CCNA 2 Module 6

Routing and Routing Protocols

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1

Objectives

• Introduction to static routing

• Dynamic routing overview

• Routing protocols overview

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Overview

• Routing is a set of directions to get from one network to another.

• These directions are known as routes

• Routes can be dynamically given to the router by another router

• (or) Route can be statically assigned to the router by an administrator.

• A network administrator chooses a dynamic routing protocol based upon many considerations.

– the size of the network, the bandwidth of available links, the processing power of the routers, the brands and models of the routers

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Introduction to Routing: Route Types

• Routing is the process that a router uses to forward packets toward the destination network.

• A router makes decisions based upon the destination

IP address of a packet.

• To make the correct decisions, routers must learn how to reach remote networks.

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Specifying Outgoing Interface (First command)

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Specifying the Next-hop IP Address (Second

Command)

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Specifying the Next-hop IP Address

Continued

• The administrator could enter either of two commands to accomplish this objective: specifies the outgoing interface or specifies the next-hop IP address of the adjacent router .

• The only difference between the two is in the administrative distance assigned to the route by the router as it is placed in the routing table

The administrative distance is an optional parameter that gives a measure of the reliability of the route.

• A lower value for the administrative distance indicates the more reliable route.

• Thus, a route with a lower administrative distance will be installed before an identical route with a higher administrative distance.

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Specifying the Next-hop IP Address

Continued

• The default administrative distance when using next-hop address is 1 , while the default administrative distance when using the outgoing interface is 0 .

• If an administrative distance other than the default is desired, a value between 0 - 255 is entered after the next-hop or outgoing interface as follows:

• waycross(config)# ip route 172.16.3.0

255.255.255.0 172.16.4.1 130

• To use a static route as a backup, set a higher administrative distance than the dynamic routing protocol.

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Configuring Static Routes

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Use a Local Interface as a Gateway

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Using a Next-Hop Device

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Non-directly Connected Networks (Default Route)

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Verifying Static Route Configuration

• The command show running-config is used to view the active configuration in RAM to verify that the static route was entered correctly.

• The show ip route command is used to make sure that the static route is present in the routing table

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Verifying Static Route Configuration

Continued

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Troubleshooting Static Route Configuration

• The show ip route command

• The ping command

• The traceroute command

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Routed Versus Routing Protocol

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Routed Versus Routing Protocol

Continued

• A routing protocol allows one router to share information with other routers regarding the networks it knows about.

• The information a router gets from another router, using a routing protocol, is used to build and maintain a routing table.

• A routed protocol provides enough information to allow a packet to be forwarded from one host to another based on the addressing scheme

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Autonomous Systems

An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy.

• The American Registry of

Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying number to each AS.

• This autonomous system number is a 16 bit number.

• Routing protocols, such as Cisco’s IGRP, require assignment of a unique, autonomous system number.

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Dynamic Routing Operations

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Purpose of Routing Protocol

• The goal of a routing protocol is to build and maintain a routing table.

• This table contains the learned networks and associated ports for those networks.

• Routers use routing protocols to manage information received from other routers and its interfaces.

• The routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid.

• The router uses the information in the routing table to forward routed protocol packets.

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Purpose of Routing Protocol

• The routing algorithm is fundamental to dynamic routing.

• Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledgebase (routing table) must also change.

• The network knowledgebase needs to reflect an accurate view of the new topology.

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Dynamic Routing Operations

Continued

• When all routers in an internetwork are operating with the same knowledge, the internetwork is said to have converged .

• Fast convergence is desirable because it reduces the period of time in which routers would continue to make incorrect routing decisions.

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Classes of Routing Protocols

• The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork.

• The link-state routing approach , also called shortest path first, recreates the exact topology of the entire internetwork.

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Distance Vector Concepts

• Distance vector based routing algorithms are also known as

Bellman-Ford algorithms

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Distance Vector Concepts

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Distance Vector Concepts

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Routing Metric Components

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Link-State Concepts

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Link-State Concepts

Continued

• Link-state algorithms are also known as Dijkstras algorithm or as SPF (shortest path first) algorithms.

• Link-state routing uses:

• Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers.

Topological database

A topological database is a collection of information gathered from LSAs.

• SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree.

• Routing tables – A list of the known paths and interfaces.

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Link-State Network Discovery

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Link-State Topology Changes

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Link-State Concerns

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Path Determination

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Routing with Network Addressing

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IP Routing Configuration Tasks

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Using the router and network

Commands

• An example of a routing configuration is:

GAD(config)# router rip

GAD(config-router)# network 172.16.0.0

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Routing Protocols: Interior and Exterior Routing

Protocols

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Routing Protocols

Continued

• Examples of IP routing protocols include:

• RIP – A distance vector interior routing protocol

• IGRP – Cisco's distance vector interior routing protocol

• OSPF – A link-state interior routing protocol

• EIGRP – Cisco’s advanced distance vector interior routing protocol

• BGP – A distance vector exterior routing protocol

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Routing Protocols

RIP

• Routing Information Protocol (RIP) was originally specified in RFC 1058. Its key characteristics include the following:

• It is a distance vector routing protocol.

• Hop count is used as the metric for path selection.

• If the hop count is greater than 15, the packet is discarded.

• Routing updates are broadcast every 30 seconds, by default.

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Routing Protocols

IGRP

• Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco.

Some of the IGRP key design characteristics emphasize the following:

• It is a distance vector routing protocol.

• Bandwidth, load, delay and reliability are used to create a composite metric.

• Routing updates are broadcast every 90 seconds, by default.

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Routing Protocols

OSPF

• Open Shortest Path First (OSPF) is a nonproprietary link-state routing protocol. The key characteristics of OSPF are as follows:

• It is a link-state routing protocol.

• Open standard routing protocol described in

RFC 2328.

• Uses the SPF algorithm to calculate the lowest cost to a destination.

• Routing updates are flooded as topology changes occur.

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Routing Protocols

EIGRP

• EIGRP is a Cisco proprietary enhanced distance vector routing protocol. The key characteristics of EIGRP are as follows:

• It is an enhanced distance vector routing protocol.

• Uses load balancing.

• Uses a combination of distance vector and linkstate features.

• Uses Diffused Update Algorithm (DUAL) to calculate the shortest path.

• Routing updates are broadcast every 90 seconds or as triggered by topology changes.

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Routing Protocols

BGP

• Border Gateway Protocol (BGP) is an exterior routing protocol. The key characteristics of

BGP are as follows:

• It is a distance vector exterior routing protocol.

• Used between ISPs or ISPs and clients.

• Used to route Internet traffic between autonomous systems.

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Interior/Exterior Routing Protocols

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Interior/Exterior Routing Protocols

Continued

• Interior routing protocols are designed for use in a network whose parts are under the control of a single organization. The design criteria for an interior routing protocol require it to find the best path through the network.

• An exterior routing protocol is designed for use between two different networks that are under the control of two different organizations. These are typically used between ISPs or between a company and an ISP.

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Summary

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