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Knowing And Learning Routing Loop

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RIP is a distance vector routing protocol that is used in thousands of networks worldwide. The fact that RIP is an open protocol standard and easy to implement, making it attractive to some network administrators. In addition to RIP, IGRP is also a distance vector routing protocol. Unlike RIP, IGRP is a Cisco proprietary protocol is more than standards-based protocols. IGRP is also very simple to implement. However, IGRP is a routing protocol that is more complex than RIP and can use several factors to determine the best route to a destination network. This module will introduce IGRP configuration and troubleshooting.

Upadate-Distance Vector routing updates

Routing table updates occur periodically or when there is a change in a distance vector protocol network. Just as the process of network discovery, topology change updates are processed systematically from router to router. Distance vector algorithms call each router to send the contents of the routing table to each neighbor that is connected directly. Routing table information about the total price including the path (total path cost). Price path defined by the metric and the logical address of the first router on the path to each network in the table.

Distance vector routing loop issues

Routing loops can occur when inconsistent routing tables are updated or not caused by the slow konfergensi the network changes.


Defines a maximum count

Update the invalid about Network 1 will take place over and over until some other process to stop looping. This condition, which is known as the count to infinity (count of infinity), repeating packets in the network surrounding the denial of the fact that the destination network, the Network 1, is down (dead). During the routers count to infinity, an invalid information causes a routing loop.

To prevent routing loops is prolonged, distance vector protocols define infinity (infinity) to a maximum number. This number refers to a routing metric, which is simply a matter of hops (jumps).

With this approach, allowing the routing protocol for routing loops occurred until the metric goes beyond the maximum value allowed. Figure shows the metric value is 16 hops. This goes beyond the distance vector default maximum of 15 hops that packets discarded by the router. When the metric value exceeds the maximum value, Network 1 is considered unreachable.

Overcome routing loops through split-horizon

Some routing loop occurs when incorrect information is sent back to a router denies the right information that has been distributed the previous router. Split horizon reduces incorrect routing information and routing load.


If information about Network 1 comes from router A, then the router B and D should not be informed about Network 1 to router A.

Route Poisoning

Route Poisoning (poison routes) used by various distance vector protocols to overcome routing loops are spacious and offer detailed information when a subnet or network can not be accessed. To resolve this, hop count is usually set to more than one maximum.

One way to prevent updates that are not consistent is route poisoning. When Network 5 dows, Router E will set a distance of 16 for Network 5 to poison route. This indicates that the network can not be achieved.


When the poisoned route, Router C is not affected by the update that is not true about the route to Network 5. After Router C receives a route poisoning from Router E, it sends an update, called a poison reverse, back to Router E. This is to convince all the routers in the segment have received the poisoned route information.

When used with a route poisoning triggered updates it will mepercepat convergence time because neighboring routers do not need menuggu 30 seconds before they announced the poisoned route.

Preventing Routing Loops with Trigerred Updates

New routing tables are sent to neighboring routers based on a determination. For example, RIP updates occur every 30 seconds. However a trigered updates sent immediately in response to some changes in the routing table. The router that detects a topology change immediately sends an update message to adjacent routers. These routers build triggered updates to notify changes to their neighbors. When a route fails, an update posted as soon as possible. Trigered update, used in conjunction with route poisoning, ensure that all routers know of failed routes before the holddown timer runs out.

Preventing Routing Loops with Holddown Timers

When a router receives an update from a neighbor, which indicates that a previously accessible network is no longer accessible, the router marks the route as inaccessible and start the timer holddows. Before the holddown timer runs out, if an update is received from the same neighborhood, which indicates that the network is accessible, the router marks the network as accessible and discard 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 discard holddown timer.
If an update is received from a different router with a higher metric before the holddown timer runs out, updates will be ignored. This update is ignored to give more time for pegetahuan about a change for the mess to spread throughout the network.

RIP Routing Process

Version of the modern open standard RIP, which is sometimes known as IP RIP, is formally didetailkan in two different documents. The first is known as Request for Comment (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 enhance the following points:

Ability to carry additional packet routing information
Authentication mechanism to secure table updates
Support for variable-length subnet masks (VLSM)

Konfigursi RIP

The command router rip to enable RIP as a routing protocol. Then the network command is used to notify to the router on which interfaces to run RIP. RIP sends routing-update messages at fixed intervals. When a router receives a routing update that contains changes to an entry, it updates its routing table to describe the new route. Matrix values ​​are acceptable for line coupled with 1, and the interface is indicated as the source of the update in the next hop routing table. Routers only manage the best route to a destination but can manage multiple paths that have equal-cost (price-the same) to a destination.

To enable RIP, use the following commands in global configuration mode:

Router (config) # router rip - Enables the RIP routing process
Router (config) # network network-number-Combining a network with a RIP routing prosess

Using the ip classless command

Sometimes a router receives packets destined for a subnet of a network that is not known which has a directly connected subnet. Use the ip classless global configuration command to instruct the Cisco IOS software to forward these packets to the best supernet route. A router is a supernet route that handles a broad range of subnets with a single entry. For example, if an enterprise uses a whole subnet, then a supernet route for will be. Ip classless command is enabled by default in Cisco IOS software release 11.3 and above. To disable the feature, use the no form of this command.

Load Balancing with RIP

Load balancing is a concept that allows a router to take advantage of some of the best path given for a purpose. These pathways can be static defined by an administrator or calculated with a dynamic routing protocol such as RIP.

RIP offers load balancing capabilities such as multiple channels such as 6 equal-cost paths (which has the same price). RIP to do what is known as the "round robin" load balancing. This means that the RIP to turn to forward packets through parallel pathways.

Load Balancing through a few points

When a router learn several routes to a particular network, route with the lowest administrative distance will be placed in the routing table.

Sometimes the router must select a route among multiple routes available, learned through the same routing process with the same distance administraive. In this case, routers choose the path with the lowest cost or to the destination matrix. Each routing process calculates its own costnya differently and the cost may have been configured manually in the event to achieve load balancing.

If a route is received and put up some lines with the administrative distance and cost the same to a destination, load-balancing can occur. Cisco IOS set a limit of up to six equal-cost routes in a routing table, but some of the IGP they have their own limitations. EIGRP allows up to four equal-cost routes.

By default, most routing protocols install a maximum of four parallel routes in a routing table. Static routes always put a 6 route. Exceptions BGP, which by default only allow one path to a destination.

Features IGRP

IGRP is a distance vector IGP. Routing protocols distance vector distance measure to compare the matimatika route. This measurement is known as distance vector. Routers that use distance vector protocols must send all or part of their routing tables in a routing update message at fixed intervals for each router. As routing information spreads throughout the network, routers perform the following functions:

Recognizing the new destination
Studying failure

IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at intervals of 90 seconds. Updates are announced all the tissues to a similar U.S.. Karakateristik IGRP key design are as follows:

Have the ability to automatically handle complex topologies and not terdefenisi.
Flexibility needed to segment with different bandwidth and delay characteristics.
Scalability for functioning in a network-wide network.

By default, IGRP routing protocol uses bandwidth and delay as a matrix.

Additionally, IGRP can be configured to use a combination of variables-variables to determine a composite matrix. These variables are:


IGRP Metrics

Using IGRP composite metric (composite metric). Matrix composite matrix is ​​more accurate than that used RIP hop count to have a path to a destination. The pathway that is used has a value matrix least of which is the best route.

IGRP uses some of the following matrix:

Bandwidth - The lowest bandwidth value in the path
Delay - cumulative interface delay along the path
Reliability - The reliability on the link to the objectives as set by the exchange of keepalives.
Load - the load on a link to the destination based on bits per second.

Delay and bandwidth is not a measured value, but on set with delay and bandwidth interface commands. A link with the highest bandwidth will have a low matrix and a route with a lower cumulative delay will have a lower matrix.

IGRP routes

This section will introduce three types of routes IGRP announced:




The interior is routes routes between subnets or a network connected to a router interface. If the network is connected to a router not in subnet, IGRP will not announce routes interior.


Route-route system is a route-route to the tissues within an autonomous system. Cisco IOS software routes the system obtained from the network interface that is connected directly and system route information provided by other IGRP routers or access servers. Route-rotue system does not include subnet information.


Adalah routes exterior routes to networks outside the autonomous system that is considered when a final destination gateway recognized. Cisco IOS Software gateway to choose a final destination from the list of exterior routes that IGRP is provided. Software uses the gateway if the ultimate goal of a better route is not found and the destination is not connected to the network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway final destination.

IGRP stability features

Three features are designed to increase the stability of IGRP:

Split horizon
Poison reserve updates


Holddown used to prevent update messages that remain of a route that is not stable. When a router is down, neighboring routers detect this from the absence of regularly scheduled update messages.

Split horizon

Split horizon comes from the idea that it is not useful to send information about a route back in the direction from which the information is coming. Split horizon rule helps prevent routing loops between adjacent routers.

Poison reverse updates

Poison reverse updates are used to prevent routing loops is more widespread. The addition of the routing matrix usually indicate routing loops. Poison reverse updates are then sent to eliminate the route and place it in holddown. With IGRP, poison reverse updates sent only if a matrix route has increased by a factor of 1.1 or greater.

Configuring IGRP

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

Command syntax is as follows:

RouterA (config) # router IGRP as-number

RouterA (config) # no router IGRP as-number

AS numbers identify the IGRP process

To determine a list of networks for the IGRP routing process, use the network router configuration command. To mehilangkan an entry, use the no form of the command.

Migrating RIP to IGRP

When Cisco IGRP been created early in 1980, he was the first company that solves problems using RIP to the merger route the datagram-datagram between interior routers. IGRP check the bandwidth and delay of the networks between routers to determine the best path through an internetwork. IGRP faster convergence than RIP. It prevents routing loops caused by the absence of persesetujuan via a next hop routing. Further, IGRP does not share the limitations of RIP hop count. As a result, and other advances through RIP, IGRP creating a more extensive, complex, the topology of various tiger internetworks to be developed.

Use the following steps to convert from RIP to IGRP:

Enter the show ip route to verify that RIP is a routing protocol on routers that will be converted.
Configuring IGRP on routers A and B
Enter the command show ip protocols on Router A and B
Enter the command show ip route on router A and router B


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