1. CSC427
Welcome to Week-11
Link-State Routing Protocols:
OSPF Prof. Nasir Faruk
Department of Information
Technology
faruk.n@slu.edu.ng
CSC427
Welcome to Week-11
Link-State Routing
Protocols: OSPF
Prof. Nasir Faruk
Department of Information
Technology
faruk.n@slu.edu.ng
2. Course content/Lecture
Schedule
2
PROBLEM SETS
Lect. No. Date Topic
1 Week 1 Course Administration, Networking Concepts and Communication pattern
2 Week 2 NETWORKING LAYERS, MODELS AND ARCHITECTURES
3 Week 3 Network Types, Size and Topologies
4 Week 4 Physical Layer and Media
5 Week 5 Data link Layer Protocols
6 Week 6 LAN and IP Addressing
7 Week 7 (Test 1) Switched LANs, VLANS and STP
8 Week 8 Network Layer: Protocols, IP header and Subnetting
9 Week 9 Network Layer: Routing Protocols I (Static and Dynamic Routing )
10 Week 10 Network Layer: Routing Protocols II (Distance Vector Routing Protocols )
11 Week 11 (Test 2) Network Layer: Routing Protocols III (Link-State Routing Protocols)
12 Week 12 Transport Layer Protocols: TCP and UDP
13 Week 13 Application Layer Protocols: DNS
14 Week 14 (Test 3) Introduction to Advanced Network Technologies, ATM, MPLS and QoS
P.S
No.
Topic Date
Assigned
Duration
1 Physical Layer and Media Week 4 1 Week
2 Switched LANs, VLANS and STP Week 7 1 week
2 Network Layer: Routing Protocols I Week 9 1 Week
4 Network Layer: Routing Protocols II Week 10 1 week
5 Network Layer: Routing Protocols III Week 11 1 week
3. Today’s Class
3
— In this week you will be introduce to:
— Review of DistanceVector and Link StateAlgorisms
— Introduction of OSPF Routing Protocol
— OSPFAlgorithm
— Building RoutingTable with OSPF
— Comparing RIP and OSPF routing tables for enterprise
networks
4. Weaknesses of Distance
Vector Routing Algorithms I
— Limit network size
— Cost of 16 for unreachable
networks limits network diameter
— Maximum number of hops across
network
— Do not support variable subnetting *
— Use same address block size for all
subnets
— Lowest hop count not necessarily
best path
— Have no built-in security
— Permits router spoofing
* Usually calledVLSM:
variable length subnet masks
5. Weaknesses of Distance
Vector Routing Algorithms II
— Noisy protocol
— Broadcasting advertisements interrupts all systems on a LAN
— Routers do not maintain a neighbour table
— Are stateless
— Do not seek verification of arrival of routing updates from neighbours
— Slow convergence (slow reaction to topology changes)
— Do not necessarily advertise failure immediately
— Merely omit failed networks in subsequent periodic advertisements
— Takes time for all routing tables to reach agreement following a topology change
— Entries for same network can differ while convergence is taking place
— Susceptible to packet looping, instability and count-to-infinity
7. OSPF: Introduction
— 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).
— It was developed as a replacement for the distance vector routing protocol RIP.
— RIP was an acceptable routing protocol in the early days of networking and the
Internet, but its reliance on hop count as the only measure for choosing the best
route quickly became unacceptable in larger networks that needed a more
robust routing solution.
— OSPF is a classless routing protocol that uses the concept of areas for
scalability.
— The Cisco IOS uses bandwidth as the OSPF cost metric.
7
8. OSPF….
— OSPF is the most widely-used IGP in large enterprise networks.
— The most widely-used exterior gateway protocol is the Border
Gateway Protocol (BGP), which does routing protocol between
autonomous systems on the Internet.
— OSPF gathers link state information from available routers and
constructs a topology map of the network.
— The topology determines the routing table which makes routing
decisions based solely on the destination IP address found in IP
packets.
— OSPF was designed to support variable-length subnet masking
(VLSM) or Classless Inter-Domain Routing (CIDR)
addressing models.
8
9. OSPF Network Types
— OSPF distinguishes three network types: Uses this to reduce routing
information on :
— Point-to-point networks
— HDLC, PPP
— Broadcast Networks (LANs) Ethernet,Wireless, Fibre Channel
— LAN broadcast capability facilitates
concept of designated router
— Reduces number of adjacencies required
— Non-Broadcast MultiAccess (NBMA)
networks (mesh topologies)
— Frame relay,ATM,VPN
9
10. OSPF Key Features
— Fast convergence: OSPF routers maintain network topology
information
— Scalable routing: Uses areas to localize routing details
— VLSM support:Address masks are included in network
advertisements (LSAs)
— Authenticated router exchange: RFC 2328 allows use of MD5
signatures
— Selective router exchange: Multicasts routing messages to the
reserved link local address
— AllOSPFRouters: IPv4: 224.0.0.5; IPv6: ff00::5
10
11. OSPF Packet Type
— Hello - Hello packets are used to establish and maintain adjacency with other
OSPF routers.
— DBD -The DataBase Description packet contains an abbreviated list of the
sending router's LS database and is used by receiving routers to check against
the local LS database.
— LSR - Receiving routers can request more information about any entry in the
DBD by sending a Link-State Request (LSR).
— LSU - Link-State Update packets are used to reply to LSRs as well as to
announce new information. It contain seven different types of Link-State
Advertisements (LSAs).
— LSAck -When an LSU is received, the router sends a Link-State
Acknowledgement (LSAck) to confirm receipt of the LSU.
11
12. Neighbor Establishment cont...
— Before two routers can form an
OSPF neighbor adjacency, they
must agree on three values:
— Hello interval
— Dead interval and
— network type.
— The OSPF Hello interval
indicates how often an OSPF
router transmits its Hello packets.
— By default, OSPF Hello packets are sent
every 10 seconds on multi-access and
point-to-point segments and
— every 30 seconds on non-broadcast
multi-access (NBMA) segments (Frame
Relay, X.25,ATM).
12
13. OSPF Algorithm
— Each OSPF router maintains a link-
state database containing the LSAs
received from all other routers.
— Once a router has received all of
LSAs and built its local link-state
database, OSPF uses Dijkstra's
shortest path first (SPF)
algorithm to create an SPF tree.
— The SPF tree is then used to
populate the IP routing table with
the best paths to each network.
13
19. Tutorial Question
I. With examples, explain what is adaptive routing protocol? [2.5 marks]
II. RIP was an acceptable routing protocol in the early days of networking and the
Internet, explain five main reasons why OSPF is now prepared as a routing algorithm
[10 marks]
III. OSPF supports five packet types. List and briefly explain each of those [5 marks]
19
20. Study Questions_1
Figure 1 shows a set of site LANs connected by router to
core network.The core network uses fast routers and
high speed links, as indicated by the lower OSPF
costs.The dash lines are slower backup paths to be
used if the connection through the core network fails.
a) Give the RIP and OSPF routing tables for R1 and R4
(showing only the entries for the six LANs) and use
these to explain why RIP wound not be a good choice
of routing algorithm for this network [12.5 marks]
b) What would be the cost of routing packets from LAN
1 to LAN 6, and LAN 1 to LAN 5 if the connection
through the core network fails? [5 marks]
20
Figure 1
21. Study Questions_2
Figure 2 shows the proposal for a routed
internetwork to connect three branch –
office LAN into three cooperate network
at Head office. (The numbers in parentheses
are the OSPF link costs).
a) The network administrator is unsure
whether to deploy a distance vector or
link state protocol. Using three suitable
performance metrics compare these two
routing algorithms and propose a solution
to this problem. [6 marks]
b) As a network administrator employed by
the organization, you were task to propose
a simple IP addressing scheme for the
subnetworks based on the private address
space 204.15.5.0/24, show the IP address
range for each subnet and comment
briefly on your design.
[11.5 marks]
21
Figure 2
22. Further Reading
Author Chapters and main topics
Topics that will NOT be
examined from these
sections
Forouzan
(4th edition)
21 Network Layer: Delivery,
Forwarding and Routing
Multicast Routing
(covered in 400 Level)
Tanenbaum
(4th edition)
5.2 Routing Algorithms
Stallings
(7th edition)
12. Routing in Switched Networks
