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

Summary of Module 1

Summary

This page summarizes the topics discussed in this module.

Variable-Length Subnet Masks (VLSM), often referred to as "subnetting a subnet", is used to maximize addressing efficiency. It is a feature that allows a single autonomous system to have networks with different subnet masks. The network administrator is able to use a long mask on networks with few hosts, and a short mask on subnets with many hosts.  

It is important to design an addressing scheme that allows for growth and does not involve wasting addresses. To apply VLSM to the addressing problem, large subnets are created for addressing LANs. Very small subnets are created for WAN links and other special cases.

VLSM helps to manage IP addresses. VLSM allows for the setting of a subnet mask that suits the link or the segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.

Addresses are assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits. There are specific rules for a router. It must know in detail the subnet numbers attached to it and it does not need to tell other routers about each individual subnet if the router can send an aggregate route for a set of routers. A router using aggregate routes would have fewer entries in its routing tables.

If VLSM is the scheme chosen, it must then be calculated and configured correctly.

RIP v1 is considered an interior gateway protocol that is classful. RIP v1 is a distance vector protocol that broadcasts its entire routing table to each neighbor router at predetermined intervals. The default interval is 30 seconds. RIP uses hop count as a metric, with 15 as the maximum number of hops.

To enable a dynamic routing protocol, select a routing protocol, such as RIP v2, assign the IP network numbers without specifying the subnet values, and then assign the network or subnet addresses and the appropriate subnet mask to the interfaces. In RIP v2, the router command starts the routing process. The network command causes the implementation of three functions. The routing updates are multicast out an interface, the routing updates are processed if they enter that same interface, and the subnet that is directly connected to that interface is advertised. The version 2 command enables RIP v2.

The show ip protocols command displays values about routing protocols and routing protocol timer information associated with the router. Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging.

Default routes

Default routes
1.2.7 This page will describe default routes and explain how they are configured.
By default, routers learn paths to destinations three different ways:

• Static routes – The system administrator manually defines the static routes as the next hop to a destination. Static routes are useful for security and traffic reduction, as no other route is known.
• Default routes – The system administrator also manually defines default routes as the path to take when there is no known route to the destination. Default routes keep routing tables shorter. When an entry for a destination network does not exist in a routing table, the packet is sent to the default network.
• Dynamic routes – Dynamic routing means that the router learns of paths to destinations by receiving periodic updates from other routers.

In Figure , the static route is indicated by the following command:

Router(config)#ip route 172.16.1.0 255.255.255.0 17.16.2.1

The ip default-network command establishes a default route in networks using dynamic routing protocols: 

Router(config)#ip default-network 192.168.20.0

Generally after the routing table has been set to handle all the networks that must be configured, it is often useful to ensure that all other packets go to a specific location. This is called the default route for the router. One example is a router that connects to the Internet. All the packets that are not defined in the routing table will go to the nominated interface of the default router.
The ip default-network command is usually configured on the routers that connect to a router with a static default route. 
In Figure , Hong Kong 2 and Hong Kong 3 would use Hong Kong 4 as the default gateway. Hong Kong 4 would use interface 192.168.19.2 as its default gateway. Hong Kong 1 would route packets to the Internet for all internal hosts. To allow Hong Kong 1 to route these packets it is necessary to configure a default route as:

HongKong1(config)#ip route 0.0.0.0 0.0.0.0 s0/0

The zeros in the IP address and mask portions of the command represent any destination network with any mask. Default routes are referred to as quad zero routes. In the diagram, the only way Hong Kong 1 can go to the Internet is through interface s0/0.
This page concludes this lesson. The next page will summarize the main points from this module.

Troubleshooting RIP v2

Troubleshooting RIP v2
1.2.6 This page explains the use of the debug ip rip command.
Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging.
The example shows that the router being debugged has received updates from one router at source address 10.1.1.2. The router at source address 10.1.1.2 sent information about two destinations in the routing table update. The router being debugged also sent updates, in both cases to the multicast address 224.0.0.9 as the destination. The number in parentheses is the source address encapsulated into the IP header.
Other outputs sometimes seen from the debug ip rip command includes entries such as the following:

RIP: broadcasting general request on Ethernet0

RIP: broadcasting general request on Ethernet1

These outputs appear at startup or when an event occurs such as an interface transition or a user manually clears the routing table.
An entry, such as the following, is most likely caused by a malformed packet from the transmitter:

RIP: bad version 128 from 160.89.80.43

Examples of debug ip rip outputs and meanings are shown in Figure .
The Lab Activities will help students become more familiar with debug commands.
The next page will discuss default routes.