1. Kashif Latif

2. What is OSPF…?

 Open Shortest Path First (OSPF) is an adaptive
routing protocol for Internet Protocol (IP) networks.
It uses a link state routing algorithm and falls into
the group of interior routing protocols, operating
within a single autonomous system (AS).
 OSPF is perhaps the most widely-used interior
gateway protocol (IGP) in large enterprise networks.
IS-IS, another link-state dynamic routing protocol, is
more common in large service provider networks.

3. Count…

 OSPF was designed to support variable-length
subnet masking (VLSM) or Classless Inter-Domain
Routing (CIDR) addressing models.
 OSPF detects changes in the topology, such as link
failures, very quickly and converges on a new loop-
free routing structure within seconds.
 An OSPF network may be structured, or subdivided,
into routing areas to simplify administration and
optimize traffic and resource utilization.

4. Count…

 OSPF does not use a TCP/IP transport protocol
(UDP, TCP), but is encapsulated directly in IP
datagrams with protocol number 89.
 OSPF uses both unicast and multicast to send
packets and link state updates.
 The OSPF protocol, when running on IPv4, can
operate securely between routers.
 OSPF version 3 introduces modifications to
the IPv4 implementation of the protocol.

5. Neighbor Relationships

 As a link state routing protocol, OSPF establishes
and maintains neighbor relationships in order to
exchange routing updates with other routers.
 The neighbor relationship table is called an
adjacency database in OSPF. Provided that OSPF
is configured correctly, OSPF forms neighbor
relationships only with the routers directly
connected to it.
 In order to form a neighbor relationship between
two routers, the interfaces used to form the
relationship must be in the same area.

6. Area Types

Areas are logical groupings of hosts and networks,
including their routers having interfaces connected to any
of the included networks.
Several special area types are defined:
1. Backbone Area
2. Stub Area
3. Not-so-Stubby Area
4. Proprietary Extensions
5. Transit Area

7. Path Preference

OSPF uses path cost as its basic routing metric, which was defined
by the standard not to equate to any standard value such as speed,
so the network designer could pick a metric important to the
design.
Four types of metrics are recognized:
1. Intra-area
2. Inter-area
3. External Type 1, which includes both the external path cost
and the sum of internal path costs to the ASBR that advertises
the route,
4. External Type 2, the value of which is solely that of the
external path cost

8. OSPF Router Types

The router type is an attribute of an OSPF process. A
given physical router may have one or more OSPF
processes.
OSPF defines the following router types:
1. Area Border Router (ABR)
2. Autonomous System Boundary Router (ASBR)
3. Internal Router (IR)
4. Backbone Router (BR)

9. OSPF Packet Header

Every OSPF packet starts with a common 24 byte
header. This header contains all the necessary
information to determine whether the packet should be
accepted for further processing.

10. OSPF in Broadcast and Non-
Broadcast Networks

In broadcast multiple-access networks, neighbor
adjacency is formed dynamically using multicast hello
packets to 224.0.0.5.
For non-broadcast multiple-access networks
(NBMA), RFC 2328 defined the following two official
modes for OSPF:
1. No broadcast
2. Point-to-multipoint

11. Implementations

1. BIRD implements both OSPFv2 and OSPFv3
2. GNU Zebra, a GPL routing suite for Unix-like systems
supporting OSPF
3. Netware implements OSPF in its Multi Protocol Routing
module.
4. OpenBSD includes an OpenOSPFD implementation within
the OpenBGPD protocol.
5. XORP, a routing suite implementing RFC2328 (OSPFv2) and
RFC2740 (OSPFv3) for both IPv4 and IPv6.
6. Windows NT 4.0 Server, Windows 2000 Server and Windows
Server 2003 implement OSPFv2 in the Routing and Remote
Access Service, although the functionality was removed
in Windows Server 2008.

12. Applications

 OSPF was the first widely deployed routing
protocol that could converge a network in the low
seconds, and guarantee loop-free paths.
 OSPF can provide better load-sharing on external
links than other IGPs.
 It has many features that allow the imposition of
policies about the propagation of routes that it may
be appropriate to keep local, for load sharing, and
for selective route importing more than IS-IS
(Intermediate System To Intermediate System).

13. Kashif Latif