ca_ex_s2m01_introduction to routing and packet forwarding
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Chapter 1 - Introduction to Routing and
Packet Forwarding
CCNA Ex loration 4.0
Please purchase apersonal license.
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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.
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packets
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Labs
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Inside the Router
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Routers are Computers
Routers have many of the same hardware and software componentsthat are found in other computers including:
CPU
RAM
ROM
Operating System
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Routers are Computers
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 are Computers
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Routers are Computers
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Routers determine the best path
The primary responsibility of a router is to direct packets destined for local andremote networks by:
Determining the best path to send packets
Forwarding packets toward their destination
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Routers are Computers
Routers determine the best path
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Routers are Computers
Routers determine the best path
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Routers are Computers
Routers determine the best path
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Router as a Computer
Describe the basic purpose of a routerComputers that specialize in sending packets over
the data network. They are responsible forinterconnecting networks by selecting the best pathfor a packet to travel and forwarding packets totheir destination
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Routers are the network centerRouters generally have 2 connections:
WAN connection (Connection to ISP)
LAN connectionNot common:
PSTN connection
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Router as a Computer
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"How Routers Work"http://computer.howstuffworks.com/router.htm
Activity 1.1.1.4
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Evolution of Routers
Multiprotocol devices providing pure data delivery with
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.
Intelligent Network Services, this allowed us to providesecurity 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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Extra: Integrated Services Routers
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The network router is quickly evolving from a device dedicated to connecting disparatenetworks to an integrated services device capable of multiple functions beyond routing.
Now, Integrated Services Routers can deliver voice, video, data and Internet access,wireless, and other applications.
Benefits of the Integrated Services Network Router
One Device, Multiple Functions Same Access at Headquarters and Remote Sites
Centralized Management
Integrated Network Security
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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 whenpower is off
Read-only memory (ROM) - Holds diagnostic software used whenrouter is powered up. Stores the routers bootstrap program.
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on-vo a e - ores s ar up con gura on. s
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 toconnect network. Examples of interface types:
Ethernet / FastEthernet interfaces Serial interfaces
Management interfaces
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Router Components
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Router Components
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Cisco router 1841s Architecture - Front Panel
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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 Blinking
green
Flash memory is busy. Do not remove the CompactFlash memory card
when this light is on.
http://www.cisco.com/cdc_content_elements/flash/nextgen/webversion/1800/kaon/1841/index.html?NO_NAV
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Cisco router 1841s Architecture - Back Panel
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Cisco router 1841s Architecture - Internal
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A SO-DIMM, or small outline dual in-line memory module, is a type ofcomputer memory integrated circuit
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Demo: View Interactive 3D Model of the Cisco1841 Integrated Services Router
http://www.cisco.com/cdc_content_elements/flash/nextgen/webversion/1800/kaon/1841/index.html?NO_NAV
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Router Components
Activity 1.1.2.2
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Internetwork Operating System
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Cisco IOS manages the hardware and software resources of the router,including memory allocation, processes, security, and file systems. Cisco IOS
is a multitasking operating system that is integrated with routing, switching,internetworking, and telecommunications functions.
Cisco creates many different types of IOS images, depending upon the modelof the router and the features within the IOS.
The command line interface (CLI) is a much more common method of
configuring Cisco routers.
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Cisco IOS Software Architecture for 1800 Series
Identified IOS Software includes: Feature and Version
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Cisco IOS Packaging
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Baseentry level image (IP Base, Enterprise Base) Servicesaddition of IP Telephony Service, MPLS, Voice over IP
(VoIP), Voice over Frame Relay (VoFR), and ATM (SP Services,Enterprise Services)
Advancedaddition of VPN, Cisco IOS Firewall, 3DES encryption,SSH, Cisco IOS IPsec and Intrusion Detection Systems (IDS)(Advanced Security, Advanced IP Services)
Enterpriseaddition of multi-protocols, including IBM, IPX, AppleTalk(Enterprise Base, Enterprise Services)
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Product Research Tools
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Router Boot-up Process
1. Performing the POST2. Loading the bootstrap program3. Locating and loading the Cisco IOS software
4. Locating and loading the startup configuration file or entering setupmode
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Router Boot-up Process
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Router Boot-up Process
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1. ROM
1. POST
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM)
Forth Hex : 0 = ROM Monitor mode
1 = ROM IOS2 - 15 = startup-config in NVRAM
2. Check for IOS boot system commands in startup-config file (NVRAM)
If boot system commands in startup-config
Extra: Router Boot Process
1, 2
3
4
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a. Run boot system commands in order they appear in startup-config to locate the IOS
b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)
3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config
a. Flash (sequential)
b. TFTP server (netboot) - The router uses the configuration register value to form a filename fromwhich to boot a default system image stored on a network server.
c. ROM (partial IOS) or keep retrying TFTP depending upon router model- If no IOS located, get partial IOS version from ROM
4. Locate and load startup-config
a. If startup-config found, copy to running-config
b. If startup-config not found, prompt for setup-mode
c. If setup-mode bypassed, create a skeleton default running-config (no startup-config)
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Extra: Router Boot Process
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Extra: Check for IOS boot system commands in startup-config file (NVRAM)
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R
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Router Interfaces
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R d h N k L
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Routers and the Network Layer
The main purpose of a router is to connect multiple networks and forwardpackets destined either for its own networks or other networks.
A router is considered a Layer 3 device because its primary forwardingdecision 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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R t d th N t k L
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Routers and the Network Layer
Routers Operate at Layers 1, 2, and 3
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CLI Configuration and Addressing
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Implementing Basic Addressing Schemes
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Implementing Basic Addressing Schemes
When designing a new network or mapping an existingnetwork, document the network. At a minimum, thedocumentation should include a topology diagram that
indicates the physical connectivity and an addressing tablethat lists all of the following information:
Device names
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Interfaces used in the design IP addresses and subnet masks
Default gateway addresses for end devices, such asPCs
Implementing Basic Addressing Schemes
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Implementing Basic Addressing Schemes
Activity 1.2.1.1
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Implementing Basic Addressing Schemes
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Implementing Basic Addressing Schemes
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Basic Router Configuration
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Basic Router Configuration
Basic Router Configuration
Router name
Passwords
Banner
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Basic Router Configuration
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Basic Router Configuration
Router Interface Configuration
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Basic Router Configuration
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Basic Router Configuration
Activity 1.2.2.3 Verifying Basic Router Configuration
Issue the show running-config command
Save the basic router configuration by Issuing the
copy running-config startup-config command
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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 interface brief - Displays abbreviated interfaceconfiguration information
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Building the Routing Table
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Introducing the Routing Table
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Introducing the Routing Table
The primary function of a router is to forward a packettoward its destination network, which is the destination IPaddress of the packet.
Routing Table is stored in RAM and contains informationabout:
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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 sourceof routing information, Network address & subnet mask,and ip address of next-hop router, exit interface
show ip route command is used to view a routing table
Introducing the Routing Table
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Introducing the Routing Table
Directly Connected Routes Static Routes Dynamic Routes
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Directly-Connected Networks
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Directly Connected Networks
Once the interface is "up," the network of that interface isadded to the routing table as a directly connected network.
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Static Routing
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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
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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 ISPHub-and-spoke topology is used on a large network
Static Routing
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Static Routing
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Dynamic Routing
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y g
Dynamic routing protocols are used by routers to share informationabout the reachability and status of remote networks. Dynamic routingprotocols perform several activities, including:
Network discovery Updating and maintaining
routing tables
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Dynamic Routing
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Dynamic routing features: Automatic Network Discovery: allows the routers to automatically
learn about these networks from other routers.
Maintaining Routing Tables: automatically share routinginformation with other routers and compensate for any topologychanges without involving the network administrator.
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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)
Routing Table Principles
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g p
Activity 1.3.5.1 3 principles regarding routing tables:
1. Every router makes its decisions alone, based on the information ithas in its routing table.
2. One routers information in the routing table may not be the sameas another routers routing table.
3. Routing information about a path to a network does not provide
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n orma on on ow o re urn o e or g na ne wor .
Routing Table Principles
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g p
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Routing Table Principles
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Routing Table Principles
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Routing Table Principles
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Path Determinationand Switchin Functions
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Packet Fields and Frame Fields
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Internet Protocol (IP) packet format contains fields that provide informationabout 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
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Precedence & type of service
Packet length
Packet Fields and Frame Fields
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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
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Data and pad
Frame check sequence
Best Path and Metric
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A Metric is a numerical value used by routing protocols to assess agiven route
Metrics may be based on a single path variable or multiple variables ofa path
2 types of metrics used by routing protocols are:
1. Hop count - this is the
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packet must travelthrough to get to itsdestination
2. Bandwidth - this is thespeed of a link also
known as the datacapacity of a link
Router Paths and Packet Switching
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Equal cost metric is a condition where a router has multiple paths tothe 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 sendpackets over multiple networks even when the metric is not thesame if it is using a routing protocol that has this capability.
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Path Determination
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The path determination function is the process of how the routerdetermines which path to use when forwarding a packet by
searching its routing table.
One of three path determinations results from this search: Directly connected network
Remote network
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No route determined
Switching Function
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Switching Function of Router is the process used by arouter to switch a packet from an incoming interface to anoutgoing interface on the same router.
A packet received by a router will do the following: Strips off layer 2 headers.
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header to find best route to destination. Re-encapsulates layer 3 packet into layer 2 frame.
Forwards frame out exit interface.
Switching Function
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Step 1: PC1 has a packet to be sent to PC2
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Switching Function
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Step 1: PC1 has a packet to be sent to PC2
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Switching Function
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Step 1: PC1 has a packet to be sent to PC2
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 2: Router R1 receives the Ethernet frame
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 3: Packet arrives at router R2
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Switching Function
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Step 4: The packet arrives at R3
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Switching Function
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Step 4: The packet arrives at R3
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Switching Function
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Step 4: The packet arrives at R3
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Switching Function
S 4 Th k i R3
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Step 4: The packet arrives at R3
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Switching Function
St 4 Th k t i t R3
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Step 4: The packet arrives at R3
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Switching Function
St 4 Th k t i t R3
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Step 4: The packet arrives at R3
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Step 5:The Ethernet Frame with encapsulated IP packet
arrives at PC2
Switching Function
Step 5: The Ethernet Frame with encapsulated IP packet
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Step 5: The Ethernet Frame with encapsulated IP packetarrives at PC2
1. PC2 examines the destination MAC address, which
matches the MAC address of the receiving interface,its Ethernet NIC. PC2 will therefore copy the rest of theframe into its buffer.
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2. PC2 sees that the Ethernet Type field is 0x800, whichmeans that the Ethernet frame contains an IP packetin the data portion of the frame.
3. PC2 decapsulates the Ethernet frame and passes the
IP packet to the IP process of its operating system.
Summary
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Configuration Register Setting 1800 Series
The order in which the router looks for system bootstrap information
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The order in which the router looks for system bootstrap informationdepends on the boot field setting in the configuration register.
The configuration register is a 16-bit register in NVRAM. To ensure that the upper 12 bits are not changed, first retrieve the
current values of the configuration register using the show versioncommand.
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the last hexadecimal digit.
Configuration Register
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Configuration Register
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Boot Field Configuration Register Bit Descriptions
Boot Field
(Bits 3, 2,
1, and 0)
Meaning
0000
(0x0)
At the next power cycle or reload, the router boots to the ROM monitor.
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(0x01)
oo s e rs mage n as memory as a sys em mage.
0010 - 1111
(0x02 - 0xF)
At the next power cycle or reload, the router sequentially processes each
boot system command in global configuration mode that is stored in the
configuration file until the system boots successfully.
If no boot system commands are stored in the configuration file, or if
executing those commands is unsuccessful, then the router attempts to
boot the first image file in flash memory.
Configuration Register
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