Link-state routing protocols use Dijkstra's algorithm to calculate the shortest path to all destinations based on a link-state database containing the full network topology. Each router runs the same algorithm locally to determine the optimal path. Key aspects include link-state advertisements to share connectivity information, the topological database to store network maps, and shortest path first calculations to derive routes. Common link-state protocols are OSPF and IS-IS. They provide fast convergence and scalability but require more resources than distance-vector protocols.

1. Link-state routing
Protocol
Aung Thu Rha Hein
Chinnawat Noosrikong

2. Outline
• Overview
• Features of Link-State routing protocols
• link-state routing algorithm
• Dijkstra’s algorithm
• OSPF
• IS-IS
• Advantages and Disadvantages of LS routing protocols

3. Overview
• One of two main classes of interior gateway routing protocols
• OSPF, IS-IS
• Performed by every switching node in the network
Basic Concept
• every node creates a map of the connectivity to the network
• This graph shows which nodes are connected to which other
nodes
• Each node calculates the next best logical path to the destination
• Collection of best paths form the routing table of the node

4. Features of Link-state routing
protocols
Link-state advertisement (LSA) or Link-state Packet(LSP)
• a small packet of routing information that is sent between
routers
Topological database or Link-state Database
• a collection of information gathered from LSAs
SPF algorithm
• a calculation performed on the database that results in the
SPF tree
Routing table
• a list of the known paths and interfaces

5. link-state routing algorithm
1. Each router is responsible for meeting its neighbors and
learning their names.
• Used a Hello Protocol, which send a data packet contains RID
and address of the network on which the packet is being sent
2. Each router constructs a LSP/LSA which consists of a list of
names and cost for each of its neighbors.
3. The LSP/LSA is transmitted to all other routers. Each router
stores the most recently generated LSP/LSA from each
other router.
• Link-state flooding: Sequencing and Aging procedures
• Each routers store the identical Link State Database
4. Each router uses complete information on the network
topology to compute the shortest path route to each
destination node.
• Use SPF or Dijkstra’s algorithm to calculate the shortest path

6. Dijkstra’s algorithm
Also known as the shortest path first (SPF) algorithm
Nodes 2 3 4 5(b) 6
1(a) 7 9 ∞ ∞ 14
2 7 9<7+10 7+15 ∞ 14
3 7 9 22<11+ ∞ 14<7+1
10+7 0+2
6 7 9 22 9+2+10 14
+7

7. Dijkstra’s algorithm(Cont.)

8. Open Shortest Path First (OSPF)
• A routing protocol developed for Internet Protocol
networks by the Interior Gateway Protocol (IGP)
• Based on the Dijkstra’s Algorithm
• Serving large, heterogeneous internetworks
• OSPF Version 1 (1988)
• OSPF Version 2 (1998) Supported IPv4
• OSPF Version 3 (2008) Supported IPv6

9. Open Shortest Path First
(OSPF)
OSPF can operate within a hierarchy
Collection of networks under a common administration that share a
common routing strategy

10. Additional OSPF Features
• Equal-cost
• Multipath routing
• Routing based on upper-layer type-of-service (TOS) requests

11. IS-IS
• Intermediate System-to-Intermediate System (IS-IS)
• That floods the network with link-state information to build a
complete, consistent picture of network topology
• Each IS-IS router independently builds a database of the
network's topology
• IS-IS uses Dijkstra's algorithm for computing the best path
through the network
• IS-IS Metrics
• A single required default metric with a maximum path value of
1024
• Three optional metrics (costs):
• Delay
• Expense
• Error

12. Integrated IS-IS
• Integrated IS-IS is a version of the OSI IS-IS
• To support more network layer protocols than just CLNP
• Dual IS-IS
• Implementations send only one set of routing updates
• Supporting multiple network layer protocols in a router

13. Advantages and Disadvantages of LS
routing protocols
Advantages
• Smaller routing tables
• Only a single optimal route for each network ID is stored in
the routing table.
• Low network overhead
• do not exchange any routing information when the
internetwork has converged.
• Ability to scale.
• scale well to large and very large internetworks.
• Lower convergence time.
• have a much lower convergence time and the internetwork is
converged without routing loops.

14. Advantages and Disadvantages of LS
routing protocols(cont.)
Disadvantages
• Complex
• much more complex and difficult to understand than DV
routing protocols.
• More difficult to configure.
• implementation requires additional planning and
configuration.
• Resource intensive.
• For very large internetworks, the LS database and the
calculation of routing table entries can be memory and
processor intensive.