Open shortest path first (ospf)

Routing protocol
 A routing protocol specifies how routers communicate with each
other
 Routing algorithms determine the specific choice of route.

Suffered from problems of Distance Vector Protocol
Count to Infinity Problem
Slow Convergence
Large update packets
Slow response to topological changes
 Need for a Link State Protocol.

OSPFOSPF Open Shortest Path First is a link-state routing protocol that
was developed in 1991.
OSPF was developed as a replacement for the distance vector
routing protocol RIP .
Based on Bellman-Ford Algorithm
Worked well in small systems

Areas
An OSPF network can be divided into sub-domains called areas.
 An area is a logical collection of OSPF networks, routers, and links
that have the same area identification.
Areas limit the scope of route information distribution. It is not
possible to do route update filtering within an area.

The main benefit of creating areas is a reduction in the number of
routes to propagate—by the filtering and the summarization of routes.
 Areas are identified by an area ID.
 All network inside an area must be connected.
 At the border of an area, special router called area border routers.
 It summarize the information and send to other areas

Backbone Area
 A backbone area—which combines a set of independent areas into
a single domain.
The backbone has the reserved area ID of 0.0.0.0. The OSPF backbone
area is also known as area 0.
The backbone acts as a hub for inter-area transit traffic and the
distribution of routing information between areas
Each non-backbone area must be directly connected to the backbone
area.
The backbone area must not be partitioned—divided into smaller
pieces—under any failure conditions

The backbone serves as primary area and the other areas as
secondary areas.
The router inside backbone area is known as backbone routers.

OSPF (type of links)
•In OSPF , a connection called a link.
•Four types of links have been defined they are:

Point-to-Point Link
A point-to-point link is a dedicated link that connects exactly two
communication facilities.
It connect two routers without any other host or router in between.
Eg : Telephone line .
No need to assign a network address to this type of link.
Router - Nodes
Link – Bidirectional edge

 Transient link
•Several routers attached to it.
•Data can enter or retrive
through any routers.
•Each routers have many no: of
neighbors.

 Stub Link
•Connected only to one router.
•Data enter and leave through this single router.
•Only one direction

 Virtual Link
 When link between two routers are broken, the administrator
create a virtual link between them
Using a longer path, which pass through several routers.

Graphical Representation
 An AS can represent graphically.

OSPF Packets
 It uses five different packets:
 hello
 database description
 link state acknowledgment

Common Header
 All packets have common headers.

Version 8 bit field define the version of OSPF
protocol
Type 8 bit field
Define type of packet
Message length 16 bit field,
define length of total length including
header
Source router IP address 32 bit field
define IP address of router that send
packets
Area identification 32 bit field
Checksum Used for error detection on entire packet
excluding authentication type and
authentication data field
Authentication type 16 bit
Define authentication protocol used in
this area; two types
0 – none
1 – password
Authentication 64 bit

 Heart of the OSPF operation.
 Used by a router to advertise the state of its links.
•Each update packet contain several different LSAs.
•All having same general headers

Link state age Received E T Link state type
Link state ID
Advertisement Router
Link state Sequence number
Link state checksum Length

Other Packets
A. OSPF Hello Protocol

Hello subprotocol is intended to perform the following tasks
within OSPF:
Dynamic neighbor discovery
Detect unreachable neighbors
Ensure two-way communications between neighbors
Ensure correctness of basic interface parameters between
neighbors
Provide necessary information for the election of the Designated
and Backup Designated routers on a LAN segment (coming)

B. Database Description Message
 Is used mostly during the database exchange
 The first DBD packet is used to elect the master and slave
relationship and to set the initial sequence number elected by the
master.
The router with the highest router ID becomes the master and
initiates the database synchronization.
The master sends the sequence number, and the slave
acknowledges it.
 After the master and the slave are elected, the database
synchronization starts; in this process, the headers of all the LSAs
are exchanged with neighbors.

I Bit— When set to 1, this means that this is the first packet in DBD exchange.
M Bit— When set to 1, this means that more packets will follow.
MS Bit— Use this for master and slave. When this bit is set, it means that the
router is a master in the DBD exchange process. If this bit is set to 0, it means
that the router is the slave.
DBD Sequence Number— This field contains a unique value set by the
master. This sequence number is used during database exchange. Only a
master can increment the sequence number.
LSA Header— This field consists of a list of the link-state database headers.

C. Link-State Request Packets
A link-state request packet, is sent if part of the database is missing or
out-of-date.
The link-state request packet is used to retrieve that precise piece of
database information that is missing.
Link-state packets are also used after the DBD exchange is finished to
request the LSAs that have been seen during the DBD exchange.

Fig illustrates the link-state request packet format.
LS Type— Identifies what type of LSA is being requested.
Link-State ID— Represents the link-state ID of that specific LSA.
Link-state ID is discussed later in this chapter.
Advertising Router— Contains the router ID of the router that is
originating this LSA.

D. Link-State Acknowledgment
PacketThe link-state acknowledgment packet, is used to acknowledge each
LSA.
This packet is sent in response to link-state update packets.
 Multiple LSAs can be acknowledged in a single link-state
acknowledgment packet.
This packet is responsible for the reliable delivery of link-state update
packets.
Figure illustrates the link-state acknowledgment packet format.

Open shortest path first (ospf)

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Open shortest path first (ospf)