Comparison of Route Distribution in Routing Protocols

Introduction

Paragraph 1 – Protocol Selection

In this document, I will cover route redistribution between the two Interior Gateway Protocols (IGP), Open Shortest Path First (OSPF) and the Enhanced Interior Gateway Routing Protocol (EIGRP) covering some of their strengths and weaknesses.

There are a number of reasons for my selection of these two routing protocols. The first is the dominance of OSPF and EIGRP in today’s medium to large size corporate networks (Komal Gehlot et al, 2014), along with the fact that they also belong to a different protocol classes, with OSPF representing a Link-state protocol and EIGRP representing a Distance-vector protocol. The final reason for the selection of OSPF and EIGRP is personal familiarity and the fact that I have had more experience and exposure and am therefore most familiar with that these protocols.

Paragraph 2 – Redistribution Overview

The preferred option for any network would be to use a single routing protocol however the reality is that most modern enterprise networks feature complex network designs containing multiple routers running one or more routing protocol.

A router running an Interior Gateway Protocol (IGP) such as OSPF and EIGRP is only able to exchange information with peer routers running the same routing protocol and in the same autonomous system as (Patel, H 2014) states “having a multiple routing protocol and different autonomous system in networks then without route redistribution we cannot advertise route from source to destination.”

Consider the network diagram depicted in figure 1 below. The network illustrated shows two routing instances or domains one running OSPF and one EIGRP. Routers in the OSPF domain have no visibility of the addresses or subnets in the EIGRP instance and vice versa. To allow the exchange of routing information between different routing instances. By configuring route redistribution routers on the boundary between the two domains known as Autonomous System Broder Routers (ASBR’s) in this case R4 and R5 they will be able to exchange route information between protocols enabling routers in the OSPF AS to route to networks in the EIGRP AS and visa versa.

Figure 1.

Route redistribution is common in environments need to support proprietary protocols, legacy equipment, or complex network structures that may have been created by merging networks that are running different routing protocols. In these situations there may be a requirement for routing domains to exchange a routing information about connected networks between two or more different routing protocols. Routers are capable of running multiple routing protocols simultaneously and route redistribution allows them to redistribute or inject routes from one routing protocol or process into another, for example EIGRP routes can be injected into an OSPF process and the EIGRP route then becomes an OSPF route to all other OSPF routers in routing domain.

OSPF and EIGRP Strengths and Weaknesses of Route Redistribution

Paragraph 3 – Benefits of Redistribution

Route redistribution can offer a number of benefits to network administrator and engineers to assist them in maintain and managing a large corporate network.

First amongst these advantages is the ability to segment or divide a large network into separate smaller autonomous systems or domains, this offers the ability of creating different management or administrative divisions as well as creating security boundaries which can reduce the size and complexity of the network topology and reduce routing tables and in turn reduce the workload placed on routers since routers inside an Autonomous system that are not operating as ASBR’s will only need to know the topology of their own piece of the network as well as summaries of the redistributed routes learned from outside.

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In addition route redistribution provides easier configure of routing allowing the connection of two or more separate routing instances and exchanging the routing information dynamically with relatively simple configuration. A process which were it not for route redistribution would be an inherently more demanding complex and time consuming task.

The greatest strength of route redistribution in OSPF and EIGRP is the ability of both protocols to be able to manipulate parameters and metrics and to influence the routing decisions and the path of data through the network.

Paragraph 4 – Weaknesses of Redistribution

Despite the advantages offered by route redistribution there are also a number of weaknesses or negative consequence or behaviours that have the ability to introduce or create unwanted or unexpected behaviours in the operation of the network. Particularly if the network has not been designed or implemented in a careful and thoughtful manner, therefore careful consideration needs to be given in the planning stage to the potential effects that any changes could have on routes and routing and on the efficient operation of the network. 

There are three primary weaknesses of Route Redistribution between OSPF and EIGRP

Routing Loops and Suboptimal Routes

Two-Way route redistribution between two domains with two or more ASBR’s can cause loops

You must be careful when configuring route redistribution and redistributing routes into another protocol since it is possible to inadvertently create persistent routing loops, suboptimal routes or route oscillations. (Franck Le et al 2007)

Incompatible Routing Metrics

Different and incompatible metrics and calculation methods can result in suboptimal routes between routing protocols

Different routing protocols use different routing metrics, so this must be into account when redistributing routes from one protocol into another.

Open Shortest Path First (OSPF) for example is a link-state routing protocol in which each router works independently to calculate its own shortest route towards the destination network using the Dijkstra algorithm and is based on which route offers least cost path to the destination network.

Enhanced Interior Gateway Routing Protocol (EIGRP) by contrast is a distance vector protocol. Unlike OSPF, EIGRP router hold no information about the network, they do not calculate or even know the entire path to the destination network only the address of the next hop router to route traffic to. EIGRP instead uses a composite metric calculated using the DUAL algorithm and is based on five metrics or K values, these are bandwidth, reliability, load, delay, and MTU size to assign different weight to each route.

OSPF has no knowledge of EIGRP’s K value calculation and no way to interpret them or use them in it’s own route calculation, just as EIGRP has no way of doing the same with OSPF’s shortest path cost calculation. As a result of these differences and incompatibilities in the metrics, algorithms’ and calculations when redistributing routes between OSPF and EIGRP all of the metrics associated with a route are lost and you must be manually specifying the cost metric for each routing domain or protocol. (Patel, N 2014)

Convergence Time

EIGRP converges faster than OSPF

There is also the need to consider differences in convergence time between different protocols. In simulations carried out by Thorenoor, S. (2010) and Wijaya, C. (2011) point out that EIGRP convergence times are much faster than OSPF which can cause convergence issues in the network. Thorenoor, S (2010) stated “simulation study is performed on the network with different routing protocols and it has been shown that EIGRP provides a better network convergence time, less bandwidth requirements and better CPU and memory utilization compared to OSPF and RIP”

Conclusion

The conclusion more or less repeats the main idea from the introduction

References

• Le, Franck, Xie, Geoffrey G. & Zhang, Hui Understanding Route Redistribution
• Franck Le, Geoffrey G. Xie Zhang, Understanding Route Redistribution, April 2007, CMU-CS-07-122, Carnegie Mellon University
• Thorenoor, S. (2010) and Wijaya, C. (2011)
• Sheela Ganesh Thorenoor, Dynamic Routing Protocol implementation decision between EIGRP, OSPF and RIP based on Technical Background Using OPNET Modeler
• Haresh N. Patel Extensive Reviews of OSPF and EIGRP Routing Protocols based on Route Summarization & Route Redistribution Int Journal of Engineering Research & Applications ISSN 2248-9622 Sep 2014, p.141-144
• 2013 Y.Navaneeth Krishnan , Dr Shobha G Performance Analysis of OSPF and EIGRP Routing Protocols for Greener Internetworking Operation and Comparison of EIGRP and OSPF Routing Protocol. EIGRP uses less system resources when compared to OSPF.
• Chandra Wijaya, Performance Analysis of Dynamic Routing Protocol EIGRP and OSPF in IPv4 and IPv6 Network 2011 Parahyangan Catholic University
• Komal Gehlot, N.C. Barwar , Performance Evaluation of EIGRP and OSPF Routing Protocols in Real Time Applications , 2014