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CCNA 2 v3.1 Module 6

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CCNA 2

Module 6

Routing & Routed Protocols

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Introducing Routing • Routers must learn the direction to remote

networks in order to forward packets.

• There are 2 ways to learn this information:

Dynamic Routing

Static Routing

• Routers use the routing process to

Forward packets toward the destination network

Decisions based upon the destination IP address

• Dynamic routing

Routers learn information from other routers

Scalable – each change learned from another router

• Static routing

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Static Routing • Configured manually by the network administrator

Add and remove static routes when topology changes

• Static Route Operations are divided into three parts:

Network administrator configures the route

Router installs the route in the routing table

Packets are routed using the static route

• Command

Router#config terminal

Router(config)#ip route 172.16.1.0 255.255.255.0 s0

Router(config)#ip route destination subnet outgoing

network maskinterface

or

next

hop

555

Static Routes using Outgoing Interface

Specifies OutgoingInterface

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Static Route Using Next Hop

SpecifiesNext Hop

777

• The Difference between the 2 options is

Administrative distance assigned to the route

• Administrative distance

Optional

Measures the reliability of the route (0 – 255)

The lower the number the more reliable the route

Set to 1 for next hop

Set to 0 for outgoing interface

• Routers choose the route with the lowest administrative distance

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• To set a static route that is not 1 or 0ip route 192.21.121.0 255.255.255.0 192.21.122.1 130

• Static routes

Can be used as a backup if dynamic route fails

Must have higher admin no. to dynamic routeTry the interactive media lab CCNA 2 Module 6 Page 6.1.2Try the interactive media lab CCNA 2 Module 6 Page 6.1.3

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Configuring a Default Route

• Default routes

Route packets with destinations that do not match any of the other routes in the routing table

Often used for

internet-bound traffic

Non-directly connected networks

• Special form of a static route

ip route 0.0.0.0 0.0.0.0 [next-hop-address or outgoing if]

Example ip route 0.0.0.0 0.0.0.0 s0Try the interactive media lab CCNA 2 Module 6 Page 6.1.4

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Verifying static route configuration

• show running-config

Views the active configuration in RAM to verify that the static route was entered correctly

• show ip route

Make sure that the static route is present in the routing table

• Ping <ip address>

check a connection

• Traceroute <ip address>

Shows the path to the ip address

Can be used to identify where the connection fails

Try Interactive media labs CCNA 2 Module 6 Page 6.1.5

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Dynamic Routing

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Routing Protocols• Allow routers to share information with other

routers regarding

the networks it knows about

its proximity to other routers

• Used to build and maintain a routing table

• Examples:

Routing Information Protocol - RIP

Interior Gateway Routing Protocol - IGRP

Enhanced Interior Gateway Routing Protocol -EIGRP

Open Shortest Path First - OSPF

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• Directs user traffic

• 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

Internet Protocol (IP)

Internetwork Packet Exchange (IPX)

Routed Protocol

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Autonomous Systems

• A collection of networks under

a common administration – e.g., MBNA_Europe

sharing a common routing strategy

• American Registry of Internet Numbers -ARIN

Assigns an identifying number to each AS

It is a 16 bit number

• IGRP and EIGRP have autonomous system numbers

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Purpose of a routing protocol and autonomous systems

• Routing protocol

Learns all available routes

Places the best routes into the routing table

Removes routes that are no longer valid

• Router use information in the routing table

To forward routed protocol packet

• Routing protocols change/update routing tables

When the network topology changes

• Internetwork is converged when

All routers in an internetwork are operating with the same knowledge

Fast convergence is desirable

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Classes of routing protocols

1. Distance vector

Uses distance and direction

2. Link-state

Shortest path first

Recreates the exact topology of the entire internetwork

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Distance vector routing protocol features

• Router passes periodic copies of a routing table

to directly connected router

These updates communicate topology changes

Updates do not allow router to know exact internetwork topology

it only sees its neighbours routers

• Also called Bellman-Ford algorithms

• When a router receives an update where there is an information change

It uses this information to alter the distance and update the routing table

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Distance Vector Concept

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Link-state routing protocol features

• Link-state algorithms are also known as

Dijkstras algorithm

or SPF (shortest path first) algorithms

• A link-state routing algorithm

Maintains a complete database of the topology

A full knowledge of distant routers and how they interconnect

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Link State Routing use

• Link-state advertisements (LSAs)

A small packet of routing information sent between routers

• Topological database

A collection of information gathered from LSAs.

• SPF algorithm (shortest path first)

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 Routing Protocol Features

• Network discovery processes for link state routing

LSAs are exchanged between directly connected routers with information about directly connected networks

These LSAs are accumulated on each router and a topological database is constructed

The SPF algorithm uses this database to calculate shortest path

It then builds a tree, with itself as the root, consisting of all possible paths to each network

It sorts these paths Shortest Path First (SPF)

Lists the best paths and ports to these destination networks in the routing table

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• To achieve convergence

Each router keeps track of its neighbor routers

Router, Name, Interface status, Cost of the link

The first router to become aware of a topology change

Forwards the information to all other routers

Using a Link State Advertisement (LSA)

When a router receives a LSA it

It adds the information to the routers database and the SPF algorithm is run again

• Link-state concerns:

Processor overhead

Memory requirements

Bandwidth Consumption

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Path determination

• A router determines the path using:

A path determination function

A switching function

• Path determination

Occurs at network layer

Router uses the routing table to determine the best path

• Switching

Router accepts a packet on one interface and forward it to a second interface on the same router

Router encapsulates the packet in the appropriate frame type for the next data link

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Routing Configuration

• To enable IP routing protocol on a router

Setting global parameters

and routing parameters

• Global tasks

Selecting a routing protocol (RIP, IGRP, EIGRP or OSPF)

Specify the network

• Routing metric

Help routers find the best path to each network

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• Command Layout

Router(config)#router <protocol> <option>

Router(config-router)#network <network no>

• <protocol> e.g., rip, igrp, ospf…

• <option> e.g., autonomous system no

• <network no> directly connected network

• Commands

Router(config)# router rip

Router(config-router)#network 192.101.1.0

Try lab activity CCNA 2 Module 6 Page 6.3.2

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Routing Protocols

• RIP – 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)

Originally specified in RFC 1058

Interior gateway protocol

Distance vector routing protocol

Metric for path selection is hop count

If hop count is greater than 15, packet discarded

Routing updates broadcast every 30 seconds

• Interior Gateway Routing Protocol (IGRP)

Proprietary protocol developed by Cisco

Distance vector routing protocol

Metrics used are

Bandwidth, load, delay and reliability

Routing updates broadcast every 90 seconds

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• Open Shortest Path First (OSPF)

Nonproprietary CISCO protocol

Link-state routing protocol

Originally described in RFC 2328.

Uses SPF algorithm to calculate the lowest cost to a destination

Routing updates are flooded as topology changes occur

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• EIGRP - Enhanced Interior Gateway Routing Protocol

Cisco proprietary protocol

Enhanced distance vector routing protocol

Uses load balancing

Combination of distance vector and link-state

Calculates shorted path first using Diffused Update Algorithm (DUAL)

Routing updates

Broadcast every 90 seconds

or as triggered by topology changes

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• Border Gateway Protocol (BGP)

Exterior Gateway Protocol

Distance vector routing protocol

Used between ISPs or ISPs and clients

Used to route Internet traffic between autonomous systems

Try Interactive Lab CCNA 2 Module 6 Page 6.3.3

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Exterior Gateway Protocols

• EGP designed for use between

two different networks that are under the control of different organizations, ISPs.

• In order for routing to begin you need:

A list of neighbor routers with which to exchange routing information

A list of networks to advertise as directly reachable

The autonomous system number of the local router

Try interactive lab CCNA 2 Module 6 Page 6.3.4

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Distance vector (RIP or IGRP)• Known as Bellman-Ford algorithm

• Routing decisions based upon information provided by its neighboring routers

• Inform neighbors of your routing table on periodic basis

• Advantages

use few resources

• Disadvantages

Slow convergence

Metrics don’t scale well

• Decisions made based on

Finding the distance (number of hops)

and vector (direction) to any link on the internetwork

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Link-state (OSPF or IS-IS)

• Shortest Path First algorithm

• Flood routing information to all routers

• Each router gets a complete view of internetwork

• Routing updates sent on topology change (LSA)

They are event triggered

Resulting in quick convergence

• Quick convergence

Prevents routing loops

Prevents routing errors

Use more system resources

• Expensive to implement but scalable