Articles Tagged ‘ospf’

Analysis of OSPF Link State Update (LSU) - Link State Advertisement (LSA) Packet Structure. Common LSA Types

Part 4 of our OSPF Routing Protocol Series covers how OSPF uses Link State Advertisement (LSA) to exchange information about the network topology between routers. When a router receives an LSA, it is stored in the Link-State DataBase(LSDB). Once the LSDBs between routers are in sync, OSPF uses the Shortest Path First (SPF) algorithm to calculate the best routes for each network. It is important to understand that LSAs are information about a route that is transported inside Link State Update (LSU) packets.

Each single Link State Update (LSU) packet can contain one or more LSAs inside it and when an LSU is sent between OSPF routers, it floods the LSA information through the network.

It is very important for any network engineer to understand how LSAs are contained within an LSU. We’ll use the example below, where an OSPF router sends an LSU to the OSPF Designated Router (DR) containing LSA information about a new network:

OSPF Link State Update (LSU) packet containing a Link State Advertisement (LSA)

Figure 1. OSPF Link State Multicast Update (LSU) packet containing a Link State Advertisement (LSA)

As shown above, LSAs are contained within LSUs, which are all part of an OSPF packet encapsulated within an Ethernet frame (assuming an Ethernet network).

Our diagram of the LSU/LSA packet structure is confirmed by capturing an OSPF Ethernet frame below. We’ve highlighted each section (LSA, LSU, OSPF Header) using the same colors:

OSPF Link State Update and List State Advertisement within an Ethernet frame

Figure 2. OSPF Link State Update and List State Advertisement within an Ethernet frame

Notice that the destination IP address is multicast address, as expected since routers send updates to the Designated Router (DR) using this multicast address. This is also analyzed under the Working Inside a Single Area section in our article How OSPF Protocol Works & Basic Concepts: OSPF Neighbor, Topology & Routing Table, OSPF Areas & Router Roles, Theory & Overview

Most Popular OSPF LSA Types

OSPF currently defines 11 different LSA types, however, despite the large variety of LSAs only around half of them are commonly found in OSPF networks. Table 1 below shows the most popular LSA types, the type of OSPF routers (DR, ABR, ASBR etc) that generate them along with their function and the OSPF areas they affect:

How OSPF Protocol Works & Basic Concepts: OSPF Neighbor, Topology & Routing Table, OSPF Areas & Router Roles, Theory & Overview

ospf-operation-basic-advanced-concepts-ospf-areas-roles-theory-overview-00This article covers basic OSPF concepts and operation. We explain how OSPF works, how OSPF tables are built on an OSPF-enabled router and their purpose (Neighbour Table, Topology Table, Routing Table), OSPF areas and their importance. Next we cover OSPF Link State Packet types used to exchange data between OSPF routers: Link State Advertisement (LSA), Link State Database (LSDB), Link State Request (LSR), Link State Update (LSU) and Link State Acknowledgment (LSAcK). Finally, we take a look at the OSPF roles: Area Boarder Router (ABR), Autonomous System Boundary Router (ASBR), Designated Router (DR), Backup DR and more.

 What is OSPF and How Does it Work?

OSPF is a Link State protocol that’s considered may be the most famous protocol among the Interior Gateway Protocol (IGP) family, developed in the mid 1980’s by the OSPF working group of the IETF.

When configured, OSPF will listen to neighbors and gather all link state data available to build a topology map of all available paths in its network and then save the information in its topology  database, also known as its Link-State Database (LSDB). Using the information from its topology database. From the information gathered, it will calculate the best shortest path to each reachable subnet/network using an algorithm called Shortest Path First (SFP) that was developed by the computer scientist Edsger W. Dijkstra in 1956. OSPF will then construct three tables to store the following information:

  • Neighbor Table: Contains all discovered OSPF neighbors  with whom routing information will be interchanged
  • Topology Table: Contains the entire road map of the network with all available OSPF routers and calculated best and alternative paths.
  • Routing Table: Contain the current working best paths that will be used to forward data traffic between neighbors.

Understanding OSPF Areas

OSPF offers a very distinguishable feature named: Routing Areas. It means dividing routers inside a single autonomous system running OSPF, into areas where each area consists of a group of connected routers.

The idea of dividing the OSPF network into areas is to simplify administration and optimize available resources. Resource optimization is especially important for large enterprise networks with a plethora of network and links.  Having many routers exchange the link state database could flood the network and reduce its efficiency – this was the need that led to the creation of concept Areas.

Areas are a logical collection of routers that carry the same Area ID or number inside of an OSPF network, the OSPF network itself can contain multiple areas, the first and main Area is called the backbone area “Area 0”, all other areas must connect to Area 0 as shown in the diagram below:

Figure 1. OSPF Areas, Area 0 (Backbone Area), ABR and ASBR OSPF routers

All routers within the same Area have the same topology table -Link State Database- but different routing table as OSPF calculates different best paths for each router depending on its location within the network topology while they will all share the same Link State topology.

The goal of having an Area is to localize the network as follow:

OSPF Routing Protocol

This section covers the Open Shortest Path First (OSPF) protocol. We analyse the OSPF protocol, explaining why the OSPF protocol exists, how OSPF works, OSPF concepts, OSPF Areas, OSPF router roles and much more.

Our OSPF section has been designed to cover engineers who are new to OSPF (CCNA level) and progressively covers OSPF CCNP professional level. Our articles include examples of OSPF configuration for Cisco routers, ensuring a complete coverage of the OSPF protocol.

Additional OSPF articles will be progressively added as we continuously expand the OSPF section.


Routing is one of the most important features in a network that needs to connect with other networks. In this page we try to explain the difference between Routed and Routing protocols and explain different methods used to achieve the routing of protocols.The fact is that if routing of protocols was not possible, then we wouldn't be able to comminucate using computers because there would be no way of getting the data across to the other end !


Routing is used for taking a packet (data) from one device and sending it through the network to another device on a different network. If your network has no routers then you are not routing. Routers route traffic to all the networks in your internetwork. To be able to route packets, a router must know the following :

  • Destination address
  • Neighbor routers from which it can lean about remote networks
  • Possible routes to all remote networks
  • The best route to each remote network
  • How to maintain and verify routing information

Before we go on, I would like to define 3 networking terms :

Convergence: The process required for all routers in an internetwork to update their routing tables and create a consistent view of the network, using the best possible paths. No user data is passed during convergence.

Default Route: A "standard" route entry in a routing table which is used as a first option. Any packets sent by a device will be sent first to the default route. If that fails, it will try alternative routes.

Static Route: A permanent route entered manually into a routing table. This route will remain in the table, even if the link goes down. It can only be erased manually.

Dynamic Route: A route entry which is dynamically (automatically) updated as changes to the network occur. Dynamic routes are basically the opposite to static routes.

We start off with the explanation of the IP routing process and move onto routed protocols, then tackle the routing protocols and finally the routing tables. There is plenty to read about, so grab that tea or coffee and let's start!

Routing Protocols

Distance Vector, Link State RIP, IGRP, EIGRP, OSPF

Routing protocols were created for routers. These protocols have been designed to allow the exchange of routing tables, or known networks, between routers. There are a lot of different routing protocols, each one designed for specific network sizes, so I am not going to be able to mention and analyse them all, but I will focus on the most popular.

The two main types of routing: Static routing and Dynamic routing

The router learns about remote networks from neighbor routers or from an administrator. The router then builds a routing table, the creation of which I will explain in detail, that describes how to find the remote networks. If the network is directly connected then the router already knows how to get to the network. If the networks are not attached, the router must learn how to get to the remote network with either static routing (administrator manualy enters the routes in the router's table) or dynamic routing (happens automaticlly using routing protocols).

The routers then update each other about all the networks they know. If a change occurs e.g a router goes down, the dynamic routing protocols automatically inform all routers about the change. If static routing is used, then the administrator has to update all changes into all routers and therefore no routing protocol is used.

Only Dynamic routing uses routing protocols, which enable routers to:

  • Dynamically discover and maintain routes
  • Calculate routes
  • Distribute routing updates to other routers
  • Reach agreement with other routers about the network topology

Statically programmed routers are unable to discover routes, or send routing information to other routers. They send data over routes defined by the network Administrator.

A Stub network is so called because it is a dead end in the network. There is only one route in and one route out and, because of this, they can be reached using static routing, thus saving valuable bandwidth.

Dynamic Routing Protocols

There are 3 types of Dynamic routing protocols, these differ mainly in the way that they discover and make calculations about routes:

  1. Distance Vector. Distance Vector routers compute the best path from information passed to them from neighbors
  2. Link State. Link State routers each have a copy of the entire network map and compute best routes from this local map
  3. Hybrid

The Table below shows the main characteristics of a few different types of dynamic routing protocols:


You can also clasify the routing protocols in terms of their location on a network. For example, routing protocols can exist in, or between, autonomous systems.

Exterior Gateway Protocols (EGP's) are found between autonomous systems, whereas Interior Gateway Protocols(IGP's) are found within autonomous systems:


Example of an EGPis the Border Gateway Protocol (BGP) which is also used amongst the Internet routers, whereas examples of IGP protocols are RIP, IGRP, EIGRP.

Articles To Read Next:


Cisco Routers

  • SSL WebVPN
  • Securing Routers
  • Policy Based Routing
  • Router on-a-Stick

VPN Security

  • Understand DMVPN
  • GRE/IPSec Configuration
  • Site-to-Site IPSec VPN
  • IPSec Modes

Cisco Help

  • VPN Client Windows 8
  • VPN Client Windows 7
  • CCP Display Problem
  • Cisco Support App.

Windows 2012

  • New Features
  • Licensing
  • Hyper-V / VDI
  • Install Hyper-V


  • File Permissions
  • Webmin
  • Groups - Users
  • Samba Setup