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.

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 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.

The distance vector metric of hop count increases each time the packet passes through another router.

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.

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.

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.

New routing tables are sent to neighbor routers on a regular basis. For example, RIP updates occur every 30 seconds.

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.

Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire.

Triggered updates do not wait for update timers to expire. They are sent when routing information has changed.

Example:

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.

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.

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 implements split horizon and holddown mechanisms to prevent the propagation of incorrect routing information.

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 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.

Example to configure router BHM:

• 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

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.

Question:

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.

Solution:

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.

To reduce routing loops and counting to infinity, RIP uses the following techniques:

• Split horizon
• Poison reverse
• Holddown counters
• Triggered updates

Maximum hop count:

RIP permits a maximum hop count of 15.

This maximum hop count greatly restricts the use of RIP in large internetworks but prevents counts to infinity and endless network routing loops.

Split horizon :

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 following command is used to disable split horizon:

GAD(config-if)#no ip split-horizon

Holddown timers:

To help prevent counting to infinity but also increase convergence time.

The default holddown for RIP is 180 seconds.

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.

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 ]

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.

disable routing updates :

A network administrator can use the passive-interface command to disable routing updates on specified interfaces.

In a non-broadcast network:

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 4.

Version:

By default, the Cisco IOS software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets.

To configure the router to send and receive packets from only one version, use the commands in Figure 5.

To control how packets received from an interface are processed, use the commands in Figure 6.

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 debug ip rip command displays RIP routing updates as they are sent and received.

The example in Figure 1 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.

Problems such as discontiguous subnetworks or duplicate networks can be diagnosed with this debug ip rip 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

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.

In Figure 1, 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.

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 1 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 2 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.

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.

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. IGRP uses bandwidth to determine how to load balance.

RIP networks must have the same hop count to load balance,

In Figure 2, 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.

Per-packet:

If process switching is enabled, the router will alternate paths on a per-packet basis.

Per-destination:

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.

By default the router uses per-destination load balancing, also called fast switching.

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.

Static routes are user-defined routes that force packets to take a set path from a source to a destination.

Static routes 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.

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.

Example:

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 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.

In Figure 2 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 3 in global configuration mode.

IGRP is a distance vector IGP.

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.

IGRP can be configured to use a combination of variables to determine a composite metric:

• Bandwidth
• Delay
• Load
• Reliability

The show ip protocols command displays parameters, filters, and network information about the routing protocols.

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.

This page will introduce the three types of routes that IGRP advertises:

• Interior
• System
• Exterior

This page will describe three features that are designed to enhance the stability of IGRP:

• Holddowns
• Split horizons
• Poison reverse updates

To configure the IGRP routing process, use the router igrp configuration command.

        RouterA(config)#router igrp as-number

To shut down an IGRP routing process, use the no form of this command.

RouterA(config)#no router igrp as-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 2 shows an example of how to configure IGRP for AS 101.

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.

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.

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

The commands illustrated in Figures 3 - 5 verify the network statements, IP addressing, and routing tables.

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 1 shows output from the debug ip igrp events command.

Figure 2 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 3.

The Lab Activity will show students how to use the IGRP debug commands.