Compare and contrast distance vector and link-state routing

Compare and contrast distance vector and link-state routing
2.1.6 This page will compare distance vector and link-state routing protocols.

All distance vector protocols learn routes and then send these routes to directly connected neighbors. However, link-state routers advertise the states of their links to all other routers in the area so that each router can build a complete link-state database. These advertisements are called link-state advertisements or LSAs. Unlike distance vector routers, link-state routers can form special relationships with their neighbors and other link-state routers. This is to ensure that the LSA information is properly and efficiently exchanged.

The initial flood of LSAs provides routers with the information that they need to build a link-state database. Routing updates occur only when the network changes. If there are no changes, the routing updates occur after a specific interval. If the network changes, a partial update is sent immediately. The partial update only contains information about links that have changed. Network administrators concerned about WAN link utilization will find these partial and infrequent updates an efficient alternative to distance vector routing protocols, which send out a complete routing table every 30 seconds. When a change occurs, link-state routers are all notified simultaneously by the partial update. Distance vector routers wait for neighbors to note the change, implement the change, and then pass the update to the neighbor routers. 

The benefits of link-state over distance vector protocols include faster convergence and improved bandwidth utilization. Link-state protocols support CIDR and VLSM. This makes them a good choice for complex and scalable networks. In fact, link-state protocols generally outperform distance vector protocols on any size network. Link-state protocols are not implemented on every network because they require more memory and processor power than distance vector protocols and can overwhelm slower equipment. Another reason they are not more widely implemented is the fact that link-state protocols are quite complex. Link-state routing protocols require well-trained administrators to correctly configure and maintain them.

This page concludes this lesson. The next lesson will introduce a link-state routing protocol called OSPF. The first page will provide an overview. 

Advantages and disadvantages of link-state routing

Advantages and disadvantages of link-state routing
2.1.5 This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols: 

• Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths.
• Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times.
• Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur.
• Routers use the latest information to make the best routing decisions.
• The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence.
• Every router, at the very least, maps the topology of its own area of the network. This attribute helps to troubleshoot problems that can occur.
• Link-state protocols support CIDR and VLSM.

The following are some disadvantages of link-state routing protocols: 

• They require more memory and processor power than distance vector protocols. This makes it expensive to use for organizations with small budgets and legacy hardware.
• They require strict hierarchical network design, so that a network can be broken into smaller areas to reduce the size of the topology tables.
• They require an administrator who understands the protocols well.
• They flood the network with LSAs during the initial discovery process. This process can significantly decrease the capability of the network to transport data. It can noticeably degrade the network performance.

The next page will continue the comparison of link-state and distance vector protocols.e

Link-state routing algorithms

Link-state routing algorithms
2.1.4 Link-state routing algorithms maintain a complex database of the network topology by exchanging link-state advertisements (LSAs) with other routers in a network. This page describes the link-state routing algorithm.
Link-state routing algorithms have the following characteristics:

• They are known collectively as SPF protocols.
• They maintain a complex database of the network topology.
• They are based on the Dijkstra algorithm.

Link-state protocols develop and maintain full knowledge of the network routers and how they interconnect. This is achieved through the exchange of LSAs with other routers in the network.
Each router constructs a topological database from the LSAs that it receives. The SPF algorithm is then used to compute the reachability of destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growth.
An LSA exchange is triggered by an event in the network instead of periodic updates. This speeds up the convergence process because there is no need to wait for a series of timers to expire before the routers can converge. If the network shown in Figure uses a link-state routing protocol, there is no concern about connectivity between routers A and D. Based on the protocol that is employed and the metrics that are selected, the routing protocol can discriminate between two paths to the same destination and use the best one. In Figure there are two routing entries in the table for the route from Router A to Router D. In this figure, the routes have equal costs so the link-state routing protocol records both routes. Some link-state protocols provide a way to assess the performance capabilities of the two routes and choose the best one. If the preferred route through Router C experiences operational difficulties such as congestion or component failure, the link-state routing protocol can detect this change and route packets through Router B.
The next page will describe some advantages of link-state protocols.

How routing information is maintained

How routing information is maintained

2.1.3 This page will explain how link-state protocols use the following features:

• The LSAs
• A topological database
• The SPF algorithm
• The SPF tree
• A routing table of paths and ports to determine the best path for packets 

Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols. For example, distance vector protocols only exchange routing updates with immediate neighbors while link-state routing protocols exchange routing information across a much larger area.
When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs with a special multicast address throughout an area. This process sends information out all ports, except the port on which the information was received. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router then forwards the LSA to all neighbor devices. LSAs cause every router within the area to recalculate routes. For this reason, the number of link-state routers within an area should be limited.
A link is the same as an interface on a router. The state of the link is a description of an interface and the relationship to the neighbor routers. For example, a description of the interface would include the IP address of the interface, the subnet mask, the type of network that it is connected to, the routers connected to that network, and so on. The collection of link-states form a link-state database which is sometimes called a topological database. The link-state database is used to calculate the best paths through the network. Link-state routers apply the Dijkstra shortest path first algorithm against the link-state database. This builds the SPF tree with the local router as the root. The best paths are then selected from the SPF tree and placed in the routing table.
The next page will discuss the link-state routing algorithm.

Link-state routing protocol features

Link-state routing protocol features
2.1.1 This page will explain how link-state protocols route data.
Link-state routing protocols collect route information from all other routers in the network or within a defined area of the network. Once all of the information is collected, each router calculates the best paths to all destinations in the network. Since each router maintains its own view of the network, it is less likely to propagate incorrect information provided by any of its neighboring routers.
The following are some link-state routing protocol functions:

• Respond quickly to network changes
• Send triggered updates only when a network change has occurred
• Send periodic updates known as link-state refreshes
• Use a hello mechanism to determine the reachability of neighbors 

Each router multicasts hello packets to keep track of the state of the neighbor routers. Each router uses LSAs to keep track of all the routers in its area of the network. The hello packets contain information about the networks that are attached to the router. In Figure, P4 knows about its neighbors, P1 and P3, on the Perth3 network. The LSAs provide updates on the state of links that are interfaces on other routers in the network.
Routers that use link-state routing protocols have the following features:

• Use the hello information and LSAs received from other routers to build a database about the network
• Use the SPF algorithm to calculate the shortest route to each network
• Store the route information in the routing table

The next page will provide more information about link-state protocols.

Link-State Routing Protocol / Overview of link-state routing

Link-State Routing Protocol
Overview of link-state routing

2.1.1 Link-state routing protocols perform differently than distance vector protocols. This page will explain the differences between distance vector and link-state protocols. This information is vital for network administrators. One essential difference is that distance vector protocols use a simpler method to exchange route information. Ooutlines the characteristics of both distance vector and link-state routing protocols.
Link-state routing algorithms maintain a complex database of topology information. While the distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers, a link-state routing algorithm maintains full knowledge of distant routers and how they interconnect.
The Interactive Media Activity will help students identify the different features of link-state and distance vector protocols.
The next page will describe link-state routing protocols.

Module 2: Single-Area OSPF (Overview)

Overview

The two main classes of IGPs are distance vector and link-state. Both types of routing protocols find routes through autonomous systems. Distance vector and link-state routing protocols use different methods to accomplish the same tasks.
Link-state routing algorithms, also known as shortest path first (SPF) algorithms, maintain a complex database of topology information. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. In contrast, distance vector algorithms provide nonspecific information about distant networks and no knowledge of distant routers.
It is important to understand how link-state routing protocols operate in order to configure, verify, and troubleshoot them. This module explains how link-state routing protocols work, outlines their features, describes the algorithm they use, and points out the advantages and disadvantages of link-state routing.
Early routing protocols such as RIP v1 were all distance vector protocols. There are many distance vector routing protocols in use today such as RIP v2, IGRP, and the hybrid routing protocol EIGRP. As networks have grown larger and more complex, the limitations of distance vector routing protocols have become apparent. Routers that use a distance vector routing protocol learn about the network topology from the routing table updates of neighbor routers. Bandwidth usage is high because of the periodic exchange of routing updates, and network convergence is slow which results in poor routing decisions.
Link-state routing protocols differ from distance vector protocols. Link-state protocols flood route information, which allows every router to have a complete view of the network topology. Triggered updates allow efficient use of bandwidth and faster convergence. Changes in the state of a link are sent to all routers in the network as soon as the change occurs.
OSPF is one of the most important link-state protocols. OSPF is based on open standards, which means it can be developed and improved by multiple vendors. It is a complex protocol that is a challenge to implement in a large network. The basics of OSPF are covered in this module.
OSPF configuration on a Cisco router is similar to the configuration of other routing protocols. Similarly, OSPF must be enabled on a router and the networks that will be advertised by OSPF must be identified. OSPF has a number of features and configuration procedures that are unique. These features make OSPF a powerful choice for a routing protocol, but also make it a challenge to configure.
In large networks, OSPF can be configured to span many areas and several different area types. The ability to design and implement large OSPF networks begins with the ability to configure OSPF in a single area. This module also discusses the configuration of single-area OSPF.
This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams. 
Students who complete this module should be able to perform the following tasks: 

• Identify key link-state routing protocol features
• Explain how link-state routing information is maintained
• Discuss the link-state routing algorithm
• Examine the advantages and disadvantages of link-state routing protocols
• Compare and contrast link-state routing protocols with distance vector routing protocols
• Enable OSPF on a router
• Configure a loopback address to set router priority
• Modify the cost metric to change OSPF route preference
• Configure OSPF authentication
• Change OSPF timers
• Describe the steps to create and propagate a default route
• Use show commands to verify OSPF operation
• Configure the OSPF routing process
• Define key OSPF terms
• Describe the OSPF network types
• Describe the OSPF Hello protocol

Identify the basics steps in the operation of OSPF