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 ...