Reference Guide
Open shortest path rst
OSPF routing is a link-state routing protocol that allows sending of link-state advertisements (LSAs) to all other routers within the same
autonomous system (AS) area. Information about attached interfaces, metrics used, and other attributes are included in OSPF LSAs. OSPF
routers accumulate link-state information, and use the shortest path rst (SPF) algorithm to calculate the shortest path to each node.
Autonomous system areas
OSPF operates in a type of hierarchy. The largest entity within the hierarchy is the autonomous system (AS). The AS is a collection of
networks under a common administration that share a common routing strategy. OSPF is an intra-AS, Interior Gateway Routing Protocol
(IGRP) that receives routes from and sends routes to other AS.
You can divide an AS into several areas, which are groups of contiguous networks and attached hosts administratively grouped. Routers
with multiple interfaces can participate in multiple areas. These routers, called area border routers (ABRs), maintain separate databases for
each area. Areas are a logical grouping of OSPF routers that an integer or dotted-decimal number identies.
Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas are valuable in that they allow
subnetworks to hide within the AS—minimizing the size of the routing tables on all routers. An area within the AS may not see the details
of another area’s topology. An area number or the router’s IP address identies AS areas.
Areas, networks, and neighbors
The backbone of the network is Area 0, also called Area 0.0.0.0, the core of any AS. All other areas must connect to Area 0. An OSPF
backbone is responsible for distributing routing information between areas. It consists of all area border routers, networks not wholly
contained in any area and their attached routers.
Layer 3
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