ccna exp2 - chapter01 - introduction to routing and packet forwarding

8/3/2019 CCNA Exp2 - Chapter01 - Introduction to Routing and Packet Forwarding

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Chapter 1-Introduction to Routing and PacketForwarding

CCNA Exploration 4.0



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Objectives

• Identify a router as a computer with an OS and hardwaredesigned for the routing process.

• Demonstrate the ability to configure devices and applyaddresses.

• Describe the structure of a routing table.

• Describe how a router determines a path and switchespackets




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





Routers are at the network center

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

• Each network that a router connects to typically requires a separateinterface. These interfaces are used to connect a combination of bothLocal Area Networks (LANs) and Wide Area Networks (WANs).





Routers determine the best path

• The primary responsibility of a router is to direct packets destined forlocal 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 overthe data network. They are responsible forinterconnecting networks by selecting the best pathfor 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

• …



Evolution of Routers

• Multiprotocol devices providing pure data delivery withminimal access control and QoS.

• Intelligent Network Services, this allowed us to providesecurity capabilities like integrated firewall and encryption.

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

• Today the router is an integrated platform for providingadvanced services in security, content delivery, and voice.



Cisco Router Series



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 lostwhen power is off

Read-only memory (ROM) - Holds diagnostic software usedwhen router is powered up. Stores the router’s bootstrap

program. Non-volatile RAM (NVRAM) - Stores startup configuration.

This may include IP addresses (Routing protocol, Hostnameof 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



Router Components(refer to 1.1.2.1)



Router Components



Cisco router 1841’s Architecture - Front Panel

LED Color Status

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 Blinkinggreen

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



Cisco router 1841’s Architecture - Back Panel



Router Components

• Router components



Cisco IOS Software Architecture for 1800 Series

• Identified IOS Software includes: Feature and Version



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



Verify the router boot-up process

• The show version command is used to view

information about the router during the bootupprocess. 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



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

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

Verify the router boot-up process



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:

Fixed: Fastethernet

Modular:

• Serial

• DSL• ISDN

• Fastethernet




Router Interfaces

• Two major groups of Router Interfaces

– LAN Interfaces:

Are used to connect router toLAN network

Has a layer 2 MAC address

Can be assigned a Layer 3 IPaddress

Usually consist of an RJ-45 jack

• WAN Interfaces

Are used to connect routers to external networks that

interconnect LANs. Depending on the WAN technology, a layer 2

address may be used.

Uses a layer 3 IP address




Router Interfaces




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 asrouting.




Routers and the Network Layer

• Routers Operate at Layers 1, 2, and 3




Implementing Basic Addressing Schemes

• At a minimum, the documentation should include a topology diagram

that indicates the physical connectivity and an addressing table thatlists 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.




Basic Router Configuration





• 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 interfaceconfiguration information




Introducing the Routing Table

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

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

Remotely connected networks - this is a network that is notdirectly connected to a particular router

Detailed information about the networks include source ofinformation, Network address & subnet mask, and Ip addressof next-hop router

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




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.




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 directlyconnected network.




Static Routing

• Static routes in the routing table

– Includes: network address and subnet mask and IP address of nexthop router or exit interface.

– Denoted with the code S in the routing table.

– Routing tables must contain directly connected networks used toconnect 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.




Static Routing




Dynamic Routing

• Dynamic routing protocols are used by routers to share information

about the reachability and status of remote networks. Dynamic routingprotocols perform several activities, including:

– Network discovery.

– Updating and maintaining routing tables.

D i R i




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).

R i T bl P i i l




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 provideinformation on how to return to the original network.

R i T bl P i i l





R ti T bl P i i l





P k t Fi ld d F Fi ld




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:

– Destination IP address.

– Source IP address.

– Version & TTL.

– IP header length.

– Precedence & type of service.

– Packet length.

P k t Fi ld d F Fi ld




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

B t P th d M t i




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.

Router Paths and Packet Switching




Router Paths and Packet Switching

• Equal cost metric is a condition where a router has multiple pathsto the same destination that all have the same metric

• To solve this dilemma, a router will use Equal Cost LoadBalancing. This means the router sends packets over themultiple exit interfaces listed in the routing table.

• Equal Cost Paths and Unequal Cost Paths: a router can sendpackets over multiple networks even when the metric is not the

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

Path Determination




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 finda route to destination

– Next hop determined.

– Exit interface determined.

– No route determined.

Switching Function




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 thesame 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.

Switching Function(refer to 1 4 5)




Switching Function(refer to 1.4.5)

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

Switching Function




Switching Function


Switching Function




Switching Function

• Step 2: Router R1 receives the Ethernet frame

Switching Function




Switching Function


Switching Function




Switching Function

• Step 3: Packet arrives at router R2

Switching Function




Switching Function


Switching Function




Switching Function

• Step 4: The packet arrives at R3

Switching Function




Switching Function


Switching Function




Switching Function


Summary




Summary

• Routers are computers that specialize in sending data over a

network.• Routers are composed of: – Hardware i.e. CPU, Memory, System bus, Interfaces – Software used to direct the routing process

IOS

Configuration file• Routers need to be configured. Basic configuration consists of: – Router name – Router banner – Password(s) – Interface configurations i.e. IP address and subnet mask

• Routing tables contain the following information – Directly connected networks – Remotely connected networks – Network addresses and subnet masks – IP address of next hop address

Summary




Summary

• Routers determine a packets path to its destination by

doing the following Receiving an encapsulated frame & examining

destination MAC address.

If the MAC address matches then Frame is de-encapsulated so that router can examine the destination

IP address.

If destination IP address is in routing table or there is astatic route then Router determines next hop IP address.Router will re-encapsulate packet with appropriate layer

2 frame and send it out to next destination. Process continues until packet reaches destination.

Note - only the MAC addresses will change the sourceand destination IP addresses do not change.

ccna exp2 - chapter01 - introduction to routing and packet forwarding

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