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types of routers

What are the OSPF LSA Types? – Explained

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ospf lsa types
ospf lsa types

OSPF, as we already know, is a Link- State routing protocol, and being a Link state routing protocol, each router in a network exchange information about the state of their directly connected links with all the routers in a network.  

It looks fine when we talk about a limited or small number of routers in a network but what happens when we have a significant number of routers? 

A lot of information will be flooded within a network leading to unnecessary network congestion, poor performance, and wastage of bandwidth. For that purpose, we use the OSPF area concept. 

But, by dividing a network into areas we are restricting router information flooding, and hence network would not have access to the necessary information required to find the best path. 

For that purpose, OSPF has a “Brahmastra “which is known as Link State Advertisements. 

In this article, we will discuss Link State Advertisements and various OSPF LSA types. 

ALSO READ: OSPF Interview Questions if you’re preparing to crack a Network Engineer job position.  

What is OSPF LSA?

In the Open Shortest Path First (OSPF) routing protocol, LSA also known as Link-State Advertisements helps to spread the routing information within an OSPF network. Routers configured with OSPF exchange LSAs to build a map of the network topology and find the best path possible to reach from one network prefix to another. All the information collected using LSA is stored in a database known as Link-State Database (LSDB) which is used to calculate the shortest path to different destinations. 

Understanding OSPF LSA Types: A Comprehensive Overview

Link State Advertisements are of different types with different functionality which are listed below:- 

 
 
 
 
 
 

Router LSA 

 
 
 
 
 

LSA Type 1 

 
 
 
 
 

Network LSA 

 
 
 
 

LSA Type 2 

 
 
 
 
 

Summary LSA 

 
 
 
 

LSA Type 3 

 
 
 
 
 

Summary ASBR LSA  

 
 
 
 

LSA Type 4 

 
 
 
 
 

Autonomous System external LSA 

 
 
 
 

LSA Type 5 

 
 
 
 
 

Multicast OSPF LSA 

 
 
 
 

LSA Type 6 

 
 
 
 
 

NSSA External LSA 

 
 
 
 

LSA Type 7 

What is LSA Type 1?

All routers generate their own Router LSA. It describes the state of its own links/interface. Type 1 LSA is flooded within an area and does not cross it. Router LSA includes information like Router ID, the status of links, and interface IP details. 

router lsa

What is LSA Type 2?

It represents the multi-access network segment. Network LSA is flooded by Designated routers only. It contains information about all the routers that are directly connected to multi-access network segments, information about DR and BDR routers, and subnet information. Similar to Router LSA, Network LSA is flooded within the same OSPF area and ensures that all the routers within an area have an accurate understanding of the network topology. 

network lsa

What is LSA Type 3?

Since Router LSA and Network LSA remain within the area, we need something to share information about a particular area with another area. For that purpose, Summary LSA or Inter-Area LSA is used. Type 3 LSA advertises routes between OSPF areas. These LSAs are generated by Area Border Router i.e. ABR. In the OSPF routing table, they are denoted as O IA. 

summery lsa

What is LSA Type 4?

These LSAs are generated by ABR to inform routers in an area about the existence of an Autonomous System Boundary Router (ASBR) in another area. This LSA informs about the path to reach external routes. 

summery asbr LSA

What is LSA Type 5?

These LSAs are generated by an ASBR to advertise external routes into OSPF. They are flooded throughout the OSPF domain.

external LSA

What is LSA Type 6?

Group Membership LSA or Type 6 LSAs are used in OSPFv3 for multicast group memberships within the link. 

What is LSA Type 7?

It is also called NSSA External LSA. In Not-So-Stubby-Area (NSSA) configurations, these LSAs are used to carry external route information. Since in Stub areas, LSA 5 is not allowed hence we use LSA 7 in this case. ABR routers convert LSA 7 into LSA 5 when it is flooded outside the stub area and convert LSA 5 to LSA 7 when LSA 5 enters the stub area. 

NSSA External LSA
OSPF LSA

We can see that LSA is the backbone of OSPF. It helps to understand network topology within OSPF routers. It also helps to calculate the shortest path to reach the destination. Not only it helps to find the shortest path but also finds the redundant path quickly whenever the primary path goes down for any reason. Hence it ensures fast route convergence which helps the network to quickly adapt to changes.

To make sure that you thoroughly understand OSPF, do watch out this video by Atul sir wherein he dives deep into the topic with simple explanations for a better understanding of the topic.

Let us quickly see the difference between each LSA. 

LSA TYPE 

Description 

Originator 

Flooded To 

Purpose 

Type 1 

Router LSA 

Router 

Same Area 

Describes router’s Link 

Type 2 

Network LSA 

DR 

Same Network 

Describes routers on a network segment 

Type 3 

Summary LSA 

ABR 

Other Area 

Advertise routes between areas 

Type 4 

Summary ASBR LSA 

ABR 

 

Informs about an ASBR 

Type 5 

Autonomous System external LSA 

ASBR 

OSPF Domain 

Advertises external routes 

Type 6 

Multicast OSPF LSA 

OSPFv3 router 

Same Area 

Used for multicast group memberships 

Type 7 

NSSA External LSA 

ASBR in NSSA 

NSSA 

Carries external routes in NSSA configuration 

What is a Router in Networking and How They Can Connect You to the Internet?

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What is a Router in Networking?
What is a Router in Networking?

Networking is a comparatively less familiar field of networking, but one that is evolving rapidly as people are getting to know, how lucrative it is. Networking is a domain in the Information Technology landscape that has a very wide horizon when employment and levelling up are concerned. 

This domain of information technology has various components working simultaneously that make a topology and each has its importance for a designated function.
In this blog, we shall have a look at the working of a router in Networking. 

What is a router?

router in Networking

A router can be defined as a device that establishes a connection between two or more packet-switched networks or subnetworks. Routers can also be defined as networking devices that are operating at layer 3 or a network layer of the OSI model.

Primary functions of a router in Networking:

A router indeed performs many united functions at once, few of its primary functions are: 

  1. It manages traffic between the networks by forwarding data packets to their intended IP addresses.
  2. It also allows multiple devices to use the same internet connection. 
  3. They also are responsible for receiving, analysing and further forwarding data packets amongst all the connected computer networks. As soon as the data packet arrives, the router begins the inspection of the destination address. 
  4. The router consults, the received packets’ routing tables to decide the optimal route and then further begins to transfer the packets along that route.

What are routing protocols?

We saw the kinds of routers we have and routing techniques but now we must be aware of the routing protocols. Routing Protocols are referred to the set of defined rules used by the routers to communicate between the source and the destination. They are responsible for moving the information from the source to a destination, rather they only update the routing table that contains the information. Network Router protocols help a user to specify the way in which the routers can communicate amongst themselves. It also makes way for the network to select routes between any two nodes on a given system network.

Types of a router in Networking:

There are different routes in a router. A route can be defined as a pairing between a destination and the attributes of the path to that destination. Thus it also has the name path-vector routing. The routers further receive a vector that contains paths to a set of destinations. 

Types of a router in Networking:

There are various different types of routers in Networking, a few of them are as follows: 

  • Static 
  • Default
  • Dynamic

We shall now see them in detail: 

Router in Networking

Static Routing:

Static routing can also be defined as a way of network routing technique. It is not necessarily a routing technique. Rather it is the manual configuration and also it selects a network route. It is most commonly administered by the network administrator.   It is employed in certain scenarios where the network parameters and environment are expected to remain constant.

This kind of routing is only optimal in a few situations. Network degradation, latency and congestion are some of the consequences that are inevitable due to the non-flexible nature of static routing.

Advantages:

  • In this form of routing there exists no overhead on the router, or CPU. 
  • In static routing, only the administrator is capable of adding routes. 
  • There exists no bandwidth between links. 

Disadvantages: 

  • It takes up excessive time, hence it isn’t advised for large organisations. 
  • There is a need for the administrator to be aware of how each router is connected. 
  • A link failure will lead to complete network failure, which doesn’t sit well, especially in small networks.

Default Routing:

Router in Networking

An instance of default routing

  • The default route is a route that is effective as soon as there is no other route available, for an IP destination address.
  •  Whenever a packet is received on a routing device, the device begins to check and see if the IP destination address is on one of the device’s local subnets.

Advantages: 

  • They establish routes that do not go down. 
  • They are beneficial as they keep the routing tables small. 

Dynamic Routing:

Diagram showing how a router connects to the internet in networking.

Working in dynamic routing. 

  •  Dynamic routing is defined as the technique of finding the best path for the data to travel over a network in this process, a router can transmit data through various different routes and reach its destination on the basis of conditions at that point in time of communication circuits.

Advantages: 

  • It is advised for all large networks, as it is easier to configure. 
  • In case of a link going down, this routing will enable, dynamically choosing a different route. 
  • It enables a user to load balance within multiple links.

Disadvantage: 

  • It happens to consume bandwidth as the updates are shared between the routers. 
  • All the routing protocols are bound to put an additional load on the router, CPU or RAM. 

The basic components needed to route:

A diagram illustrating the process of a building management system that includes a router in networking.

There are basically four important components of a router. The ones that have been identified as important are as follows:

1. Input ports:
These ports perform the physical layer function of terminating a physical link that is incoming at a router. 

A diagram of a pc motherboard featuring various display ports including VGA, DVI, and HDMI for connecting to monitors or other display devices.

2. Output Ports:
These ports store packets that are received from the fabric. Then these packets are further transmitted on the link that is outgoing.

3. Switching Fabric:

This establishes a connection between the router’s input ports to its output ports.

router in Networking

3. Routing Processor:
The routing processor works to execute routing protocols, maintain routing tables and link state information. It also is responsible for computing the forwarding table.

The functions of a router’s control plane functions are usually implemented in software and further executed on the routing processor.