OSPF LSA(Link State Advertisement) | Explained In Simple Terms.

Explaining OSPF LSA(Link State Advertisement)

Guyz we have already learned what is OSPF in my previous post just in case you haven't here is the link(OSPF), because to learn OSPF LSA(Link State Advertisement) you need to know OSPF. Also,  before we start with LSA we need to know some important OSPF terminologies.

Explaining OSPF Terminologies To Understand LSA(Link State Advertisement).

Here are the two important terms you need to know before understanding OSPF LSA:-

Link- A link can be a physical component or logical component. A physical link is like a router interfaces or nodes that provides a connection between two devices(routers) and when the link is used to connect to networks and has been assigned an IP address then that is a logical link.

Topological Database-  The information in the LSA packets that are received from the routers in an area is stored in the Topological database in the form tables. These formations from the topological database are used by the router to input it into Dijkstra algorithm for calculating the shortest path to the remote network. Also, the link state advertisements are used to maintain and update the topological database.

LSA Flooding- OSPF uses this method to distribute routing information via link-state updates(LSU) packets that contain the link status so that all the OSPF routers have the same topology map.

Designated Routers-  When OSPF routers are connected to the same broadcast networks, then an election is done on the basis of priority level and the one having the highest priority is selected as a designated router, also referred as DR and the objective of DR is to minimize the number of adjacencies formed and also, it is used to distribute the received routing information to the remaining routers and from the remaining routers on the broadcast network or link.

Backup Designated Router- As the name suggests, it is a backup or a hot standby for DR. Though BDR gets all the routing information from the OSPF adjacent routers but it will not disperse the LSA updates like DR does. But when the DR is down BDR will start giving the routing updates.

Area Border Router(ABR)- ABR concept is used when the routers are in two different areas ie in multi-area OSPF configuration. ABR routers are those routers that are at the border between an OSPF area and a backbone area(area 0). So In simpler terms, ABR is the member of both an  OSPF area and the backbone area. 

Autonomous System Boundary Router(ASBR)- ASBR concept is used to bring the connectivity between routers in an OSPF domain and routers running different routing protocol by using Autonomous System. So basically ASBRs are those routers that are members of both OSPF as well as the other routing protocol

Now getting back to our main topic of discussion, what is OSPF LSA(Link State Advertisements).

What Is (LSA)Link State Advertisement?

We are already aware that unlike Eigrp and RIP that uses routing by rumor, OSPF does routing by intelligence. Why intelligence because OSPF keeps track and keeps verifying the status of the links within their respective areas. The link status is stored in the LSA and if by chance a link is down, then the OSPF has to recalculate the whole route by rerunning the SPF algorithm, which consumes CPU.

In Simpler terms, an LSA(Link State Advertisement) is OSPF data packet that contains the link status and the routing information taken from the neighbor of all the routers in an area. Also, one important thing I must tell you is that an OSPF  will only exchange LSA packets with a router to which it has form adjacencies.

Now let's talk about what are the different types of OSPF LSA(Link State Advertisements).

What Are The Types Of  LSA(Link State Advertisement)?

There are 5 types of  OSPF LSA(Link State Advertisement) and each type has an important role to perform in propagating routing information. 

• Type 1(Router LSA)-  All the links that are local to a router along with their status and the cost of those links are listed in Type 1 LSA. All the OSPF routers generate this LSA and then it is flooded to all the routers within the same area.

• Type 2(Network LSA)- This Type of LSA is generated by the designated router(DR)  which contains the list of all the routers attached to it.

• Type 3(Network Summary link LSA)-  This type of LSA is generated at an area by ABR and then they forward it to the other OSPF areas connected to the backbone area, so unlike Type 1 and Type 2, this LSA crosses boundaries and carry information send by one ABR at one end to other ABR at the other end of the boundary via backbone area. These LSAs contain a summary of network and link of the OSPF area along with the IP address and RID of the ABR that is generating and advertising the Type 3 LSA.

• Type 4(ASBR Summary LSA)- This type of LSA is generated by ABRs and then they forward this to the area external to the one in which they were generated much like how is done in LSA Type 3, but there is a big difference and that is, this type LSA is specifically used to inform the rest of the OSPF areas how to get to ASBRs. 

• Type 5(AS External Link LSA)- This LSA is generated and advertised by ASBRs that contains the information of the routes external to the OSPF and are flooded everywhere. A type 5 LSA is generated for each individual external network that ASBR advertises.

Also Read This Hot Topic:- What Is OSPF?

Also Read This Hot Topic:- Eigrp Vs OSPF

So, guys, this is the end of my explanation for the topic what is OSPF LSA(Link State Advertisement). I have explained this complicated topic at the best of my capabilities, so if you'll like my explanation please share it and follow me on facebook or google plus for more. Also If you have any questions or doubts please comment it down below. See you guyz soon...

EIGRP Vs OSPF- Protocol Showdown

EIGRP Vs OSPF is the hot concern for network admins when they are confused which protocol they should use in their enterprise. If you certainly look at the fact both these protocols are very good, both have high convergence rate, use up less bandwidth and are free from loops. Then what are the real points that make these two so distinguishable from each other, those points are my concern in this session. Also for this topic, you need to know what is EIGRP AND OSPF.  



In this EIGRP Vs OSPF topic, I will consider all the main advantages and disadvantages of these two protocols and pit them against each other to see which one fairs better. So let's get started.

• Starting with my first point, EIGRP is a Cisco Proprietary so it can only be used in Cisco routers, but what if there are multiple vendor routers, in that case, we will have no choice but to use OSPF which is an open standard protocol and can be used by other vendors as well. 

• Now talking about implementation, Implementing OSPF is a little bit more challenging as compared to EIGRP,  as OSPF involves areas for dividing the network, the network administrator should have a solid understanding of data communication and network in order to make OSPF run well. But since OSPF is getting more and more popular this could not be considered a big problem anymore.

• OSPF is specially designed for larger networks, as it has unlimited hop count, unlike Eigrp which has hop count limit of max 255(100 by Default). Hop count here means to how many routers it can send routing information. This one is a severe drawback for EIGRP protocol and that is why it can not be implemented in larger networks where thousands of routers are concerned. 

• Eigrp uses Dual(Diffusing Update Algorithm) Algorithm, due to this, it also maintains a Feasible Successor(backup) route along with Successor one in its topological database, so whenever a route is down it will automatically use the feasible successor as a route to reach the destination so no need to rerun the algorithm again and hence saving CPU consumption. Whereas in OSPF if the route goes down it has to rerun the SPF algorithm again and recalculate the route and hence here CPU consumption is considerably more. 

• OSPF does not support auto-summary, you will have to manually summerise each network, whereas Eigrp supports auto-summarization by default, you can disable auto-summarization to enable manual summarization.

• Since Eigrp is Advance Distance vector Protocol, it uses Routing-by-Rumour approach ie there is no authenticity to the routing information from the neighbor router about the network. Whereas in OSPF, which is a link state protocol, checks the routing information contained in the topological table in form the LSA's(Link state Advertisement). 

• Eigrp has an Administrative distance of 90(Internal Eigrp) and 170(External Eigrp) whereas OSPF has an AD of 110.

• Eigrp Supports IP multiple protocols like IP, IPx and AppleTalk, whereas OSPF only supports IP.

• Eigrp Supports both Equal and Unequal cost Load balancing whereas OSPF only supports Equal cost load balancing.

EIGRP Vs OSPF:- Verdict

I think the network administrator should absolutely go with using OSPF protocol, as he won't have to worry that all the routers should be of Cisco. Also, OSPF is suitable for large enterprises where thousands of routers are involved. But if the company is of small-scale and all the routers are of Cisco then definitely using Eigrp will be more suitable, as Eigrp is less complex to implement and has more convergence rate then OSPF.

So I think choosing which protocol to use depends upon the situation or scenario you are facing. I am very sure that this Article of EIGRP VS OSPF will help the administrators to choose the protocol suitable for them

So I guess this is it in this article of EIGRP Vs OSPF. If you guys have some suggestions for this post, write it down in the comment section, also after reading this article don't forget to share and subscribe...As sharing is caring!!

What Is OSPF In Networking- Top Standout Characteristics

Explaining OSPF In Networking-

Guyz we have already learned Eigrp and its various terms and characteristics in our previous sessions. So this session is dedicated to learning OSPF in networking. So let's begin.

OSPF in networking stands for open shortest path first and is a link state protocol which uses the shortest path first algorithm(SPF) also called as Dijkstra's algorithm to find the best path. OSPF is open standard protocol and so this protocol is currently being used by many vendors including Cisco. This open standard characteristic of OSPF helps in connecting two different routers from different vendors and that makes it very popular and flexible.

OSPF uses an area for employing hierarchical network design, so the routers will only communicate and form adjacency neighbor relationship with routers in the same area, this characteristic of OSPF in networking is also called as single area OSPF.

Note:- You can also form neighbor relationship with routers in different areas using backbone area also called as area 0, since its a lengthy topic  that would need a whole another dedicated session.

Since OSPF is a link state protocol it doesn't advertise distance to the directly connected networks, instead, it sends the link status to the neighbors using link-state advertisement(LSA's). Remember OSPF also sends change triggered update, it sends updates(LSA's), when there is a change to one of its link status.

OSPF maintains three main type of table-

1. Routing Table- Contains only the best route to the remote networks
2. Topology Table- Contains list of all the possible routes to all the known network 
3. Neighbor Table- Contains list of all the neighboring routers along with their router ID

  OSPF In Networking- Top Standout Characteristics And Features

• OSPF supports both IPv4 and IPv6 networks
• The administrative distance of OSPF is 110
• It is link state protocol that uses cost as its metric which is calculated on the basis of bandwidth of the link.
• OSPF is an open standard protocol that uses SPF also called as Dijkstra's algorithm for path selection
• Unlike Eigrp which has limited hop count for sending routing updates, OSPF has unlimited hop count and its the better solution for big enterprise whereas thousands of routers are involved.
• OSPF uses an area to identify routers using common routing strategy.
• It minimizes routing update traffic, as the update is change triggered not a periodic one.
• OSPF uses multicast address either of 224.0.0..05(For all OSPF routers)  or 224.0.0.6(All designated routers).
• OSPF supports VLSM and CIDR
• OSPF is open standard protocol and hence it supports multiple vendors.
• It only supports manual summary, There is no auto-summary in OSPF.

READ ALSO:- WHAT IS EIGRP PROTOCOL AND ITS STANDOUT FEATURES?

READ ALSO:- WHAT IS SPLIT HORIZON

READ ALSO:- WHAT IS STATIC ROUTING?

READ ALSO:- WHAT IS DYNAMIC ROUTING?

The above mentioned are the main standout characteristics of OSPF in networking. Learn it and you are good to go for an interview. Also In my next session, I will explain some important terms of OSPF like LSA's and etc. So guys after reading this if you have some doubts or question ask me in the comment section. Also, don't forget to share and subscribe for more such awesome posts. See you guys in next class.