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Table of contents
1.
Introduction
2.
What is Unicast Routing?
3.
Types of Protocols Used in Unicast Routing
4.
What is a Link State Routing Algorithm?
5.
Important Points Related to the Link State Routing Algorithm
6.
Link State Routing Protocols
7.
Phases of Link State Routing
7.1.
Flooding Phase 
7.2.
Calculation Phase
8.
Features of Link State Routing Protocols
9.
Algorithm for Link State Routing
10.
Characteristics of Link State Protocol
11.
Advantages of Link State Routing
12.
Disadvantages of Link State Routing
13.
Example to Understand Link State Routing Algorithm
13.1.
Step 1: Discovery phase
13.2.
Step 2: LSA flooding
13.3.
Step 3: SPF calculation
13.4.
Step 4: Updating LSAs
13.5.
Step 5: SPF calculation
14.
Frequently Asked Questions
14.1.
What is Link State Routing?
14.2.
What is the Link State Database (LSDB)?
14.3.
What is the Shortest Path First (SPF) algorithm?
14.4.
What are some examples of Link State Routing protocols?
15.
Conclusion
Last Updated: Mar 27, 2024
Medium

Link State Routing

Author Riya Singh
1 upvote
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Ashwin Goyal
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Introduction

Hello ninjas, Have you ever heard something about link-state routing? If not, then don't worry, we will clear all your doubts in this blog.

Link-state routing is a method in which each node creates a map to find its network connectivity. Subsequently, each node calculates the best logical path to every possible destination in the network independently. This approach ensures that each node has an up-to-date view of the network topology, making it easier to determine the best path to take for data transmission.

link state routing

In this article, we will further discuss link state routing in detail. We will also discuss its features, phases, advantages and disadvantages, and some examples. Before diving deeper into the topic, let us understand unicast routing first.

What is Unicast Routing?

Unicast routing is a type of network routing. In unicast routing, a packet is sent from one source to one specific destination. It is a one-to-one communication model. Because in this, a packet is sent from a source to a destination IP address.

The routing tables are maintained by routers in unicast routing. These tables contain information about the network topology. They are used to determine the best path for packets to travel from the source to the destination. The routers use this information to forward the packet to the next hop in the path toward the destination.

Unicast routing protocols include protocols like Open Shortest Path First (OSPF), Routing Information Protocol (RIP), and Border Gateway Protocol (BGP). These protocols allow routers to exchange information about the network topology and determine the best path for packets to travel.

Unicast routing is commonly used in applications such as email, web browsing, and file transfers, where data is transmitted between a single sender and a single receiver.

Also see,  Personal Area Network

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Types of Protocols Used in Unicast Routing

There are various types of protocols used in unicast routing. 

Some of the most common types include:

  1. Distance-vector routing protocols: These protocols determine the best path to a destination by using a metric. This metric can be the number of hops or distance to measure the path's cost.
     
  2. Link state routing protocols: These protocols create a map of the entire network. It includes information about each router and the links between them. This allows the routing devices to calculate the best path to a destination based on the most up-to-date information.
     
  3. Path-vector routing protocols: These protocols are similar to distance-vector protocols. But they also consider additional information, such as policy-based routing decisions.
     
  4. Hybrid routing protocols: These protocols combine the features of both distance-vector and link state routing protocols. It provides a balance between speed and accuracy.

What is a Link State Routing Algorithm?

A Link State Routing Algorithm is a type of routing algorithm used in computer networks to determine the shortest path from one node to another. It operates by constructing a complete topological map of the network, representing each node and link, and then using this map to calculate the shortest paths.

Important Points Related to the Link State Routing Algorithm

  • Complete Topological Map: It constructs a complete map of the network, including all nodes and links.
  • Dijkstra's Algorithm: It often employs Dijkstra's algorithm to calculate the shortest paths between nodes.
  • Link State Packet: Each router sends periodic Link State Packets (LSPs) containing information about its local links to all other routers in the network.
  • Topology Database: Routers maintain a topology database containing information from received LSPs, allowing them to construct the network map.
  • Shortest Path Calculation: After constructing the network map, routers use Dijkstra's algorithm to calculate the shortest paths to all other nodes.
  • Link State Updates: When there are changes in the network topology, routers generate new LSPs to inform other routers, ensuring that all routers have up-to-date information.

Link State Routing Protocols

Link state routing protocol is a type of network routing protocol. It maps the entire network, including the routers and the links between them. This map is called the link state database or topology table. The LSDB(Link State Database) is a very important component of a Link State Routing Protocol. It is a data structure that stores the topology of a network as known by a specific router running a Link State Routing Protocol.

Link state routing protocols exchange LSAs(Link State Advertisements) between routers. It contains information about the state of the links connected to each router. The LSAs are used to build and maintain a complete and up-to-date network map. It enables routers to calculate the shortest path to a destination. 

Phases of Link State Routing

There are two primary phases in link state routing:

Flooding Phase 

In this phase, each router floods its own LSA to all other routers in the network. The LSA contains information about the router's own links. It also contains the state of its neighboring routers. Routers use the LSA to build their own LSDB and to update the database as changes occur in the network.

Calculation Phase

Once each router has received all LSAs from its neighbors and has built its own LSDB, it performs a shortest-path-first (SPF) calculation. It performs SPF to determine the best path to each destination in the network. 

The SPF calculation considers the cost of each link. It also considers the state of each router in the LSDB. The result of the SPF calculation is used to build the forwarding table. This table contains the best path to each destination. This table is used to forward packets to their intended destination.

These two phases are continuous and occur simultaneously in the link state routing protocol. Routers continue to flood LSAs and update their LSDBs as changes occur in the network. They recalculate the SPF algorithm to determine the best path to each destination based on the most up-to-date network topology information.

Features of Link State Routing Protocols

The following are some of the features of link state routing protocols:-

  • Shortest Path Calculation: These protocols use graph algorithms such as Dijkstra's algorithm to compute the shortest path to each destination.
     
  • Loop Avoidance: Routing loops are avoided by using the complete information about the network topology.
     
  • Support for Multiple Parameters: These protocols can take various parameters such as bandwidth and delay into route calculations, making the network more versatile.
     
  • Predictable Convergence: Due to the network topology awareness and different efficient algorithms, the time required for the network to converge is more predictable when compared to other protocols.
     

In the next section, we will discuss the algorithms used for link state routing.

Algorithm for Link State Routing

The algorithm for Link State Routing involves the following steps:

  1. Discovery phase: Each router sends out Hello packets to discover its neighbors and to establish a neighbor adjacency. Once a neighbor adjacency is established, routers exchange LSAs to learn about the state of the network.
     
  2. LSA flooding: Each router floods its own LSA to all other routers in the network. The LSA consists of information about the router's own links and the state of its neighboring routers. Routers use the LSA to build their own LSDB and to update the database as changes occur in the network.
     
  3. Shortest Path First (SPF) calculation: Once each router has received all LSAs from its neighbors and has built its own LSDB, it performs an SPF calculation which determines the best path to each destination in the network. 
    The SPF calculation considers the cost of each link and the state of each router in the LSDB. The result of the SPF calculation is used to build the forwarding table. This table contains the best path to each destination. This table is used to forward packets to their intended destination.
     
  4. Updating LSAs: When a change occurs in the network, such as a link failure or adding a new router, the affected router floods a new LSA to all other routers in the network. This triggers a recalculation of the SPF algorithm, and the forwarding table is updated to reflect the new best path to each destination.
     
  5. Aging LSAs: To prevent outdated LSAs from remaining in the LSDB, each router assigns a time-to-live (TTL) value to each LSA. When the TTL value expires, the router removes the LSA from its LSDB.
     

The above steps are repeated continuously to maintain an up-to-date view of the network topology and to determine the best path to each destination. 

Characteristics of Link State Protocol

Let's take a look at some of the characteristics of the Link State Protocol:-

  • Resource Intensive: Link-state protocol requires more memory and processing power to maintain and distribute detailed link-state databases.
     
  • Fast Convergence: Link-state protocol has fast network convergence when changes occur because routers know about link-state changes immediately.
     
  • Vulnerable to Attacks: Constant exchange of detailed information related to routing can expose the network to cyber-attacks.
     
  • Hierarchical Design: Some link-state protocols like OSPF support hierarchical network design which makes managing large networks more feasible.
     
  • Initial Overhead: Establishing adjacencies and exchanging initial link-state information adds some overhead to the initial network setup.

 

Now, let's take a look at the advantages of link state routing.

Advantages of Link State Routing

Link state protocols offer several advantages over other routing protocols, such as distance vector protocols. These advantages include the following:

  • Since each router has a complete network map, link state routing protocols can converge much faster than distance vector protocols, which rely on periodic updates.
     
  • Link state routing protocols only send updates when there is a change in the network topology. It reduces the amount of bandwidth used compared to distance vector protocols.
     
  • Link state routing protocols are better suited to large networks. They can handle a more significant number of routers and links without suffering from routing loops or other issues.
     

Examples of link state routing protocols include Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS).

Also, check out - Phases of Compiler

Disadvantages of Link State Routing

While link state routing protocols offer several advantages over other types of routing protocols, they also have some disadvantages:

  • Link state routing protocols require more computational power. It requires more memory to maintain the link state database and to perform the shortest path calculation. This can limit their scalability, especially in more extensive networks.
     
  • The link state routing protocol is more complex than other routing protocols. This complexity makes the protocol more challenging to configure and manage.
     
  • It generates more network traffic than distance vector protocols. It sends frequent updates about changes in the network topology.
     
  • These protocols are vulnerable to routing attacks, such as spoofed LSA attacks. It can cause routers to make incorrect routing decisions.

Example to Understand Link State Routing Algorithm

Here is an example of how the Link State Routing algorithm works. 

Consider the following network topology:

Example to Understand Link State Routing Algorithm

Assume that each link costs 1. All the routers start with an empty LSDB(Link State Database). 

The following steps illustrate how the Link State Routing algorithm would operate in this network:

Step 1: Discovery phase

Each router sends Hello packets to discover its neighbors. Based on the topology, each router learns the following information:

P: Q, R //P discovers its neighbors Q and R

Q: P, S, T

R: P, U

S: Q

T: Q, U

U: R, T

 

Step 2: LSA flooding

Each router floods its own LSA to all other routers in the network. The LSA contains information about the router's own links and the state of its neighboring routers. 

After flooding is complete, the LSDB for each router will look like this:

P: P, Q, R, S, T, U

Q: P, Q, R, S, T, U

R: P, Q, R, T, U

S: Q, S

T: Q, T, U

U: R, T, U
 

Step 3: SPF calculation

Each router performs an SPF calculation to determine the best path to each destination. The result of the SPF calculation is used to build the forwarding table. For example, Router P's forwarding table will look like this:

Destination Next Hop
Q Q
R R
S Q
T Q
U R

Step 4: Updating LSAs

Suppose that Link P-R fails. Router R would detect the failure and send a new LSA to all other routers in the network. 

After flooding is complete, the LSDB for each router will look like this:

P: P, Q, S, T, U

Q: P, Q, S, T, U

R: P, Q, S, T, U

S: Q, S

T: Q, T, U

U: R, T, U
 

Step 5: SPF calculation

Each router would perform another SPF calculation to determine the best path to each destination. The result of the SPF calculation is used to update the forwarding table. For example, Router P's forwarding table would look like this:

Destination Next Hop
Q Q
R -
S Q
T Q
U R

In this example, the Link State Routing algorithm is used. It is used to maintain an up-to-date view of the network topology. It is also used to determine the best path to each destination. The algorithm is designed to quickly adapt to changes in the network, such as link failures, and to provide a reliable and efficient way to route packets.

Also see, Basic Networking Commands

Frequently Asked Questions

What is Link State Routing?

Link State Routing is a type of routing protocol. It is used in computer networks to determine the shortest path between two nodes. In Link State Routing, each router maintains a detailed view of the entire network, and routers exchange information about the state of their links.

What is the Link State Database (LSDB)?

The Link State Database (LSDB) is a database in link state routing. It contains complete information about the network topology a router maintains in a Link State Routing protocol. 

What is the Shortest Path First (SPF) algorithm?

The Shortest Path First (SPF) algorithm is used in Link State Routing. It is used to calculate the shortest path between two nodes in the network. It considers the cost of each link and the state of each router in the LSDB to determine the shortest path.

What are some examples of Link State Routing protocols?

Some examples of Link State Routing protocols include Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS), which are commonly used in enterprise networks, and Link State Routing Protocol (LSRP), which is used in some legacy networks.

Conclusion

This article delves into the concept of link state routing. We have also discussed the unicast routing protocol and the different phases in link state routing. Additionally, we explored the advantages and disadvantages associated with link state routing. You can check out our other blogs to enhance your knowledge:

We hope this blog helped you to understand link state routing protocols. You can refer to our guided paths on the Coding Ninjas Studio platform. You can check our course to learn more about DSADBMSCompetitive ProgrammingPythonJavaJavaScript, etc. 

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