OSPF is a link-state interior gateway protocol that uses shortest path first algorithm to calculate routes. It elects a designated router that exchanges link-state advertisements and database information with other routers to choose the best routes. OSPF supports equal cost multi-path routing, uses different types of link-state advertisements, and allows dividing networks into areas to reduce routing overhead. It authenticates messages to prevent routing attacks.

1. OSPF
ROUTING PROTOCOL

2. INTRODUCTION
TO OSPF
• Developed by IETF – RFC1247
• OSPF v2 described in RFC2328/STD54
• OSPF v3 described in RFC2740 - IPv6
• Link state/Shortest Path First Technology
• Dynamic Routing
• Interior gateway protocol
• Fast Convergence
• ECMP
• Route authentication

3. HOW IT WORKS
Become neighbours - Two routers
running OSPF on the same link agree to
form a neighbour relationship
Exchange database information - The
neighbouring routers exchange their
LSDB information with each other
Choose the best routes – Each router
chooses the best routes to add to its
routing table based on the learned LSDB
information

4. OSPF METRIC
• It is calculated by dividing reference bandwidth (100 mbps
by default) value by interface’s bandwidth
• Anything faster than Fast Ethernet will be a cost of 1.
• It can be changed using the command : auto-cost refence-
bandwidth mbps or ip ospf cost

5. ECMP
• Equal Cost Multi-Path or ECMP is a
routing strategy where packets
towards a single destination IP
address are load-balanced over
multiple best paths with equal
metrics.

6. HELLO MESSAGES
• When OSPF is activated on an interface , the router starts
sending OSPF hello messages out of the interface at regular
intervals (10 s by default)
• They are multicast to 224.0.0.5
• Dead timer - how long the router will wait without hearing a
HELLO message before it assumes the neighbor is dead (40 s
by default)
• The passive interface command tells the router to stop
sending OSPF HELLO messages out of the interface

7. LOOPBACK INTERFACE
• It is a virtual interface in the router .
• It is always up/up .
• It provides a consistent IP address that can be used to reach the router .

8. ROUTER ID
Mannually assigned
Highest Ip status loopback
interface Ip address
Highest Ip status non-
loopback interface Ip
address

9. OSPF NEIGHBORS

10. OSPF MESSAGES

11. DR AND BDR
• Only DR and BDR form full
adjacencies with other routers
• The remaining routers remain in a “2-
way” state with each other
• If DR or BDR “disappear”, re-election
of missing router takes place

12. OSPF NEIGHBOR REQUIREMENTS
Area must
match
Interfaces must
be in the same
subnet
OSPF process
must not be
shutdown
Hello and Dead
timers must
match
Authentification
settings must
match
IP MTU settings
must match
OSPf network
type must
match

13. OSPF AREAS
• It is uses to divide up the network
• Small networks can be single-areas without any negative effects on performance .
• In Larger networks , a single-area design can have negative effects :
 SFP algorithm takes more time to calculate routes
 Larger LSDB takes up memory
 Any small change in the network causes every router to flood LSAs and run the SPF algorithm again
• The backbone area is an area that all other areas must connect to .

14. CLASSIFICATION OF
ROUTERS
• Internal Router (IR)
• Area Border Router (ABR)
• Backbone Router (BR)
• Autonomous System Border Router
(ASBR)

15. LSA 1
• This LSA contains a list with all the directly
connected links of this route , it always
stays within the area

16. LSA 2
• The network LSA or type 2 is created
for each multi-access network ,
generated by the DR , In this LSA we
will find all the routers that are
connected to the multi-access
network, the DR and of course the
prefix and subnet mask , the network
LSA always stays within the area

17. LSA 3 SUMMARY
• This LSA generated by the ABR will
flood into all the other areas of our
OSPF network. This way all the
routers in other areas will know
about the prefixes from other areas.

18. TYPE 4 SUMMARY
ASBR LSA
• This LSA will also be flooded in all
other areas and is required so all
OSPF routers know where to find the
ASBR

19. LSA TYPE 5
• Generated by the ASBR to advertise
external redistributed routes into the
OSPF’s AS.

20. STANDARD AREA
• OSPF packets can be normally
transmitted. It supports 1, 2, 3, 4, and
5 LSAs

21. STUB AREA
• This area does not accept any
external routes of non-OSPF network
, this area supports 1, 2, and 3 LSAs

22. TOTALLY STUBBY
AREA
• this area does not accept external
routes and does not accept routes
from other areas , This area supports
Type 1, Type 2 LSAs, and Type 3 LSAs
with default routes

23. NO SO STUBBY AREA
• this mode allows the introduction of
external routes in a completely stub
area environment

24. OSPF NETWORK TYPES
• Broadcast : enabled by default on
Ethernet and FDDI interfaces .
• Point to point : enabled by default on
PPP and HDLC interfaces (no DR/DBR
election).

25. VIRTUAL LINK
• By using a virtual link, we can extend area
0 through area 1 so area 2 will be “directly
connected” to area 0

26. OSPF AUTHENTIFICATION
• OSPF can be configured to authenticate every OSPF message. This is usually done to prevent
a rogue router from injecting false routing information and therefore causing a Denial-of-Service
attack.
• Two types of authentication can be used
 clear text authentication
 MD5 authentication