OSPF Hello protocol

OSPF Hello protocol
2.2.6 This page will introduce hello packets and the Hello protocol.
When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol.
At Layer 3 of the OSI model, the hello packets are addressed to the multicast address 224.0.0.5. This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds.
On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR).
Although the hello packet is small, it consists of the OSPF packet header. For the hello packet the type field is set to 1.
The hello packet carries information that all neighbors must agree upon before an adjacency is formed, and link-state information is exchanged.
The Interactive Media Activity will help students identify the fields in an OSPF packet header.
The next page will describe the OSPF routing process.

OSPF network types

OSPF network types
2.2.5 This page will introduce the three types of OSPF networks.
A neighbor relationship is required for OSPF routers to share routing information. A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is connected. OSPF routers determine which routers to become adjacent to based on the type of network they are connected to. Some routers may try to become adjacent to all neighbor routers. Other routers may try to become adjacent to only one or two neighbor routers. Once an adjacency is formed between neighbors, link-state information is exchanged.
OSPF interfaces automatically recognize three types of networks:

• Broadcast multi-access, such as Ethernet
• Point-to-point networks
• Nonbroadcast multi-access (NBMA), such as Frame Relay

A fourth type, point-to-multipoint, can be manually configured on an interface by an administrator. 
In a multi-access network, it is not known in advance how many routers will be connected. In point-to-point networks, only two routers can be connected.
In a broadcast multi-access network segment, many routers may be connected. If every router had to establish full adjacency with every other router and exchange link-state information with every neighbor, there would be too much overhead. If there are 5 routers, 10 adjacency relationships would be needed and 10 link-states sent. If there are 10 routers then 45 adjacencies would be needed. In general, for n routers, n*(n-1)/2 adjacencies would need to be formed.
The solution to this overhead is to hold an election for a designated router (DR). This router becomes adjacent to all other routers in the broadcast segment. All other routers on the segment send their link-state information to the DR. The DR in turn acts as the spokesperson for the segment. The DR sends link-state information to all other routers on the segment using the multicast address of 224.0.0.5 for all OSPF routers.
Despite the gain in efficiency that electing a DR provides, there is a disadvantage. The DR represents a single point of failure. A second router is elected as a backup designated router (BDR) to take over the duties of the DR if it should fail. To ensure that both the DR and the BDR see the link-states all routers send on the segment, the multicast address for all designated routers, 224.0.0.6, is used.
On point-to-point networks only two nodes exist and no DR or BDR is elected. Both routers become fully adjacent with each other.
The Interactive Media Activity will help students recognize the three types of OSPF networks.
The next page will describe the OSPF Hello protocol.

Shortest path algorithm

Shortest path algorithm
2.2.4 This page will explain how OSPF uses the shortest-path algorithm to determine the best path to a destination.
In this algorithm, the best path is the lowest cost path. Edsger Wybe Dijkstra, a Dutch computer scientist, formulated the shortest path-algorithm, also known as Dijkstra's algorithm. The algorithm considers a network to be a set of nodes connected by point-to-point links. Each link has a cost. Each node has a name. Each node has a complete database of all the links and so complete information about the physical topology is known. All router link-state databases, within a given area, are identical. The table in Figure shows the information that node D has received. For example, D received information that it was connected to node C with a link cost of 4 and to node E with a link cost of 1.
The shortest path algorithm then calculates a loop-free topology using the node as the starting point and examining in turn information it has about adjacent nodes. In Figure , node B has calculated the best path to D. The best path to D is by way of node E, which has a cost of 4. This information is converted to a route entry in B which will forward traffic to C. Packets to D from B will flow B to C to E, then to D in this OSPF network.
In the example, node B determined that to get to node F the shortest path has a cost of 5, through node C. All other possible topologies will either have loops or a higher cost paths.
The next page will explain the concept of OSPF networks.

Comparing OSPF with distance vector routing protocols

Comparing OSPF with distance vector routing protocols
2.2.3 This page will explain how OSPF compares to distance vector protocols such as RIP. Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or network changes. Link-state routers do not broadcast routing tables periodically as distance vector protocols do. Therefore, link-state routers use less bandwidth for routing table maintenance.
RIP is appropriate for small networks, and the best path is based on the lowest number of hops. OSPF is appropriate for large, scalable internetworks, and the best path is determined by the speed of the link. RIP and other distance vector protocols use simple algorithms to compute best paths. The SPF algorithm is complex. Routers that implement distance vector protocols need less memory and less powerful processors than those that implement OSPF.
OSPF selects routes based on cost, which is related to speed. The higher the speed, the lower the OSPF cost of the link.
OSPF selects the fastest loop-free path from the SPF tree as the best path in the network.
OSPF guarantees loop-free routing. Distance vector protocols may cause routing loops.
If links are unstable, flooding of link-state information can lead to unsynchronized link-state advertisements and inconsistent decisions among routers.
OSPF addresses the following issues:

• Speed of convergence
• Support for Variable Length Subnet Mask (VLSM)
• Network size
• Path selection
• Grouping of members

In large networks RIP convergence can take several minutes since the routing table of each router is copied and shared with directly connected routers. After initial OSPF convergence, maintaining a converged state is faster because only the changes in the network are flooded to other routers in an area.
OSPF supports VLSMs and therefore is referred to as a classless protocol. RIP v1 does not support VLSMs, however, RIP v2 does support VLSMs.
RIP considers a network that is more than 15 routers away to be unreachable because the number of hops is limited to 15. This limits RIP to small topologies. OSPF has no size limits and is suitable for intermediate to large networks.
RIP selects a path to a network by adding one to the hop count reported by a neighbor. It compares the hop counts to a destination and selects the path with the smallest distance or hops. This algorithm is simple and does not require a powerful router or a lot of memory. RIP does not take into account the available bandwidth in best path determination.
OSPF selects a path using cost, a metric based on bandwidth. All OSPF routers must obtain complete information about the networks of every router to calculate the shortest path. This is a complex algorithm. Therefore, OSPF requires more powerful routers and more memory than RIP.
RIP uses a flat topology. Routers in a RIP region exchange information with all routers. OSPF uses the concept of areas. A network can be subdivided into groups of routers. In this way OSPF can limit traffic to these areas. Changes in one area do not affect performance in other areas. This hierarchical approach allows a network to scale efficiently.
The Interactive Media Activity will help students learn the differences between link-state and distance vector protocols.
The next page will discuss the shortest path algorithm

OSPF terminology

OSPF terminology
2.2.2 There are many words and concepts for students in this TI and the figures should help to explain them. Use the interactive media activity to reinforce the terms and their abbreviations. Instructors might like to hold an acronym competition to see who can explain the words and concepts in the following table:

Link	A link is a physical and electrical connection between two network devices.
Link-state (LS)	Link-state is the status of a link between two routers. This status includes information about a router interface and its relationship to neighboring routers.
Cost	Cost is the value assigned to a link. Link-state protocols assign a cost to a link, which is based on the speed of the network connection.
Area	An area is a collection of networks and routers that has the same area identification. Each router within an area has the same link-state information. A router within an area is called an internal router.
Designated Router (DR)	A DR is one router on an OSPF multi-access network that represents all the routers in that network. Each OSPF network has a DR and BDR. These routers have special responsibilities that are discussed later in this module.
Backup Designated Router (BDR)	A BDR is a standby router that becomes the DR, if the original DR fails.
Adjacencies database (AD)	An AD is a listing of all the neighbors to which a router has established bi-directional communication.
Link-state database (LSD) or topological database	An LSD is a list of information about all other routers in the network. It shows the network topology. All routers within an area have identical link-state databases.
Routing table	The routing table, also known as the forwarding database, is generated when an algorithm is run on the link-state database. Each routing table is unique and contains information of how and where to send packets to other routers.
SPF algorithm	An SPF algorithm is a routing algorithm that iterates on length of path to determine a shortest-path spanning tree.
Link-state advertisement (LSA)	An LSA is a broadcast packet used by link-state protocols that contain information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.

Link-state routers identify neighboring routers and then communicate with the identified neighbors. OSPF has its own terminology. The new terms are shown in Figure .
OSPF gathers information from neighbor routers about the link status of each OSPF router. This information is flooded to all its neighbors. An OSPF router advertises its own link-states and passes on received link-states.
The routers process the information about link-states and build a link-state database. Every router in the OSPF area will have the same link-state database. Therefore, every router has the same information about the state of the links and the neighbors of every other router.
Each router then applies the SPF algorithm on its own copy of the database. This calculation determines the best route to a destination. The SPF algorithm adds up the cost, which is a value that is usually based on bandwidth. The lowest cost path is added to the routing table, which is also known as the forwarding database.
Each router keeps a list of adjacent neighbors, called the adjacency database. The adjacency database is a list of all the neighbor routers to which a router has established bidirectional communication. This is unique to each router.
To reduce the number of exchanges of routing information among several neighbors on the same network, OSPF routers elect a designated router (DR) and a backup designated router (BDR) that serve as focal points for routing information exchange.
The Interactive Media Activity will teach students about OSPF terminology.
The next page will compare OSPF to distance vector protocols.

Single-Area OSPF Concepts /

Single-Area OSPF Concepts
OSPF overview
2.2.1 This page will introduce OSPF. OSPF is a link-state routing protocol that is based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF). The Open in OSPF means that it is open to the public and is non-proprietary.

OSPF, when compared to RIP v1 and v2, is the preferred IGP because it is scalable. RIP is limited to 15 hops, it converges slowly, and it sometimes chooses slow routes because it ignores critical factors such as bandwidth in route determination. A drawback to using OSPF is that it only supports the TCP/IP protocol suite. OSPF has overcome these limitations and is a robust and scalable routing protocol that is suitable for modern networks. OSPF can be used and configured as a single area for small networks. It can also be used for large networks.
As shown in Figure , large OSPF networks use a hierarchical design. Multiple areas connect to a distribution area, or area 0 which is also called the backbone. The design approach allows for extensive control of routing updates. Area definition reduces routing overhead, speeds up convergence, confines network instability to an area, and improves performance.
The next page will provide more information about OSPF.
Certification-level claim: Configure routing protocols given user requirements.
Course-level claim: Describe, configure, verify, analyze, and troubleshoot the OSPF link-state routing protocol in a single area mode of operation.
Hands-on skills: none
This is a core TI.
This is an important overview of OSPF and links back to what the students already know about RIP. Ensure that the figures are discussed, especially Figures, which are animated when students press the white arrow. Remember to stress that OSPF uses areas to implement hierarchical routing as illustrated in Figure .
The following are points to emphasize when contrasting OSPF with RIP:

• OSPF only floods changes to other routers instead of the entire routing table.
• OSPF supports VLSM.
• OSPF overcomes the hop count limit of RIP.
• OSPF is event driven, whereas RIP broadcasts every 30 seconds.
• RIP sometimes picks suboptimal paths, in terms of hops rather than bandwidth.

OSPF isolates changes to areas, while changes to a RIP topology affect every router. 

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

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.