Seminar Presenting On
Open Shortest Path First (OSPF)
M-TECH (CSE) BATCH -1
Simple basic configuration
Important term used in OSPF
Drawback & Benefit
Introduction of OSPF
1. The OSPF protocol is a link state Protocol that handles routing for IP traffic.
Open Shortest Path First (OSPF) uses the Shortest Path First (SPF) algorithm, to provide
a loop-free topology.
3. Open Shortest Path First (OSPF) provides fast convergence with triggered, incremental
updates via Link State Advertisements (LSAs).
4. Open Shortest Path First (OSPF) is a classless protocol and allows for a hierarchical
design with VLSM and route summarization.
5. The two important concepts in case of OSPF are Autonomous Systems and Areas.
Important Term Related with OSPF
Loopback Interface:- A loopback interface is a logical, virtual interface on a router
o Router(Config)#int loopback 2
o Router(Config-if)#ip address 126.96.36.199 255.255.255.0
Area Border Router (ABR):-An Area border router (ABR) is a router that connects one or more
OSPF areas to the main backbone network. It is considered a member of all areas it is connected to.
Internal Router:-An Internal Router is a router that has only OSPF neighbour relationships with routers
in the same area.
Backbone Router:-Backbone Routers are part of the OSPF backbone. This includes all area border
routers and also routers connecting different areas.
Designated Router (DR) and Backup Designated Router (BDR):-A DR is the router interface elected
among all routers on a network segment, and BDR is a backup for the DR. DRs are used for reducing
network traffic by providing a source for routing updates. The DR maintains a complete topology table of
the network and sends the updates to the other routers via multicast. All routers in an area will form a
slave/master relationship with the DR .
Group of contiguous nodes/networks
Per area topology DB
Invisible outside the area
Reduces routing traffic
Backbone Area is contiguous
All others areas must connect to the
OSPF protocol configuration
Rtr(config)# router ospf process-id
Rtr(config-router)#network address wildcard-mask area area-id
Rtr(config-router)# default-information originate
Rtr(config-router)# area area authentication
Rtr(config-router)# area area authentication message-digest
Rtr(config)# interface loopback number
(Configure lo as RtrID)
Rtr(config)# interface type slot/port
Rtr(config-if)# ip ospf priority <0-255>
Rtr(config-if)# bandwidth kbps
(Modify default bandwdth)
RTB(config-if)# ip ospf cost cost
(Modify inter. cost)
Rtr(config-if)# ip ospf hello-interval seconds
Rtr(config-if)# ip ospf dead-interval seconds
Rtr(config-if)# ip ospf authentication-key passwd
Rtr(config-if)# ip ospf message-digest-key key-id md5 password
Header format of OSPF Packet
OSPF common header precedes each OSPF Packet
Configuring a Loopback Address
Rtr(config)# interface loopback 0
Rtr(config-if)# ip add 10.1.1.1 255.255.255.0
Automatically are “up” and “up”
Very useful in setting Router IDs as they never go down.
RouterID is used to identify the routers in the OSPF network
– IP address configured with the Router-ID command
– Highest loopback address
– Highest active IP address
Important for DR/BDR elections unless you use the ip ospf priority command
Extra: Also, useful to configure “virtual” networks that you can ping and route as if they
were attached networks.
The Hello Protocol
When a router starts an OSPF routing process on an
interface, it sends a hello packet and continues to send
hellos at regular intervals
◦ The rules that govern the exchange of OSPF hello packets are
called the Hello protocol
◦ Type field is set to 1 to indicate packet contains hello
More on Hello Packets…
• Hello packet timing…
– sent every 10 seconds by default on broadcast multi-access and point-to-point
– 30 seconds by default on interfaces that connect to NBMA networks, such as
Disadvantage and Benefit
Disadvantages:OSPF is very processor intensive.
OSPF maintains multiple copies of routing information, increasing the amount of memory needed.
OSPF is not as easy to learn as some other protocols.
Advantages:OSPF is hierarchical, using area 0 as the top as the hierarchy.
OSPF is a Link State Algorithm.
OSPF supports Variable Length Subnet Masks (VLSM).
OSPF uses multicasting within areas.