Chapter 22. Ospf; Ospfv2 Overview; Types Of Ospf Areas - IBM RackSwitch G8000 Application Manual

Chapter 22. OSPF

OSPFv2 Overview

Types of OSPF Areas

© Copyright IBM Corp. 2011
IBM Networking OS supports the Open Shortest Path First (OSPF) routing protocol.
The IBM N/OS implementation conforms to the OSPF version 2 specifications
detailed in Internet RFC 1583, and OSPF version 3 specifications in RFC 2740. The
following sections discuss OSPF support for the RackSwitch G8000:
• "OSPFv2 Overview" on page
concepts, such as types of OSPF areas, types of routing devices, neighbors,
adjacencies, link state database, authentication, and internal versus external
routing.
• "OSPFv2 Implementation in IBM N/OS" on page
OSPFv2 is implemented in N/OS, such as configuration parameters, electing the
designated router, summarizing routes, defining route maps and so forth.
• "OSPFv2 Configuration Examples" on page
step-by-step instructions on configuring different OSPFv2 examples:
– Creating a simple OSPF domain
– Creating virtual links
– Summarizing routes
• "OSPFv3 Implementation in IBM N/OS" on page
differences and additional features found in OSPFv3.
OSPF is designed for routing traffic within a single IP domain called an Autonomous
System (AS). The AS can be divided into smaller logical units known as areas.
All routing devices maintain link information in their own Link State Database
(LSDB). The LSDB for all routing devices within an area is identical but is not
exchanged between different areas. Only routing updates are exchanged between
areas, thereby significantly reducing the overhead for maintaining routing
information on a large, dynamic network.
The following sections describe key OSPF concepts.
An AS can be broken into logical units known as areas. In any AS with multiple
areas, one area must be designated as area 0, known as the backbone. The
backbone acts as the central OSPF area. All other areas in the AS must be
connected to the backbone. Areas inject summary routing information into the
backbone, which then distributes it to other areas as needed.
As shown in Figure 26, OSPF defines the following types of areas:
• Stub Area—an area that is connected to only one other area. External route
information is not distributed into stub areas.
• Not-So-Stubby-Area (NSSA)—similar to a stub area with additional capabilities.
Routes originating from within the NSSA can be propagated to adjacent transit
and backbone areas. External routes from outside the AS can be advertised
within the NSSA but are not distributed into other areas.
• Transit Area—an area that allows area summary information to be exchanged
between routing devices. The backbone (area 0), any area that contains a virtual
link to connect two areas, and any area that is not a stub area or an NSSA are
considered transit areas.
257. This section provides information on OSPFv2
261. This section describes how
270. This section provides
279. This section describes
257