Distance Vector Routing Protocols

Chapter 07 (Distance Vector Routing Protocols)

Overview
Dynamic routing makes it possible to avoid the configuration of static routes. Dynamic routing makes it possible to avoid the time-consuming and exacting process of configuring static routes. Dynamic routing also makes it possible for routers to react to changes in the network and to adjust their routing tables accordingly, without the intervention of the network administrator. However, dynamic routing can cause problems. Some of the problems associated with dynamic distance vector routing protocols are discussed in this module, along with some of the steps that designers of the protocols have taken to solve the problems.
RIP is a distance vector routing protocol that is used in thousands of networks throughout the world. The fact that RIP is based on open standards and is easy to implement makes it attractive to some network administrators. However, RIP lacks the power and features of more advanced routing protocols. Because of its simplicity, RIP is a good basic protocol for networking students. This module will also introduce RIP configuration and troubleshooting.
IGRP is another distance vector routing protocol. Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol. IGRP is also very simple to implement. However, IGRP is a more complex routing protocol than RIP and can use many factors to determine the best route to a destination network. This module will introduce IGRP configuration and troubleshooting.


This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams. -


Students who complete this module should be able to perform the following tasks:
• Describe how routing loops can occur in distance vector routing
• Describe several methods used by distance vector routing protocols to ensure that routing information is accurate
• Configure RIP
• Use the ip classless command
• Troubleshoot RIP
• Configure RIP for load balancing
• Configure static routes for RIP
• Verify RIP
• Configure IGRP
• Verify IGRP operation
• Troubleshoot IGRP









7.1 Distance Vector Routing
7.1.1 Distance vector routing updates
This page will explain how distance vector routing updates occur.
Routing table updates occur periodically or when the topology in a distance vector protocol network changes. It is important for a routing protocol to update the routing tables efficiently. As with the network discovery process, topology change updates proceed systematically from router to router. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost. The path cost is defined by the metrics and the logical address of the first router on the path to each network in the table.
The next page will explain how routing loops occur.









7.1.2 Distance vector routing loop issues
This page will help students understand routing loops.
Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
An example is as follows:
1. Just before the failure of Network 1, all routers have consistent knowledge and correct routing tables. The network is said to have converged. For Router C, the preferred path to Network 1 is by way of Router B, and the distance from Router C to Network 1 is 3.
2. When Network 1 fails, Router E sends an update to Router A. Router A stops routing packets to Network 1, but Routers B, C, and D continue to do so because they have not yet been informed of the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. However, Router C has not received an update. For Router C, Network 1 can still be reached through Router B.
3. Now Router C sends a periodic update to Router D, which indicates a path to Network 1 by way of Router B. Router D changes its routing table to reflect this incorrect information, and sends the information to Router A. Router A sends the information to Routers B and E, and the process continues. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.



The next page explains how a maximum count can be used to prevent routing loops.
7.1.3 Defining a maximum count
This page will teach students how to define a maximum count to prevent routing loops.



The invalid updates of Network 1 will continue to loop until some other process stops the looping. This condition, which is called count to infinity, loops packets around the network in spite of the fact that the destination network, which is Network 1, is down. While the routers count to infinity, the invalid information allows a routing loop to exist.
Without countermeasures to stop the count to infinity process, the distance vector metric of hop count increases each time the packet passes through another router. These packets loop through the network because of incorrect information in the routing tables.
Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. This number refers to a routing metric, which may simply be the hop count.



With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. The graphic shows the metric value as 16 hops. This exceeds the distance vector default maximum of 15 hops so the packet is discarded by the router. When the metric value exceeds the maximum value, Network 1 is considered unreachable.
The next page will discuss another solution for routing loops.
7.1.4 Elimination routing loops through split-horizon
This page will explain how split horizon can be used to avoid routing loops.
Some routing loops occur when incorrect information that is sent back to a router contradicts the correct information that the router originally distributed. An example is as follows:
1. Router A passes an update to Router B and Router D, which indicates that Network 1 is down. However, Router C transmits an update to Router B, which indicates that Network 1 is available at a distance of 4, by way of Router D. This does not violate split horizon rules.
2. Router B concludes, incorrectly, that Router C still has a valid path to Network 1, although at a much less favorable metric. Router B sends an update to Router A, which informs Router A of the new route to Network 1.
3. Router A now determines that it can send to Network 1 by way of Router B. Router B determines that it can send to Network 1 by way of Router C. Router C determines that it can send to Network 1 by way of Router D. Any packet introduced into this environment will loop between routers.
4. Split horizon is used to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split horizon reduces incorrect routing information and routing overhead.









The next page will introduce the concept of route poisoning.
7.1.5 Route poisoning
This page will explain what route poisoning is and why it is used.
Route poisoning is used by various distance vector protocols to overcome large routing loops and offer detailed information when a subnet or network is not accessible. To accomplish this, the hop count is usually set to one more than the maximum.
One way to avoid inconsistent updates is route poisoning. When Network 5 goes down, Router E will set a distance of 16 for Network 5 to poison the route. This indicates that the network is unreachable. When the route is poisoned, Router C is not affected by incorrect updates about the route to Network 5. After Router C receives a route poisoning from Router E, it sends an update, which is called a poison reverse, back to Router E. This makes sure all routers on the segment have received the poisoned route information.



When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before they advertise the poisoned route.
Route poisoning causes a routing protocol to advertise infinite-metric routes for a failed route. Route poisoning does not break split horizon rules. Split horizon with poison reverse is route poisoning that is placed on links that split horizon would not normally allow routing information to flow across. In either case, the result is that failed routes are advertised with infinite metrics.
The next page will discuss how triggered updates can prevent routing loops.
7.1.6 Avoiding routing loops with triggered updates
New routing tables are sent to neighbor routers on a regular basis. For example, RIP updates occur every 30 seconds. However a triggered update is sent immediately in response to some change in the routing table. The router that detects a topology change immediately sends an update message to adjacent routers. These routers generate triggered updates to notify their adjacent neighbors of the change. When a route fails, an update is sent immediately. Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire.
This page will explain how triggered updates can be used to prevent routing loops.






Triggered updates do not wait for update timers to expire. They are sent when routing information has changed. A router will immediately send a routing update on its other interfaces. This forwards the information about the route that has changed and starts the holddown timers sooner on the neighbor routers. The wave of updates propagates throughout the network.
Router C issues a triggered update, which announces that network 10.4.0.0 is unreachable. Upon receipt of this information, Router B announces through interface S0/1 that network 10.4.0.0 is down. In turn, Router A sends an update out interface Fa0/0.
The next page will explain how holddown timers can be used to prevent routing loops.
7.1.7 Preventing routing loops with holddown timers



This page will explain how holddown timers can be used to avoid a count to infinity problem:
• When a router receives an update from a neighbor, which indicates that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a holddown timer. Before the holddown timer expires, if an update is received from the same neighbor, which indicates that the network is accessible, the router marks the network as accessible and removes the holddown timer.
• If an update arrives from a different neighbor router with a better metric for the network, the router marks the network as accessible and removes the holddown timer.
• If an update is received from a different router with a higher metric before the holddown timer expires, the update is ignored. This update is ignored to allow more time for the knowledge of a disruptive change to propagate through the entire network.
This page concludes this lesson. The next lesson will define RIP. The first page introduces the RIP routing process.



7.2 RIP
7.2.1 RIP routing process
This page will provide an overview of the RIP routing process.
The modern open standard version of RIP, which is sometimes referred to as IP RIP, is formally detailed in two separate documents. The first is known as Request for Comments (RFC) 1058 and the other as Internet Standard (STD) 56.


RIP has evolved over the years from a Classful Routing Protocol, RIP Version 1 (RIP v1), to a Classless Routing Protocol, RIP Version 2 (RIP v2). RIP v2 enhancements include the following:
• Ability to carry additional packet routing information
• Authentication mechanism to secure table updates
• Support for variable-length subnet mask (VLSM)
To prevent indefinite routing loops, RIP implements a limit on the number of hops allowed in a path from a source to a destination. The maximum number of hops in a path is 15. When a router receives a routing update that contains a new or changed entry, the metric value is increased by 1 to account for itself as a hop in the path. If this causes the metric to be higher than 15, the network destination is considered unreachable. RIP includes a number of features that are common in other routing protocols. For example, RIP implements split horizon and holddown mechanisms to prevent the propagation of incorrect routing information.
The next page will teach students how to configure RIP.
7.2.2 Configuring RIP
This page will explain how RIP is configured.
The router rip command enables RIP as the routing protocol. The network command is then used to tell the router on which interfaces to run RIP. The routing process associates specific interfaces with the network addresses and begins to send and receive RIP updates on these interfaces.
RIP sends routing-update messages at regular intervals. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The received metric value for the path is increased by 1, and the source interface of the update is indicated as the next hop in the routing table. RIP routers maintain only the best route to a destination but can maintain multiple equal-cost paths to the destination. Most routing protocols use a combination of time-driven and event-driven updates. RIP is time-driven, but the Cisco implementation of RIP sends triggered updates whenever a change is detected. Topology changes also trigger immediate updates in IGRP routers, regardless of the update timer. Without triggered updates, RIP and IGRP will not perform. After updating its routing table due to a configuration change, the router immediately begins transmitting routing updates in order to inform other network routers of the change. These updates, called triggered updates, are sent independently of the regularly scheduled updates that RIP routers forward. The descriptions for the commands used to configure router BHM shown in the figure are as follows:
• BHM(config)#router rip – Selects RIP as the routing protocol
• BHM(config-router)#network 10.0.0.0 – Specifies a directly connected network
• BHM(config-router)#network 192.168.13.0 – Specifies a directly connected network
The Cisco router interfaces that are connected to networks 10.0.0.0 and 192.168.13.0 send and receive RIP updates. These routing updates allow the router to learn the network topology from a directly connected router that also runs RIP.
RIP must be enabled and the networks must be specified. All other tasks are optional. These optional tasks include the following:
• Apply offsets to routing metrics
• Adjust timers
• Specify a RIP version
• Enable RIP authentication
• Configure route summarization on an interface
• Verify IP route summarization
• Disable automatic route summarization
• Run IGRP and RIP concurrently
• Disable the validation of source IP addresses
• Enable or disable split horizon
• Connect RIP to a WAN


To enable RIP, use the following commands in global configuration mode:
• Router(config)#router rip – Enables the RIP routing process
• Router(config-router)#networknetwork-number – Associates a network with the RIP routing process
The Lab Activities will let students examine a router topology and configure RIP.
The next page will introduce the ip classless command.
7.2.3 Using the ip classless command
This page will explain what the ip classless command is and how it is used.
Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets. Use the ip classless global configuration command to instruct the Cisco IOS software to forward these packets to the best supernet route. A supernet route is a route that covers a greater range of subnets with a single entry. For example, if an enterprise uses the entire subnet 10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16. The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this feature, use the no form of this command.
When this feature is disabled any packets received that are destined for a subnet that falls within the subnetwork addressing scheme of the router will be discarded.
IP classless only affects the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built. This is the essence of classful routing. If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.


The most confusing aspect of this rule is that the router only uses the default route if the major network destination does not exist in the routing table. A router by default assumes that all subnets of a directly connected network should be present in the routing table. If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist. So the router will drop the packet even if there is a default route. To resolve this problem, configure ip classless on the router. This allows the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route. -





The Lab Activity will help students become more familiar with the ip classless command.
The next page describes some methods that are used to reduce routing loops.
7.2.4 Common RIP configuration issues
This page will provide some more information about the methods that are used to reduce routing loops.
RIP routers must rely on neighbor routers for some types of network information. A common term used to describe this functionality is Routing by Rumor. RIP uses a distance vector routing algorithm. All distance vector routing protocols have issues that are primarily created by slow convergence. Convergence is when all routers in a network have the same routing information.
Among these issues are routing loops and counting to infinity. These result in inconsistencies due to update messages with incorrect routes that are propagated around the network.
To reduce routing loops and counting to infinity, RIP uses the following techniques:
• Split horizon
• Poison reverse
• Holddown counters
• Triggered updates
Some of these methods may need to be configured.
RIP permits a maximum hop count of 15. Any destination greater that 15 hops away is tagged as unreachable. This maximum hop count greatly restricts the use of RIP in large internetworks but prevents counts to infinity and endless network routing loops.
The split horizon rule is based on the theory that it is not useful to send information about a route back in the direction from which it came. In some network configurations, it may be necessary to disable split horizon.
The holddown timer is another mechanism that may need to be configured. Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. This will prevent any inferior route from being updated but may also prevent a valid alternative route from being installed. The holddown timer can be decreased to speed up convergence but should be done with caution. Ideally, the timer should be set just longer than the longest possible update time for the internetwork. In the example in Figure , the loop consists of four routers. If each router has an update time of 30 seconds, the longest loop would be 120 seconds. Therefore, the holddown timer should be set to slightly more than 120 seconds.
The following command is used to disable split horizon:
GAD(config-if)#no ip split-horizon


Use the following command to change the holddown timer as well as the update, invalid, and flush timers:
Router(config-router)#timers basicupdate invalid holddown flush [sleeptime ]























Another configurable item that affects convergence time is the update interval. The default RIP update interval in Cisco IOS is 30 seconds. This can be configured for longer intervals to conserve bandwidth, or for shorter intervals to decrease convergence time.
Another issue with routing protocols is the unwanted advertisement of routing updates out a particular interface. When a network command is issued for a given network, RIP will immediately begin to send advertisements out all interfaces within the specified network address range. A network administrator can use the passive-interface command to disable routing updates on specified interfaces.


Because RIP is a broadcast protocol, the network administrator may have to configure RIP to exchange routing information in a non-broadcast network such as Frame Relay. In this type of network, RIP must be informed of neighbor RIP routers. To do this use the neighbor command displayed in Figure .


By default, the Cisco IOS software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets. The network administrator can configure the router to only receive and send Version 1 packets or the administrator can configure the router to send only Version 2 packets. To configure the router to send and receive packets from only one version, use the commands in Figure .


To control how packets received from an interface are processed, use the commands in Figure
The next page will show students how to verify a RIP configuration.







7.2.5 Verifying RIP configuration
This page will describe several commands that can be used to verify that RIP is properly configured. Two of the most common are the show ip route command and the show ip protocols command.






The show ip protocols command shows which routing protocols carry IP traffic on the router. This output can be used to verify most if not all of the RIP configuration. Some of the most common configuration items to verify are as follows:
• RIP routing is configured.
• The correct interfaces send and receive RIP updates.
• The router advertises the correct networks.
The show ip route command can be used to verify that routes received by RIP neighbors are installed in the routing table. Examine the output of the command and look for RIP routes signified by "R". Remember that the network will take some time to converge so the routes may not appear immediately.







Additional commands to check RIP configuration are as follows:
• show interfaceinterface
• show ip interfaceinterface
• show running-config
The Lab Activity on this page will teach students how to use the show commands to verify the configuration of a router.
The next page discusses some commands that can be used to troubleshoot RIP.
7.2.6 Troubleshooting RIP update issues
This page will teach students how to troubleshoot RIP update issues.
Most of the RIP configuration errors involve an incorrect network statement, discontiguous subnets, or split horizons. An effective command that is used to find RIP update issues is the debug ip rip command.
The debug ip rip command displays RIP routing updates as they are sent and received. The example in Figure shows the output from the debug ip rip command after a router receives a RIP update. After the router receives and processes the update, it sends the updated information out its two RIP interfaces. The output shows the router uses RIP v1 and broadcasts the update with the broadcast address 255.255.255.255. The number in parenthesis represents the source address encapsulated into the IP header of the RIP update.
There are several key indicators to look for in the output of the debug ip rip command. Problems such as discontiguous subnetworks or duplicate networks can be diagnosed with this command. A symptom of these issues would be a router that advertises a route with a metric that is less than the metric it received for that network.
























The following commands can also be used to troubleshoot RIP:
• show ip rip database
• show ip protocols {summary}
• show ip route
• debug ip rip {events}
• show ip interface brief
The Lab Activities on this page will teach students how to configure and troubleshoot RIP.
The next page will introduce the command that is used to prevent routing updates.
7.2.7 Preventing routing updates through an interface
This page will teach students how to prevent routing updates.
Route filtering regulates the routes that are entered into or advertised out of a route table. These have different effects on link-state routing protocols than they do on distance vector protocols. A router that runs a distance vector protocol advertises routes based on what is in its route table. As a result, a route filter influences which routes the router advertises to its neighbors.
Routers that run link-state protocols determine routes based on information in the link-state database, rather than the route entries advertised by neighbor routers. Route filters have no effect on link-state advertisements or the link-state database. For this reason, the information on this page only applies to distance vector IP routing protocols such as RIP and IGRP.
The passive-interface command prevents the transmission of routing updates through a router interface. When update messages are not sent through a router interface, other systems on the network cannot learn about routes dynamically. In Figure , Router E uses the passive-interface command to prevent routing updates from being sent.



For RIP and IGRP, the passive-interface command stops the router from sending updates to a particular neighbor, but the router continues to listen and use routing updates from that neighbor.
The Lab Activities will instruct students on how to prevent routing updates through an interface.
The next page will explain the concept of load balancing.
7.2.8 Load balancing with RIP
This page will describe load balancing and explain how RIP uses this feature.
Load balancing is a concept that allows a router to take advantage of multiple best paths to a given destination. These paths are either statically defined by a network administrator or calculated by a dynamic routing protocol such as RIP.


RIP is capable of load balancing over as many as six equal-cost paths. The default is four paths. RIP performs what is referred to as “round robin” load balancing. This means that RIP takes turns forwarding packets over the parallel paths.
Figure shows an example of RIP routes with four equal cost paths. The router will start with an interface pointer to the interface connected to Router 1. Then the interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1 and so on. Since the metric for RIP is hop count, the speed of the links is not considered. Therefore, the 56-Kbps path will be given the same preference as the 155-Mbps path.


The show ip route command can be used to find equal cost routes. For example, Figure is a display of the output show ip route to a particular subnet with multiple routes.
Notice there are two routing descriptor blocks. Each block is one route. There is also an asterisk (*) next to one of the block entries. This corresponds to the active route that is used for new traffic.
The next page will explain load balancing in greater detail.
7.2.9 Load balancing across multiple paths
This page will further explain how routers use load balancing to transmit packets to a destination IP address over multiple paths. The paths are derived either statically or with dynamic protocols, such as RIP, EIGRP, OSPF, and IGRP.
When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table. Sometimes the router must select a route from among many, learned through the same routing process with the same administrative distance. In this case, the router chooses the path with the lowest cost or metric to the destination. Each routing process calculates its cost differently and the costs may need to be manually configured in order to achieve load balancing.
If the router receives and installs multiple paths with the same administrative distance and cost to a destination, load-balancing can occur. Cisco IOS imposes a limit of up to six equal cost routes in a routing table, but some IGPs have their own limitations. EIGRP allows up to four equal cost routes.


By default, most IP routing protocols install a maximum of four parallel routes in a routing table. Static routes always install six routes. The exception is BGP, which by default allows only one path to a destination.
The range of maximum paths is one to six paths. To change the maximum number of parallel paths allowed, use the following command in router configuration mode:
Router(config-router)#maximum-paths [number ]
IGRP can load balance up to six unequal links. RIP networks must have the same hop count to load balance, whereas IGRP uses bandwidth to determine how to load balance.


In Figure , there are three ways to reach Network X:
• E to B to A with a metric of 30
• E to C to A with a metric of 20
• E to D to A with a metric of 45
Router E chooses the second path, E to C to A with a metric of 20, since it is a lower cost than 30 and 45.
Cisco IOS supports two methods of load balancing for IP packets. These are per-packet and per-destination load balancing. If process switching is enabled, the router will alternate paths on a per-packet basis. If fast switching is enabled, only one alternate route will be cached for the destination address. All packets that are bound for a specific host will take the same path. Packets bound for a different host on the same network may use an alternate route. Traffic is load balanced on a per-destination basis.
By default the router uses per-destination load balancing, also called fast switching. The route cache allows outgoing packets to be load-balanced on a per-destination basis rather than on a per-packet basis. To disable fast switching, use the no ip route-cache command. Using this command will cause traffic to be load balanced on a per-packet basis.
In the Lab Activities, students will configure and observe load balancing.
The next page will discuss static routes with RIP.
7.2.10 Integrating static routes with RIP
This page will explain how static routes can be configured on a router that uses RIP.
Static routes are user-defined routes that force packets to take a set path from a source to a destination. Static routes become very important if the Cisco IOS software does not learn a route to a particular destination. They are also used to specify a gateway of last resort, which is commonly referred to as a default route. If a packet is destined for a subnet that is not explicitly listed in the routing table, the packet is forwarded to the default route.
A router that runs RIP can receive a default route through an update from another router that runs RIP. Another option is for the router to generate the default route itself.
Use the no ip route global configuration command to remove static routes. The administrator can override a static route with dynamic routing information by adjusting the administrative distance values. Each dynamic routing protocol has a default administrative distance (AD). A static route can be defined as less desirable than a dynamically learned route, as long as the AD of the static route is higher than that of the dynamic route. Note that after the static route to network 172.16.0.0 through 192.168.14.2 was entered, the routing table does not show it. Only the dynamic route learned through RIP is present. This is because the AD of 130 is higher for the static route, and unless the RIP route through S0/0 goes down, the static route will not be installed in the routing table.
Static routes that point out an interface will be advertised by the RIP router that owns the static route and propagated throughout the internetwork. This is because static routes that point to an interface are considered in the routing table to be connected and thus lose their static nature in the update. If a static route is assigned to an interface that is not defined in a network command, a redistribute static command must be specified in the RIP process before RIP will advertise the route.





When an interface goes down, all static routes pointing out that interface are removed from the IP routing table. Likewise, when the software can no longer find a valid next hop for the address specified in the static route, then the static route is removed from the IP routing table.
In Figure a static route has been configured on the GAD router to take the place of the RIP route in the event that the RIP routing process fails. This is referred to as a floating static route. To configure the floating static route, an AD of 130 was defined on the static route. This is greater than the default AD of RIP, which is 120. The BHM router would also need to be configured with a default route.
To configure a static route, use the command shown in Figure in global configuration mode.
The Lab Activity will teach students how to define static routes when RIP is used.








This page concludes this lesson. The next lesson will discuss IGRP. The first page provides an overview of IGRP.
7.3 IGRP
7.3.1 IGRP features
This page will explain the main features and functions of IGRP.
IGRP is a distance vector IGP. Distance vector routing protocols measure distances to mathematically compare routes. This measurement is known as the distance vector. Routers that use distance vector protocols must send all or a portion of their routing table in a routing update message at regular intervals to each neighbor router. As routing information spreads throughout the network, routers perform the following functions:
• Identify new destinations
• Learn of failures
IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90 second intervals. These updates advertise all the networks for a particular AS. Key design characteristics of IGRP are a follows:
• The versatility to automatically handle indefinite, complex topologies
• The flexibility needed to segment with different bandwidth and delay characteristics
• Scalability for functioning in very large networks



By default, the IGRP routing protocol uses bandwidth and delay as metrics. Additionally, IGRP can be configured to use a combination of variables to determine a composite metric. These variables are as follows:
• Bandwidth
• Delay
• Load
• Reliability
The Interactive Media Activity will help students identify the features of RIP and IGRP.
The next page will introduce the IGRP metrics.
7.3.2 IGRP metrics
This page will describe the metrics that IGRP uses.
The show ip protocols command displays parameters, filters, and network information about the routing protocols in use on the router. The algorithm used to calculate the routing metric for IGRP is shown in the graphic. It defines the value of the K1 to K5 metrics and provides information about the maximum hop count. The metric K1 represents bandwidth and the metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1, and K2, K4, and K5 are set to 0.



This composite metric is more accurate than the hop count metric that RIP uses to choose a path to a destination. The path that has the smallest metric value is the best route.
IGRP uses the following metrics:
• Bandwidth – The lowest bandwidth value in the path
• Delay – The cumulative interface delay along the path
• Reliability – The reliability on the link toward the destination as determined by the exchange of keepalives
• Load – The load on a link toward the destination based on bits per second
IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. By default, only bandwidth and delay are considered. The other parameters are considered only if enabled through configuration. Delay and bandwidth are not measured values, but are set with the delay and bandwidth interface commands. The show ip route command in the example shows the IGRP metric values in brackets. A link with a higher bandwidth will have a lower metric and a route with a lower cumulative delay will have a lower metric.
The next page will discuss the IGRP routes.






7.3.3 IGRP routes
This page will introduce the three types of routes that IGRP advertises:
• Interior
• System
• Exterior





Interior
Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.
System
System routes are routes to networks within an autonomous system. The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other IGRP routers or access servers. System routes do not include subnet information.
Exterior
Exterior routes are routes to networks outside the autonomous system that are considered when a gateway of last resort is identified. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.
The Interactive Media Activity will help students understand the different types of IGRP routes.
The next page will introduce three features that increase the stability of IGRP.
7.3.4 IGRP stability features
This page will describe three features that are designed to enhance the stability of IGRP:
• Holddowns
• Split horizons
• Poison reverse updates
Holddowns
Holddowns are used to prevent regular update messages from reinstating a route that may not be up. When a router goes down, neighbor routers detect this from the lack of regularly scheduled update messages.
Split horizons
Split horizons are derived from the premise that it is not useful to send information about a route back in the direction from which it came. The split horizon rule helps prevent routing loops between adjacent routers.
Poison reverse updates
Poison reverse updates are used to prevent larger routing loops. Increases in routing metrics usually indicate routing loops. Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.
IGRP also maintains many timers and variables that contain time intervals. These include an update timer, an invalid timer, a holddown timer, and a flush timer.







The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds.
The invalid timer specifies how long a router should wait in the absence of routing-update messages about a route before it declares that route invalid. The IGRP default for this variable is three times the update period.
The holddown timer specifies the amount of time for which information about poorer routes is ignored. The IGRP default for this variable is three times the update timer period plus 10 seconds.
Finally, the flush timer indicates how much time should pass before a route is flushed from the routing table. The IGRP default is seven times the routing update timer.
IGRP lacks support for VLSM. Cisco has created Enhanced IGRP to correct this problem.
The next page will show students how to configure IGRP.
7.3.5 Configuring IGRP
This page will introduce the commands that are used to configure IGRP.
To configure the IGRP routing process, use the router igrp configuration command. To shut down an IGRP routing process, use the no form of this command.


The command syntax is as follows:
RouterA(config)#router igrpas-number
RouterA(config)#no router igrpas-number
The AS number identifies the IGRP process.
To specify a list of networks for IGRP routing processes, use the network router configuration command. To remove an entry, use the no form of the command.


Figure shows an example of how to configure IGRP for AS 101.
The Lab Activities will help students configure IGRP.
The next page will explain how to convert a router from RIP to IGRP.
7.3.6 Migrating RIP to IGRP
This page will teach students how to convert a router from RIP to IGRP.
When Cisco created IGRP in the early 1980s, it was the first company to solve the problems associated with the use of RIP to route datagrams between interior routers. IGRP examines the bandwidth and delay of the networks between routers to determine the best path through an internetwork. IGRP converges faster than RIP. This prevents routing loops that are caused by disagreement over the next routing hop. Further, IGRP does not share the hop count limitation of RIP. As a result of this and other improvements over RIP, IGRP enabled many large, complex, topologically diverse internetworks to be deployed.



Use the following steps to convert from RIP to IGRP:
1. Enter show ip route to verify that RIP is the routing protocol on the routers to be converted.
2. Configure IGRP on Router A and Router B.
3. Enter show ip protocols on Router A and Router B.
4. Enter show ip route on Router A and Router B.
The Lab Activities will show students how to configure a default route, use RIP to propagate the information, and then convert the router to IGRP.
































The next page will explain how to verify that IGRP has been configured properly.























































7.3.7 Verifying IGRP configuration
This page will teach students how to verify an IGRP configuration.
To verify that IGRP has been configured properly, enter the show ip route command and look for IGRP routes signified by an "I".
Additional commands for checking IGRP configuration are as follows:
• show interfaceinterface
• show running-config
• show running-config interfaceinterface
• show running-config | begin interfaceinterface
• show running-config | begin igrp
• show ip protocols
To verify that the Ethernet interface is properly configured, enter the show interface fa0/0 command. Figure illustrates the output.















To see if IGRP is enabled on the router, enter the show ip protocols command. Figure illustrates the output.



The commands illustrated in Figures - verify the network statements, IP addressing, and routing tables.











In the Lab Activities, students will verify an IGRP configuration and then use IGRP to set up dynamic routing.
The next page will teach students how to troubleshoot IGRP.

7.3.8 Troubleshooting IGRP
This page will introduce some commands that can be used to troubleshoot IGRP.
Most IGRP configuration errors involve a mistyped network statement, discontiguous subnets, or an incorrect AS Number.
The following commands are used to troubleshoot IGRP:
• show ip protocols
• show ip route
• debug ip igrp events
• debug ip igrp transactions
• ping
• traceroute



Figure shows output from the debug ip igrp events command.
Figure shows output from the debug ip igrp transactions command.
If the AS number is wrong and then corrected, it results in the output shown in Figure .
The Lab Activity will show students how to use the IGRP debug commands.
This page concludes this lesson. The next page will summarize the main points from this module.























Summary
This page summarizes the topics discussed in this module.


Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost as defined by the metrics and the logical address of the first router on the path to each network contained in the table.
RIP uses many techniques to reduce routing loops and counting to infinity. RIP permits a maximum hop count of 15. A destination greater than 15 hops away is tagged as unreachable.
The split horizon rule specifies that it is not useful to send information about a route back in the direction from which it came. In some network configurations, it may be necessary to disable split horizon.
Route poisoning is used to overcome large routing loops and provide information when a network is down. It also keeps a router from receiving incorrect updates.
Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. Triggered updates are also sent if routing information changes. The router sends triggered routing update on its other interfaces rather than waiting on the routing update timer to expire.
RIP v2 enhancements include the ability to carry additional packet routing information, an authentication mechanism to secure table updates, and support for VLSM. By default, routing updates are broadcast every 30 seconds.
RIP is enabled with the router rip command. The network command is then used to tell the router on which interfaces to run RIP.
A supernet route is a route that covers a greater range of subnets with a single entry. The ip classless global configuration command is used to forward packets to the best supernet route when a router receives packets destined for an unknown subnet of a network.
The two most common commands used to verify that RIP is properly configured are the show ip route and show ip protocols commands. The show ip route command shows the routes that are installed in the routing table and the status of each route. The show ip protocols command is used to verify the state of the active routing protocol as well as the installed routes specific to the protocol.
To display RIP routing updates as they are sent and received, use the debug ip rip command.
The passive-interface command prevents routers from sending routing updates through a router interface. This keeps update messages from being sent through a router interface so that other systems on a network will not learn about routes dynamically.
The show ip route command is used to find equal cost routes for load balancing. RIP uses round robin load balancing. Routers take turns to forward packets over equal cost paths.
IGRP is a distance vector routing protocol that measures distances to mathematically compare routes. It sends routing updates at 90 second intervals to advertise networks for an AS. IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability.
IGRP advertises three types of routes. These include interior, system, and exterior. There are many features such as holddowns and split horizons that provide stability.
Use the show ip protocols and the show ip route commands to verify that IGRP is properly configured. In addition, the ping and trace commands are used to troubleshoot errors.