Cnap 2 06 Routing & Routing Protocols

© 2003, The SaigonCTT – A Regional Cisco Networking Academy

CNAP Semester <x> – <Chapter name><Chapter> - 1

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CNAP Semester 2: CNAP Semester 2: Router and Routing BasicsRouter and Routing Basics

Module 6Module 6

ROUTING AND ROUTING AND

ROUTING ROUTING

PROTOCOLSPROTOCOLS

Version 3.0Cisco Regional Networking Academymailto: [email protected]



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ObjectivesObjectives

• Explain the significance of static routing • Configure static and default routes • Identify the classes of routing protocols • Identify distance vector routing protocols • Identify link-state routing protocols • Describe the basic characteristics of common

routing protocols • Identify interior gateway protocols • Identify exterior gateway protocols • Enable Routing Information Protocol (RIP) on a

router



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Table of ContentTable of Content

Routing protocols overview3Dynamic routing overview2Introduction to static route1



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INTRODUCTION TO STATIC ROUTING INTRODUCTION TO STATIC ROUTING



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Introducing Routing Introducing Routing

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

• Static route & dynamic route

• Static routes must be configured manually

• Network topology changes require the network administrator to add and delete static routes to account for the changes.

• Static route requires a tremendous amount of adm. time.

• Static routing does not have the scalability of dynamic routing.

– 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. All devices along the way use the destination IP address to point the packet in the correct direction so that the packet eventually arrives at its destination. In order to make the correct decisions, routers must learn the direction to remote networks. When routers use dynamic routing, this information is learned from other routers. When static routing is used, a network administrator configures information about remote networks manually. Because static routes must be configured manually, any network topology changes require the network administrator to add and delete static routes to account for the changes. In a large network this manual maintenance of routing tables could require a tremendous amount of administrative time. On small networks with few possible changes, static routes require very little maintenance. Because of the extra administrative requirements, static routing does not have the scalability of dynamic routing. Even in large networks, static routes that are intended to accomplish a specific purpose are often configured in conjunction with a dynamic routing protocol



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Static Route Operation Static Route Operation

Sterling Hoboken waycross

172.16.1.1/24 172.16.3.1/24 172.16.5.1/24

172.16.2.1

/24

172.16.2.2

/24

172.16.4.1

/24

172.16.4.2

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S0 S0S1 S1

Router(config) ip route des_net subnet_mask [next-hop-address | outgoing interface]

Hobeken(config)# ip route 172.16.1.0 255.255.255.0 s1Command des. Net subnetmask interface

Hobeken(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1Command des. Net subnetmask nexthop add

– Static route operations can be divided into these three parts: – Network administrator configures the route – Router installs the route in the routing table – Packets are routed using the static route – Since a static route is manually configured, the administrator must configure the static route on the router

using the ip route command. The correct syntax for the ip route command is shown in Figure .– In Figures and , the network administrator of the Hoboken router needs to configure a static route pointing

to the 172.16.1.0/24 and 172.16.5.0/24 networks on the other routers. The administrator could enter either of two commands to accomplish this objective. The method in Figure specifies the outgoing interface. The method in Figure specifies the next-hop IP address of the adjacent router. Either of the commands will install a static route in the routing table of Hoboken. 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. 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 and 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– If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed

in the routing table. This means if that interface is down, the route will not be placed in the routing table.– Sometimes static routes are used for backup purposes. A static route can be configured on a router that will

only be used when the dynamically learned route has failed. To use a static route in this manner, simply set the administrative distance higher than that of the dynamic routing protocol being used.



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Configuring Static Routes Configuring Static Routes

• Determine all desired destination networks, their subnet masks, and their gateways. Enter global configuration mode.

• Type the ip route command with a destination address and subnet mask followed by their corresponding gateway from Step one. Including an administrative distance is optional.

• Repeat Step three for as many destination networks as were defined in Step one.

• copy running-config startup-config command.

– This section lists the steps for configuring static routes and gives an example of a simple network for which static routes might be configured. Use the following steps to configure static routes:

– Determine all desired destination networks, their subnet masks, and their gateways. A gateway can be either a local interface or a next hop address that leads to the desired destination.

– Enter global configuration mode. – Type the ip route command with a destination address and subnet mask followed by their

corresponding gateway from Step one. Including an administrative distance is optional. – Repeat Step three for as many destination networks as were defined in Step one. – Exit global configuration mode. – Save the active configuration to NVRAM by using the copy running-config startup-config

command.



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Configuring Static Routes: ExampleConfiguring Static Routes: Example


172.16.1.1/24 172.16.3.1/24 172.16.5.1/24

172.16.2.1

/24

172.16.2.2

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172.16.4.1

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172.16.4.2

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S0 S0S1 S1

Hobeken(config)# ip route 172.16.1.0 255.255.255.0 s1Command des. Net subnetmask gateway

Hobeken(config)# ip route 172.16.5.0 255.255.255.0 s0Command des. Net subnetmask gateway

– The example network is a simple three-router configuration. Hoboken must be configured so that it can reach the 172.16.1.0 network and the 172.16.5.0 network. Both of these networks have a subnet mask of 255.255.255.0.

– Packets that have a destination network of 172.16.1.0 need to be routed to Sterling and packets that have a destination address of 172.16.5.0 need to be routed to Waycross. Static routes can be configured to accomplish this task.

– Both static routes will first be configured to use a local interface as the gateway to the destination networks. Since the administrative distance was not specified, it will default to 0 when the route is installed in the routing table. Note that an administrative distance of 0 is the same as a directly connected network.

– The same two static routes can also be configured using a next-hop address as their gateway. The first route to the 172.16.1.0 network has a gateway of 172.16.2.1. The second route to the 172.16.5.0 network has a gateway of 172.16.4.2. Since the administrative distance was not specified, it defaults to 1.



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Configuring Default Route ForwardingConfiguring Default Route Forwarding


172.16.1.1/24 172.16.3.1/24 172.16.5.1/24

172.16.2.1

/24

172.16.2.2

/24

172.16.4.1

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172.16.4.2

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S0 S0S1 S1

Hobeken(config)# ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

Default routes are used to route packets with destinations that do not match any of the other routes in the routing table

Hobeken(config)# ip route 0.0.0.0 0.0.0.0 s0

– Default routes are used to route packets with destinations that do not match any of the other routes in the routing table. Routers are typically configured with a default route for Internet-bound traffic, since it is often impractical and unnecessary to maintain routes to all networks in the Internet. A default route is actually a special static route that uses this format: ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

– The 0.0.0.0 mask, when logically ANDed to the destination IP address of the packet to be routed, will always yield the network 0.0.0.0. If the packet does not match a more specific route in the routing table, it will be routed to the 0.0.0.0 network.

– Use the following steps to configure default routes:Enter global configuration mode. Type the ip route command with 0.0.0.0 for the destination network address and 0.0.0.0 for the subnet mask. The gateway for the default route can be either the local router interface that connects to the outside networks or the IP address of the next-hop router. In most cases, it is preferred that the IP address of the next hop router is specified. Exit global configuration mode. Save the active configuration to NVRAM by using the copy running-config startup-configcommand.

– In the section Configuring Static Routes, static routes were configured on Hoboken to make networks 172.16.1.0 on Sterling and 172.16.5.0 on Waycross accessible. It should now be possible to route packets to both of these networks from Hoboken. However, neither Sterling nor Waycross will know how to return packets to any non-directly connected network. A static route could be configured on Sterling and Waycross for each of the non-directly connected destination networks. This would not be a scalable solution on a larger network.

– Sterling connects to all non-directly connected networks via interface Serial 0. Waycross has only one connection to all non-directly connected networks. This is through interface Serial 1. A default route on both Sterling and Waycross will provide routing for all packets that are destined for non-directly connected networks



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Verifying & Troubleshoot Static Route Configuration Verifying & Troubleshoot Static Route Configuration

• show running-config to view the active configuration

• show ip route to verify that the route that was configured is in the routing table.

• Ping• Traceroute

– After static routes are configured it is important to verify that they are present in the routing table and that routing is working as expected. 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 iproute command is used to make sure that the static route is present in the routing table. Use the following steps to verify static route configuration:

• In privileged mode enter the command show running-config to view the active configuration.

• Verify that the static route has been correctly entered. If the route is not correct, it will be necessary to go back into global configuration mode to remove the incorrect static route and enter the correct one.

• Enter the command show ip route. • Verify that the route that was configured is in the routing table.



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DYNAMIC ROUTING OVERVIEW



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Introduction To Routing Protocols Introduction To Routing Protocols

• A routing protocol allows one router to share information with other routers. – E.g: RIP, IGRP, EIGRP, OSPF

• A routed protocol provides enough information in its network layer address to allow a packet to be forwarded from one host to another based on the addressing scheme.– E.g IP,IPX,…

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

– Routing protocols are different from routed protocols in both function and task. A routing protocol is the communication used between routers. A routing protocol allows one router to share information with other routers regarding the networks it knows about as well as its proximity to other routers. The information a router gets from another router, using a routing protocol, is used to build and maintain a routing table.

– Examples of routing protocols are:• Routing Information Protocol (RIP) • Interior Gateway Routing Protocol (IGRP) • Enhanced Interior Gateway Routing Protocol (EIGRP) • Open Shortest Path First (OSPF)

– A routed protocol is used to direct user traffic. A routed protocol provides enough information in its network layer address to allow a packet to be forwarded from one host to another based on the addressing scheme.

– Examples of routed protocols are:Internet Protocol (IP) Internetwork Packet Exchange (IPX)



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Purpose Of A Routing Protocol & Autonomous SystemsPurpose Of A Routing Protocol & Autonomous Systems

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

• The routing protocol learns all available routes, places the best routes into the routing table

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

• Autonomous systems (AS) provide the division of the global internetwork into smaller and more manageable networks.

– The goal of a routing protocol is to build and maintain the 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, information learned from the configuration of its own interfaces, along with manually configured routes. 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.

– The routing algorithm is fundamental to dynamic routing. Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledgebase must also change. The network knowledgebase needs to reflect an accurate consistent view of the new topology.

– 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.

– Autonomous systems (AS) provide the division of the global internetwork into smaller and more manageable networks. Each AS has its own set of rules and policies and an AS number that will uniquely distinguish it from other autonomous systems throughout the world.



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Identifying The Classes Of Routing ProtocolsIdentifying The Classes Of Routing Protocols

• Most routing algorithms can be classified into one of two categories: – The distance vector routing approach determines the

direction (vector) and distance to any link in the internetwork.

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

– Most routing algorithms can be classified into one of two categories: o distance vector o link-state

– The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork. The link-state approach, also called shortest path first, recreates the exact topology of the entire internetwork.



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Distance Vector Routing Protocol FeaturesDistance Vector Routing Protocol Features

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0

10.1.0.0 0

Routing Table

10.2.0.0 0

10.2.0.0 0

Routing Table

10.3.0.0 0

10.3.0.0 0

Routing Table

10.4.0.0 0

10.3.0.0 1 10.1.0.0 1 10.2.0.0 1

10.4.0.0 1

10.4.0.0 2 10.1.0.0 2

– Distance vector routing algorithms pass periodic copies of a routing table from router to router. These regular updates between routers communicate topology changes. Distance vector based routing algorithms are also known as Bellman-Ford algorithms. Each router receives a routing table from its directly connected neighbor routers. Router B receives information from Router A. Router B adds a distance vector number (such as a number of hops), which increases the distance vector. Then Router B passes this new routing table to its other neighbor, Router C. This same step-by-step process occurs in all directions between neighbor routers.

– The algorithm eventually accumulates network distances so that it can maintain a database of network topology information. However, distance vector algorithms do not allow a router to know the exact topology of an internetwork as each router only sees its neighbor routers.

– Each router that uses distance vector routing begins by identifying its own neighbors. The interface that leads to each directly connected network is shown as having a distance of 0. As the distance vector network discovery process proceeds, routers discover the best path to destination networks based on the information they receive from each neighbor. Router A learns about other networks based on the information that it receives from Router B. Each of the other network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction.

– Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost as defined by its metric and the logical address of the first router on the path to each network contained in the table.

– An analogy of distance vector could be the signs found at a highway intersection. A sign points towards a destination and indicates the distance to the destination. Further down the highway, another sign points toward the destination, but now the distance is shorter. As long as the distance is shorter, the traffic is following the best path.



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6

LinkLink--state Routing Protocol Features state Routing Protocol Features

10.3.0.0 10.4.0.0

B

10.2.0.0 10.3.0.0

CA

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0

Hello Hello

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CA

10.3.0.0 10.4.0.0

B

A B C

SPF

LSU

LSU

Routing TableRouting Table

10.1.0.010.1.0.0 00

10.2.0.010.2.0.0 00

10.3.0.010.3.0.0 11

10.4.0.010.4.0.0 22

– The second basic algorithm used for routing is the link-state algorithm. Link-state algorithms are also known as Dijkstrasalgorithm or as SPF (shortest path first) algorithms. Link-state routing algorithms maintain a complex database of topology information. The distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. 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.

– Network discovery processes for link state routingLSAs are exchanged between routers starting with directly connected networks for which they have direct information. Each router in parallel with the others constructs a topological database consisting of all the exchanged LSAs.

– The SPF algorithm computes network reachability. The router constructs this logical topology as a tree, with itself as the root, consisting of all possible paths to each network in the link-state protocol internetwork. It then sorts these paths Shortest Path First (SPF). The router lists the best paths and the interfaces to these destination networks in the routing table. It also maintains other databases of topology elements and status details.

– The router that first becomes aware of a link-state topology change forwards the information so that all other routers can use it for updates. This involves sending common routing information to all routers in the internetwork. To achieve convergence, each router keeps track of its neighbor routers, the router name, interface status, and the cost of the link to the neighbor. The router constructs an LSA packet that lists this information along with new neighbors, changes in link costs, and links that are no longer valid. The LSA packet is then sent out so that all other routers receive it.

– When the router receives an LSA, the database is updated with the most recent information and computes a map of the internetwork using the accumulated data and calculates the shortest path to other networks using the SPF algorithm. Each time an LSA packet causes a change to the link-state database, SPF recalculates the best paths and updates the routing table.

– Link-state concerns:• Processor overhead • Memory requirements • Bandwidth Consumption

– Routers running link-state protocols require more memory and perform more processing than distance vector routing protocols. Routers must have sufficient memory to be able to hold all the information from the various databases, the topology tree, and the routing table. Initial link-state packet flooding consumes bandwidth. During the initial discovery process, all routers using link-state routing protocols send LSA packets to all other routers. This action floods the internetwork and temporarily reduces bandwidth available for routed traffic carrying user data. After this initial flooding, link-state routing protocols generally require only minimal bandwidth to send infrequent or event triggered LSA packets reflecting topology changes.



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ROUTING PROTOCOLS OVERVIEW



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Path Determination Path Determination

• Path determination occurs at the network layer.

• The path determination function enables a router to evaluate the paths to a destination and to establish the preferred handling of a packet

• The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router.

– A router determines the path of a packet from one data link to another, using two basic functions: – A path determination function – A switching function – Path determination occurs at the network layer. The path determination function enables a router to

evaluate the paths to a destination and to establish the preferred handling of a packet. The router uses the routing table to determine the best path and proceeds to forward the packet using the switching function. -

– The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router. A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.



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Routing Configuration Routing Configuration

Network 172.16.0.0

Network 172.30.0.0

RIP

RIP

RIP

IGRP

IGRP

Network 160.89.0.0

Global configuration

Select routing protocol(s)Specify Network(s)

Interface configuration

Verify address/subnet mask

– Enabling an IP routing protocol on a router involves the setting of both global and routing parameters. Global tasks include selecting a routing protocol, such as RIP, IGRP, EIGRP or OSPF. The major task in the routing configuration mode is to indicate IP network numbers. Dynamic routing uses broadcasts and multicasts to communicate with other routers. The routing metric helps routers find the best path to each network or subnet.

– The router command starts a routing process. – The network command is required because it enables the routing process to determine which

interfaces participate in the sending and receiving of routing updates. – An example of a routing configuration is:– GAD(config)#router rip

GAD(config-router)#network 172.16.0.0– The network numbers are based on the network class addresses, not subnet addresses or individual

host addresses. Major network addresses are limited to Class A, B, and C network numbers.



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Routing Configuration ( cont. )Routing Configuration ( cont. )

• Router(config)# router protocol {options}– Difines an ip routing protocol

– Protocol: IGRP, OSPF,EIGRP,RIP– Option: autonomous system or process id if required

• Router(config-router)# network network-number– The network subcommand is a mandatory configuration

command for each ip routing process– Network-number: specifies a directly connected

network



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IGPIGPIGP

Routing Protocols Routing Protocols

AS 100AS 200

EGP(BGP)

RIP,IGRP,EIGRP,OSPF

– At the Internet layer of the TCP/IP suite of protocols, a router can use an IP routing protocol to accomplish routing through the implementation of a specific routing algorithm. 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 Information Protocol (RIP)Routing Information Protocol (RIP)

• 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.

– 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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Interior Gateway Routing Protocol (IGRP)Interior Gateway Routing Protocol (IGRP)

• 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.

– 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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Open Shortest Path First (OSPF)Open Shortest Path First (OSPF)

• 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

– 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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Enhanced Interior Gateway Routing Protocol (EIGRP) Enhanced Interior Gateway Routing Protocol (EIGRP)

• It is an enhanced distance vector routing protocol.

• Uses load balancing.

• Uses a combination of distance vector and link-state features.

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

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

– 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 link-state 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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Summary Summary

• Routing protocols • Autonomous systems • The classes of routing protocols • Routing configuration • Routing protocols (RIP, IGRP, OSPF, EIGRP,

BGP) • Autonomous systems and IGP versus EGP • Distance vector routing • Link-state routing •



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Cnap 2 06 Routing & Routing Protocols

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