing strategy that fall under a common administrative domain. Autonomous systems are identified by a unique number. Autonomous systems' numbers can be either public or private, depending on your needs. Autonomous systems' numbers are assigned by American Registry for Internet Numbers (ARIN) in North America, Réseaux IP Européens (RIPE) in Europe, and Asia Pacific Network Information Centre (APNIC) in Asia. An AS number is not required to use OSPF, as singlehomed enterprises are considered to be part of their upstream ISP's AS. The routing of information within an AS takes place in one of three ways:

If the source and destination addresses of a packet reside within the same area, intraarea routing is used.
• If the source and destination addresses of a packet reside within different areas but are still within the AS, inter-area routing is used.
• If the destination address of a packet resides outside the AS, external routing is used.