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Chapter 5Chapter 5

ROUTING ROUTING PROTOCOL PROTOCOL

IGRPIGRP

Semester 3Semester 3

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ObjectiveObjective

Learn that a router can have more than one Internet Protocol (IP) address because it is attached to more than one network.Learn more about how routers operate and what kinds of protocols they use. Describes routing and IP routing protocols and discusses Cisco's proprietary implementation of Interior Gateway Routing Protocol (IGRP).

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THE NETWORK LAYER BASICSTHE NETWORK LAYER BASICS

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TTPath selection and packet switchingPath selection and packet switching

• A router generally relays a packet from one data link to another, using two basic functions: – A path determination function:

The router uses the network portion of the network address to make path selections to pass the packet to the next router along the path.

– A switching function: The switching function allows a router to accept a packet on one interface and forward it through a second interface.

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Path DeterminationPath Determination

• Occurs at the network layer (Layer 3).• Enables a router to evaluate the available

paths to a destination.• Establish the preferred handling of a

packet.

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Find the best pathFind the best path

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Routing processRouting process

• Routing information can be configured by the administrator or collected dynamically

• Provides best-effort end-to-end packet delivery across interconnected networks

• The network layer uses the IP routing table to send packets from the source network to the destination network

• Takes the packet that it accepted on one interface forwards to another interface

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Routing tableRouting table

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Routing packetsRouting packetsTh

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Routing packets (cont.)Routing packets (cont.)

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Routing packets (cont.)Routing packets (cont.)

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Routing packets (cont.)Routing packets (cont.)

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Routing packets (cont.)Routing packets (cont.)

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How distances are determinedHow distances are determined

• Primary objective of routing protocol is to determine the best information to include in the routing table.

• Each routing algorithm interprets what is best in its own way.

• Routing algorithm generates a number, called the metric value, for each path through the network.

• Typically, the smaller the metric number, the better the path.

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Distance in MetricsDistance in MetricsTh

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How the metric is calculatedHow the metric is calculated

• You can calculate metrics based on a single characteristic of a path.

• You can calculate more complex metrics by combining several characteristics.

• The metrics most commonly used by routers are as bandwidth, delay, load, reliability, hop count, ticks and cost.

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Network and host addressingNetwork and host addressing

• The router uses the network address to identify the destination network (LAN) of a packet within an internetwork.

• For some network layer protocols, this relationship is established by a network administrator who assigns network host addresses according to a predetermined internetwork addressing plan.

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Addressing: Addressing: Network and HostNetwork and Host

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Determining network addressDetermining network address

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ROUTED AND ROUTING ROUTED AND ROUTING PROTOCOLSPROTOCOLS

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Routed and Routing ProtocolRouted and Routing Protocol

• Routed protocol:– Provides information in its network layer to

allow forwarding a packet.– Define the field formats within a packet.

• Routing protocols– Support a routed protocol by providing

mechanisms for sharing routing information. – Allows the routers to communicate with other

routers to update and maintain tables.

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NetworkNetwork--layer protocol operations layer protocol operations

• When a host needs to send a packet to a destination on a different network:– Host send the data link frame to the router– Router's network layer process examines the

incoming packet's header– Router references the routing table which

associates networks to outgoing interfaces– The packet is encapsulated in the data link

frame that is appropriate for the outgoing port– The packet is queued for delivery to the next

hop in the path

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Network protocol operationsNetwork protocol operations

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Network protocol operations (cont.)Network protocol operations (cont.)

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Network protocol operations (cont.)Network protocol operations (cont.)

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Network protocol operations (cont.)Network protocol operations (cont.)Th

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Network protocol operations (cont.)Network protocol operations (cont.)

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Network protocol operations (cont.)Network protocol operations (cont.)

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MultiMulti--protocol routingprotocol routing

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IP ROUTING PROTOCOLS IP ROUTING PROTOCOLS

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Routing protocolsRouting protocols

• Routing is the process of determining where to send data packets destined for addresses outside the local network.

• Routers gather and maintain routing information.

• Routing information takes the form of entries in a routing table, with one entry for each identified route.

• Routing protocols allow a router to create and maintain routing tables dynamically and to adjust to network changes.

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Differentiating routing protocolsDifferentiating routing protocols

1.The particular goals of the protocol designer affect the operation of the resulting routing protocol.

2.There are various types of routing protocols. Each protocol has a different effect on network and router resources.

3.Routing protocols use a variety of metrics to identify the best routes.

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TTInterior and Exterior Routing Protocols Interior and Exterior Routing Protocols

• Interior protocols are used for routing information within networks that are under a common network administration.Example: RIP, IGRP.

• Exterior protocols are used to exchange routing information between networks that do not share a common administration.Example: EGP, BGP.

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The goals of routing protocols The goals of routing protocols

• The Optimal Route • Simplicity and Efficiency• Robustness• Rapid Convergence• Flexibility

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The problem of routing loops The problem of routing loops

• Routing loops can occur if a network's slow convergence on a new configuration causes inconsistent routing entries.

• Solutions:– Defining a Maximum– Split Horizon– Split Horizon with Poison Reverse– Route Poisoning– Hold-down Timers

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Static and Dynamic routes Static and Dynamic routes

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Why use a static route Why use a static route

• When a network is accessible by only one path (stub network), a static route to the network can be sufficient.

• Configuring static routing to a stub network avoids the overhead of dynamic routing.

• For security reasons, you may want to hide parts of an internetwork.

• Static routing enables you to specify the information you want to reveal about restricted networks.

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Stub network Stub network Th

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Default routing Default routing

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Dynamic routes Dynamic routes

• The administrator enters configuration commands to start dynamic routing.

• Routing knowledge is automatically updated by a routing process whenever new information is received from the internetwork.

• Changes in dynamic knowledge are exchanged between routers as part of the update process.

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Why dynamic routing is necessary Why dynamic routing is necessary

• Dynamic routing offers more flexibility. • When a router recognizes that the link to

other router is down, it adjusts its routing table.

• Load-sharing: Dynamic routing protocols can also direct traffic from the same session over different paths in a network for better performance.

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Classes of routing protocolsClasses of routing protocols

• Most routing algorithms can be classified as one of two basic algorithms distance vector and link state.

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

• The link-state approach re-creates the exact topology of the entire internetwork.

• The balanced hybrid approach combines aspects of the link-state and distance-vector algorithms.

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Choosing a routing protocol Choosing a routing protocol

• Network size and complexity • Network traffic levels • Security needs • Reliability needs • Network delay characteristics • Organizational policies • Organizational acceptance of change

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IGRP OPERATIONIGRP OPERATION

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TTIGRP Routing ProtocolIGRP Routing Protocol

• IGRP is a Cisco proprietary protocol and was developed to supercede RIP.

• IGRP is a distance-vector interior routing protocol.

• IGRP uses a combination of metrics. Network delay, bandwidth, reliability, and load are all factored into the routing decision.

• Network administrators can determine the settings for each of these metrics.

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Distance vector algorithm Distance vector algorithm

• Routing algorithms pass periodic copies of a routing table from router to router.

• These regular updates between routers communicate topology changes.

• Each router receives a routing table from its directly connected neighbors.

• Distance-vector algorithms do not allow a router to know the exact topology of an internetwork.

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Distance vector concepts Distance vector concepts

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Exchange routing tablesExchange routing tables

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IGRP MetricIGRP Metric

• Default: k1 = 1, k2 = 0, k3 = 1, k4 = 0, k5 = 0.• Delay is the sum of all the delays of the

links along the paths.– Delay = [Delay in 10s of microseconds]×256.

• BW is the lowest bandwidth of the links along the paths.– BW = [10000000 / (bandwidth in Kbps)]×256.

k1xBW +k2xBW

256 – Load + k3xDelayk5

Rel + k4Metric =

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Autonomous System Autonomous System

• An AS is a collection of networks under common administration sharing a common routing strategy.

• IGRP uses a combination of user-configurable metrics, including network delay, bandwidth, reliability, and load.

• IGRP advertises three types of routes: interior, system, and exterior.

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TTInterior Routes Interior Routes

• Interior routes are routes between subnets in the network attached to a router interface.

• If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

• Subnet information is not included in IGRP updates, which poses a problem for discontiguous IP subnets.

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System Routes System Routes

• System routes are routes to other major networks within the AS.

• The router derives system routes from directly connected network interfaces and system route information provided by other routers that use IGRP.

• System routes do not include subnettinginformation.

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Exterior Routes Exterior Routes

• Exterior routes are routes to networks outside the AS that are considered when identifying a gateway of last resort.

• The router chooses a gateway of last resort from the list of exterior routes that IGRP provides.

• If the AS has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.

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Autonomous System Autonomous System

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Autonomous System: Autonomous System: ExampleExample

AS = 100 AS = 200

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IGRP Routing updatesIGRP Routing updates

• Update (90s): IGRP update broadcast. • Invalid (270s): declare a route inaccessible

if it does not receive an update.• Hold-down (280s): router wait before

believing any routing information.• Flushed (630s): the router removes the

route from the routing table. • IGRP uses flash update and poison

reverse to speed up the convergence of the routing protocol.

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TTThe maximum hop countThe maximum hop count

• Maximum hop count of 255, which is normally set lower than the default 100.

• Should set the maximum hop count to something smaller.

• It should be a number at least as large as the maximum number of routers a route might ever have to go.

• If you exchange IGRP routing with an external network, the hop count must include your network plus that network.

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RIP and IGRP RIP and IGRP

23216Infinity value

NoNoMask sent

YesYesFlash updates

280s180sHolddown timer

BW, delay, load, reliability, MTUHopMetric

90s30sUpdate timer

IGRP (default)RIPv1 (default)Feature

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IGRP CONFIGURATIONIGRP CONFIGURATION

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Routing configurationRouting configuration

• router igrp autonomous-system

• network network-number• passive-interface type number• timers basic upd inv hld flush

• metric maximum-hops hop-count• show ip route• show ip protocols• debug ip igrp transactions

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LAB TopologyLAB Topology