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Học viện mạng Bach Khoa - Website: www.bkacad.com 32Basic Router Configuration • Verifying Basic Router Configuration – Issue the show running-config command – Save the basic router conf

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Chapter 1-Introduction to Routing and Packet

Forwarding

CCNA Exploration 4.0

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Học viện mạng Bach Khoa - Website: www.bkacad.com 2

• Describe the structure of a routing table.

• Describe how a router determines a path and switches

packets

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Routers are Computers

• Routers have many of the same hardware and software components that are found in other computers including:

– CPU

– RAM

– ROM

– Operating System

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Routers are at the network center

• A router connects multiple networks This means that it has multiple interfaces that each belong to a different IP network

• Each network that a router connects to typically requires a separate

interface These interfaces are used to connect a combination of both Local Area Networks (LANs) and Wide Area Networks (WANs)

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Routers determine the best path

• The primary responsibility of a router is to direct packets destined for local and remote networks by:

– Determining the best path to send packets

– Forwarding packets toward their destination

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Router as a Computer

• Describe the basic purpose of a router

– Computers that specialize in sending packets over

the data network They are responsible for

interconnecting networks by selecting the best path

for a packet to travel and forwarding packets to

their destination

• Routers are the network center

– Routers generally have 2 connections:

• WAN connection (Connection to ISP)

• LAN connection

• PSTN connection

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Evolution of Routers

• Multiprotocol devices providing pure data delivery with

minimal access control and QoS.

• Intelligent Network Services, this allowed us to provide

security capabilities like integrated firewall and encryption.

• Integrated voice and data capabilities, allowing routers to provide voice gateway features for IP telephony.

• Today the router is an integrated platform for providing

advanced services in security, content delivery, and voice.

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Cisco Router Series

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Router Components

• Router components and their functions”

 CPU - Executes operating system instructions

 Random access memory (RAM) - Contains the running copy

of configuration file Stores routing table RAM contents lost

when power is off

 Read-only memory (ROM) - Holds diagnostic software used

when router is powered up Stores the router’s bootstrap

program

 Non-volatile RAM (NVRAM) - Stores startup configuration

This may include IP addresses (Routing protocol, Hostname

of router)

 Flash memory - Contains the operating system (Cisco IOS)

 Interfaces - There exist multiple physical interfaces that are

used to connect network Examples of interface types:

• Ethernet / fast Ethernet interfaces

• Serial interfaces

• Management interfaces

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Cisco router 1841’s Architecture - Front Panel

SYS PWR Green Router has successfully booted up and the software is functional

Slow, steady blinking when system is booting or in the ROM monitor

SYS ACT Green Blinking when packets are transmitted or received on any WAN

or LAN interface, or when monitoring system activity

CF Blinking

green

Flash memory is busy Do not remove the CompactFlash memory card when this light is on

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Cisco router 1841’s Architecture - Back Panel

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• Router components

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Cisco IOS Software Architecture for 1800 Series

• Identified IOS Software includes: Feature and Version

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Router Boot-up Process

Bootup Process

• There are four major phases to the bootup process:

– 1 Performing the POST

– 2 Loading the bootstrap program

– 3 Locating and loading the Cisco IOS software

– 4 Locating and loading the startup configuration file or entering setup

mode

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Verify the router boot-up process

• The show version command is used to view

information about the router during the bootup

process Information includes:

 Platform model number

 Image name & IOS version

 Bootstrap version stored in ROM

 Image file name & where it was loaded from

 Number & type of interfaces

 Amount of NVRAM

 Amount of flash

 Configuration register

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• The show version command can be used to help verify and

troubleshoot some of the basic hardware and software components of the router.

Verify the router boot-up process

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Router Interfaces

• Router Interface is a physical connector that enables a

router to send or receive packets

• Each interface connects to a separate network

• Consist of socket or jack found on the outside of a router

• Types of router interfaces:

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 Has a layer 2 MAC address

 Can be assigned a Layer 3 IP address

 Usually consist of an RJ-45 jack

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Router Interfaces

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Routers and the Network Layer

• The main purpose of a router is to connect multiple networks and

forward packets destined either for its own networks or other networks

• A router is considered a Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet,

specifically the destination IP address This process is known as

routing

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Routers and the Network Layer

• Routers Operate at Layers 1, 2, and 3

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Implementing Basic Addressing Schemes

• At a minimum, the documentation should include a topology diagram that indicates the physical connectivity and an addressing table that

lists all of the following information:

– Device names

– Interfaces used in the design

– IP addresses and subnet masks

– Default gateway addresses for end devices, such as PCs

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Basic Router Configuration

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• Verifying Basic Router Configuration

– Issue the show running-config command

– Save the basic router configuration by Issuing the copy

running-config startup-config command

– Additional commands that will enable you to further verify

router configuration are:

 Show running-config - Displays configuration currently in RAM

 Show startup-config - Displays configuration file NVRAM

 Show IP route - Displays routing table

 Show interfaces - Displays all interface configurations

 Show IP int brief - Displays abbreviated interface configuration information

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Introducing the Routing Table

• Routing Table is stored in ram and contains information about:

 Directly connected networks - this occurs when a device is connected to another router interface

 Remotely connected networks - this is a network that is not directly connected to a particular router

 Detailed information about the networks include source of

information, Network address & subnet mask, and Ip address

of next-hop router

• Show ip route command is used to view a routing table

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Introducing the Routing Table

• The following analogies may help clarify the concept of

connected, static, and dynamic routes:

– Directly Connected Routes.

– Static Routes

– Dynamic Routes.

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Adding a Connected Network to the Routing Table

• Once the interface is " up “ and “ have an ip address” , the network

of that interface is added to the routing table as a directly

connected network.

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Static Routing

• Static routes in the routing table

– Includes: network address and subnet mask and IP address of next hop router or exit interface

– Denoted with the code S in the routing table

– Routing tables must contain directly connected networks used to connect remote networks before static or dynamic routing can be used

• When to use static routes

– When network only consists of a few routers

– Network is connected to internet only through one ISP

– Hub & spoke topology is used on a large network

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Static Routing

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Dynamic Routing

• Dynamic routing protocols are used by routers to share information

about the reachability and status of remote networks Dynamic routing protocols perform several activities, including:

– Network discovery

– Updating and maintaining routing tables

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Dynamic Routing

• Dynamic routing features:

– Automatic Network Discovery

– Maintaining Routing Tables

• IP Routing Protocols

– RIP (Routing Information Protocol)

– IGRP (Interior Gateway Routing Protocol)

– EIGRP (Enhanced Interior Gateway Routing Protocol)

– OSPF (Open Shortest Path First)

– IS-IS (Intermediate System-to-Intermediate System)

– BGP (Border Gateway Protocol)

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Routing Table Principles

• Routing Table Principles

– 3 principles regarding routing tables:

 Every router makes its decisions alone, based on the information it has in its routing table

 One router’s information in the routing table may not be the same as another router’s routing table

 Routing information about a path to a network does not provide information on how to return to the original network

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Packet Fields and Frame Fields

• Internet Protocol (IP) packet format contains fields that provide

information about the packet and the sending and receiving hosts

• Fields that are importance for CCNA students:

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Packet Fields and Frame Fields

• MAC Layer Frame Format

• MAC Frames are also divided into fields They include:

– Preamble

– Start of frame delimiter

– Destination MAC address

– Source MAC address

– Type/length

– Data and pad

– Frame check sequence

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Best Path and Metric

• A Metric is a numerical value used by routing protocols to assess a

given route

• Metrics may be based on a single path variable or multiple variables of

a path

• 2 types of metrics used by routing protocols are:

– Hop count - this is the number of routers a packet must travel

through to get to its destination

– Bandwidth - this is the “speed” of a link also known as the data

capacity of a link

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Router Paths and Packet Switching

• Equal cost metric is a condition where a router has multiple paths

to the same destination that all have the same metric

• To solve this dilemma, a router will use Equal Cost Load

Balancing This means the router sends packets over the

multiple exit interfaces listed in the routing table

• Equal Cost Paths and Unequal Cost Paths: a router can send

packets over multiple networks even when the metric is not the

same if it is using a routing protocol that has this capability

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Path Determination

• Path determination enables a router to evaluate paths to a destination

& to establish the preferred way to handle the packet

• One of 3 things happens when a router searches a routing table to find

a route to destination

– Next hop determined

– Exit interface determined

– No route determined

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Switching Function

• Switching Function of Router is the process used by a router to switch

a packet from an incoming interface to an outgoing interface on the

same router

– A packet received by a router will do the following:

 Strips off layer 2 headers

 Examines destination IP address located in Layer 3 header to find best route to destination

 Re-encapsulates layer 3 packet into layer 2 frame

 Forwards frame out exit interface

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• Step 1: PC1 has a packet to be sent to PC2

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• Step 2: Router R1 receives the Ethernet frame

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• Step 3: Packet arrives at router R2

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• Step 4: The packet arrives at R3

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• Step 4: The packet arrives at R3

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