Module 5 - SLP
STATIC AND DYNAMIC ROUTING
Dynamic routing protocols have evolved over several years to meet the demands of changing network requirements. Many organizations use dynamic routing protocols such as Open
Shortest Path First (OSPF), Routing Information Protocol (RIP).
Dynamic routing protocols have been used in networks since the early 1980s. The first version of RIP was released in 1982, but some of the basic algorithms within the protocol were used on the ARPANET as early as 1969.
One of the earliest routing protocols was RIP. RIP has evolved into a newer version: RIPv2. However, the newer version of RIP still does not scale to larger network implementations. To address the needs of larger networks, two advanced routing protocols were developed: OSPF and Intermediate System–to–Intermediate System (IS-IS). Cisco developed Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP). EIGRP also scales well in larger network implementations.
Additionally, there was the need to interconnect different internetworks and provide routing among them. Border Gateway Protocol (BGP) is now used between Internet service providers (ISP) as well as between ISPs and their larger private clients to exchange routing information.
Role of Dynamic Routing Protocol
What exactly are dynamic routing protocols? Routing protocols are used to facilitate the exchange of routing information between routers. Routing protocols allow routers to dynamically learn information about remote networks and automatically add this information to their own routing tables, as shown in Figure 1.
Figure 1. Routers Dynamically Pass Updates
Routing protocols determine the best path to each network, which is then added to the routing table. One of the primary benefits of using a dynamic routing protocol is that routers exchange routing information whenever there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths if there is a link failure to a current network.
Compared to static routing, dynamic routing protocols require less administrative overhead. However, the expense of using dynamic routing protocols is dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth. Despite the benefits of dynamic routing, static routing still has its place. There are times when static routing is more appropriate and other times when dynamic routing is the better choice. More often than not, you will find a combination of both types of routing in any network that has a moderate level of complexity.
Purpose of Dynamic Routing Protocols
A routing protocol is a set of processes, algorithms, and messages that are used to exchange routing information and populate the routing table with the routing protocol’s choice of best paths. The purpose of a routing protocol includes:
Discovering remote networks
Maintaining up-to-date routing information
Choosing the best path to destination networks
Having the ability to find a new best path if the current path is no longer available
Dynamic Routing Protocol Operation
All routing protocols have the same purpose: to learn about remote networks and to quickly adapt whenever there is a change in the topology. The method that a routing protocol uses to accomplish this depends on the algorithm it uses and the operational characteristics of that protocol. The operations of a dynamic routing protocol vary depending on the type of routing protocol and the specific operations of that routing protocol. The specific operations of RIP, EIGRP, and OSPF are examined in later chapters. In general, the operations of a dynamic routing protocol can be described as follows:
The router sends and receives routing messages on its interfaces.
The router shares routing messages and routing information with other routers that are using the same routing protocol.
Routers exchange routing information to learn about remote networks.
When a router detects a topology change, the routing protocol can advertise this change to other routers.
Dynamic Routing Advantages and Disadvantages
Dynamic routing advantages are as follows:
Administrator has less work in maintaining the configuration when adding or deleting networks.
Protocols automatically react to the topology changes.
Configuration is less error-prone.
More scalable; growing the network usually does not present a problem.
Dynamic routing disadvantages are as follows:
Router resources are used (CPU cycles, memory, and link bandwidth).
More administrator knowledge is required for configuration, verification, and troubleshooting.
Metrics are a way to measure or compare. Routing protocols use metrics to determine which route is the best path.
Purpose of a Metric
There are cases when a routing protocol learns of more than one route to the same destination.
To select the best path, the routing protocol must be able to evaluate and differentiate among the available paths. For this purpose, a metric is used. A metric is a value used by routing protocols to assign costs to reach remote networks. The metric is used to determine which path is most preferable when there are multiple paths to the same remote network.
Each routing protocol calculates its metric in a different way. For example, RIP uses hop count, EIGRP uses a combination of bandwidth and delay, and the Cisco implementation of OSPF uses bandwidth. Hop count is the easiest metric to envision. The hop count refers to the number of routers a packet must cross to reach the destination network.
For Router R3 in Figure 2, network 172.16.3.0 is two hops, or two routers, away. For Router R2, network 172.16.3.0 is one hop away, and for Router R1, it is 0 hops (because the network is directly connected).
Figure 2. Metrics
Metrics and Routing Protocols
Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol.
Two different routing protocols might choose different paths to the same destination because of using different metrics.
Figure 3 shows how R1 would reach the 172.16.1.0/24 network. RIP would choose the path with the least amount of hops through R2, whereas OSPF would choose the path with the highest bandwidth through R3.
Figure 3. Hop Count vs. Bandwidth
Metrics used in IP routing protocols include the following:
Hop count: A simple metric that counts the number of routers a packet must traverse.
Bandwidth: Influences path selection by preferring the path with the highest bandwidth.
Load: Considers the traffic utilization of a certain link.
Delay: Considers the time a packet takes to traverse a path.
Reliability: Assesses the probability of a link failure, calculated from the interface error count or previous link failures.
Cost: A value determined either by the IOS or by the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics, or a policy.
To learn more about binary trees, check the following sites:
Static vs. Dynamic Routing (http://www.inetdaemon.com/tutorials/internet/ip/routing/dyamic_vs_static.shtml)
Dynamic Routing (http://www.techopedia.com/definition/19047/dynamic-routing)
What is Dynamic Routing? (http://publib.boulder.ibm.com/infocenter/cicsts/v2r3/index.jsp?topic=/com.ibm.cics.ts23.doc/dfht1/dfht1mk.htm)
Static and Dynamic Routing (http://pic.dhe.ibm.com/infocenter/aix/v7r1/index.jsp?topic=%2Fcom.ibm.aix.commadmn%2Fdoc%2Fcommadmndita%2Ftcpip_routing_types.htm)
Static vs. Dynamic Routing (http://www.routeralley.com/ra/docs/static_dynamic_routing.pdf)
Dynamic Routing Protocols (http://www.youtube.com/watch?v=tFdkhMix-SY)
You are required to review the case materials and conduct your own research (at least three additional sources of materials. Please be sure to use appropriate citations for materials that you collect on your own), in your own words, please write a 2- to 3-page paper (excluding the cover page and reference page) to describe at least three metrics and how they are applied in dynamic routing.
SLP Assignment Expectations
Your paper should be between two and three pages. You are expected to understand the metrics and how they are applied in dynamic routing.
Upload your assignment when you are finished.