ROUTING TOPOLOGY: Linear vs looped

Author: Iloka Benneth Chiemelie
Published: 14th April 2017

Introduction

Routing Information Protocol (RIP) is described as an interior routing protocol and it has been in used for numerous years (which can be traced back to the 1980s). It is normally used for the purpose of routing IP dynamically inside a small-to-medium sized networks. Although this protocol is not commonly used these days, it is generally lectured to the new network engineers for the overall purpose of introducing the concept of dynamic routing. When compared with other dynamic routing protocols, RP is considered a simple protocol and they are easily configured to function in the Cisco systems. As such, this research is designed to expand the basic understanding of the RIP system in relation to how it is operated and how it is configured in order to function within the Cisco equipment.

Theory

In this section, numerous key points in relation to the RIP system will be discussed.

The concept of RIP

In the course of understanding RIP, the first thing to note is that it is a distance vector protocol. What is implies here is that it is known to learn routes only from the neighbors, as all of its routing information are obtained from the neighbors trusted by the system and those that are assumed to be correct. This style adopted in learning is known as routing by rumor. In terms of distance, the metric distance adopted by the RP is a hop count, with each device attached to a packet crosses on their way to destination and they are counted as an additional hop. Although RIP is seldom used, their actual application involves mounting them on small-to-limited sized networks as it is only capable of supporting jut 15 hop counts.

In any case, devices that make use of RIP have been known to be outstanding in a multicast or broadcast message advertising all the known route in about 30 seconds or so. The information obtained from the process is the utilized by the neighboring router for the purpose of building a routing table, and the information generated in this process are added to the reachable networks of the neighboring router and the resent out to the network’s interface (for every 30 seconds) with the processing continuing repeatedly. This is basically how an RIP database re built throughout all networks. Together with the update that the system sends out every 30 seconds, it should be noted that RIP also supports flash updates. Flash updates occur in the occasion that a router learns of a change in reachable route and once such happens, the router will flood out the route instantly.

In order to avoid routing route in RIP, there are different concept that must be reviewed and such is also applicable when it comes to maintaining a stable network, which include: split horizon, route poisoning, and RIP timers.

Split horizon

In the case of normal operations, RIP have been known to learn routes form its neighboring routers, which it the adds to its own database prior to sending the routing update to its RIP interfaces. The main note with split horizon is that makes it impossible for RIP to send out an advertisement over a given network that has been learned on the same interface. That is to say, if 192.168.1.0 has been learned on the interface f0/0, it will be impossible for the same network to be advertised outside out of the f0/0 interface.

Route poisoning

Routing poisoning is produced in the case that the RIP device learns that the process of routing to a given device has failed. In the event that such occurs, the RIP will take necessary measures and advice the failed destination out of its interface with a metric of 16 (which is unreachable as the highest volume of reachable metric is 15). This pass the information to all the RIP’s neighbor that the system no longer maintains route to the said network.

RIP Timers

In the course of maintaining operations, the RIP makes use of varied kinds of timers as: update, flush, invalid and hold down timers. The use of the update timer is for identifying when the RIP has been able to send out scheduled updates (which is 30 seconds by default). The use of the invalid timer is to determine how valid the entry route actually is. In the case that no update has been received in relation with the said route within 180 seconds, it will be automatically be marked with a hop of 16 metric and considered invalid by default. In the case of the flush timer, it is used for the purpose of the period that route has been removed from the RIP database. The use of the hidden timer is for stabilizing the RIP route within the network, in the event that the route experienced change in its metric, the hold down timer will be initiated and no new updates will occur within the route else the hold done time has expired (the hold down time is 180 seconds by default).

In RIP, there are two different versions and they are different in relation to their ability. The first version is able to send out updates through broadcast (255.255.255.255) and it is also capable of providing support for Variable Length Subnet Masking (VLSM). The second version is capable of sending updates through multicast (224.0.0.9), and it does support VLSM while adding support for authentication. The devices that run in the version two of the RIP have backward compatibility and individual interface that are capable of being advertised to only updates coming from version 1 and version 2.

Configuration Method

Bus typology

When it comes installation, the bus (linear) is the simplest form of router to install as all the devices on the network are connected to a single primary trunk cable. This form of networks are normally used with the contention network. In the course of configuring the bus router, it is vital to pay close attention to the termination. This is because it is important to ensure that all the end of the trunk cable are properly terminated. If the termination process are not properly effected, the signal will bounce back and this will result to collisions.

The cable adopted in bus typology is coaial cable and they are connected with the aid of BNC connectors. The T connectors are normally used for the purpose of connecting the computer to the trunk cable, and they can be connected to the computer in two sections of the cable with the bus being extended in both directions. The end devices that are created on the bus will then be offered terminators on one connector of the T cable.

This form of connection does come with numerous advantaged in the sense that they are: 1) easy to configure and install; 2) they are not expensive; and 3) they are easily extended. In any case, they are also some associated disadvantages in the sense that: 1) with time, their performance degrades; 2) the barrel connectors that are used for the purpose of extending the length of the cable have the potential of weakening the signal: and 3) it is quite difficult to troubleshoot a bus router. 

Looped (three router typology)

When it comes to configuring looped router, the following steps will be adopted in the course of configuring this form of router (assuming that the IP address configuration has already been completed). They are:

R1:

·         Enter global configuration mode - R1#configure terminal

·         Enter RIP router configuration mode - R1 (config)#router rip

·         Configure RIP version 2 - R1(config-router)#version 2

·         Configure the 172.16.0.0 networks into RIP - R1(config-router)#network 172.16.0.0

·         Configure the 192.168.1.0 network into RIP - R1(config-router)#network 192.168.1.0

R2:

·         Enter global configuration mode - R2#configure terminal

·         Enter RIP router configuration mode - R2(config)#router rip

·         Configure RIP version 2 - R2(config-router)#version 2

·         Configure the 172.16.0.0 networks into RIP - R2(config-router)#network 172.16.0.0

·         Configure the 192.168.2.0 network into RIP - R2(config-router)#network 192.168.2.0

R3:

·         Enter global configuration mode - R3#configure terminal

·         Enter RIP router configuration mode - R3(config)#router rip

·         Configure RIP version 2 - R3(config-router)#version 2

·         Configure the 172.16.0.0 networks into RIP - R3(config-router)#network 172.16.0.0

·         Configure the 192.168.3.0 network into RIP - R3(config-router)#network 192.168.3.0

Results

Linear typology (three router)

Looped

Discussion

From the above configuration, it is clear that the linear typology is much easier to configure in the essence that it involves flowing of cable through a bus line format. However, in the case that troubleshooting is required, it becomes an issue because it will require the whole routers being checked. This is different from the looped format, that is more difficult to configure but much easier to maintain in the course of troubleshooting.

Conclusion

In line with the above understanding, it is recommended that the looped format should be used. Although the process of configuring it is more complex, it is evidentially clear that it is more cable of delivering desired result from the configuration process.

References

Sean, W. (2012). RIP Concepts and Configuration. Available at: http://www.pearsonitcertification.com/articles/article.aspx?p=1868079 [Accessed on: 22/10/2016].