Ospf Overview; Ospf Areas - ZyXEL Communications ZyWALL 1050 User Manual

ZyWALL 1050 User's Guide

13.3 OSPF Overview

OSPF (Open Shortest Path First, RFC 2328) is a link-state protocol designed to distribute
routing information within a group of networks, called an Autonomous System (AS). OSPF
offers some advantages over vector-space routing protocols like RIP.
• OSPF supports variable-length subnet masks, which can be set up to use available IP
addresses more efficiently.
• OSPF filters and summarizes routing information, which reduces the size of routing
tables throughout the network.
• OSPF responds to changes in the network, such as the loss of a router, more quickly.
• OSPF considers several factors, including bandwidth, hop count, throughput, round trip
time, and reliability, when it calculates the shortest path.
• OSPF converges more quickly than RIP.
Naturally, OSPF is also more complicated than RIP, so OSPF is usually more suitable for
large networks.
OSPF uses IP protocol 89.

13.3.1 OSPF Areas

An OSPF Autonomous System (AS) is divided into one or more areas. Each area represents a
group of adjacent networks and is identified by a 32-bit ID. In OSPF, this number may be
expressed as an integer or as an IP address.
There are several types of areas.
• The backbone is the transit area that routes packets between other areas. All other areas
are connected to the backbone.
• A normal area is a group of adjacent networks. A normal area has routing information
about the OSPF AS, any networks outside the OSPF AS to which it is directly connected,
and any networks outside the OSPF AS that provide routing information to any area in
the OSPF AS.
• A stub area has routing information about the OSPF AS. It does not have any routing
information about any networks outside the OSPF AS, including networks to which it is
directly connected. It relies on a default route to send information outside the OSPF AS.
• A Not So Stubby Area (NSSA, RFC 1587) has routing information about the OSPF AS
and networks outside the OSPF AS to which the NSSA is directly connected. It does not
have any routing information about other networks outside the OSPF AS.
Each type of area is illustrated in the following figure.
258 Chapter 13 Routing Protocol