Routing Protocols and Concepts

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Introduction to Routing

and Packet Forwarding

Routing Protocols and

Concepts – Chapter 1

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Objectives

 Identify a router as a computer with an OS and

hardware designed for the routing process

 Demonstrate the ability to configure devices and

apply addresses

 Describe the structure of a routing table

 Describe how a router determines a path and

switches packets

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Router as a Computer

 Describe the basic purpose of a router

network

the best path for a packet to travel and forwarding packets to

their destination

 Routers are the network center

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 Data is sent in form of packets between 2 end devices

 Routers are used to direct packet to its destination

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 Routers examine a packet’s destination IP address and determine the best path by enlisting the aid of a routing table

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configuration file Stores routing table RAM contents lost

when power is off

when router is powered up Stores the router’s bootstrap

program

This may include IP addresses (Routing protocol, Hostname of router)

used to connect network Examples of interface types:

• Ethernet / fast Ethernet interfaces

• Serial interfaces

• Management interfaces

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 Router components

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 Major phases to the router boot-up process

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 Verify the router boot-up process:

the router during the bootup process Information includes:

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 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

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• Are used to connect router to LAN network

• Has a layer 2 MAC address

• Can be assigned a Layer 3 IP address

• Usually consist of an RJ-45 jack

• 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

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 Routers and the Network Layer

consults information in the routing table

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 Routers Operate at Layers 1, 2 & 3

destination IP address

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Configure Devices and Apply Addresses

 Implementing Basic Addressing Schemes

 When designing a new network or mapping an existing network you must provide the following information in

the form of a document:

– Topology drawing that Illustrates physical connectivity

– Address table that provides the following information:

• Device name

• Interfaces used

• IP addresses

• Default gateway

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 Basic Router Configuration

• Router name - Host name should be unique

• Banner - At a minimum, banner should warn against unauthorized use

• Passwords - Use strong passwords

• Interface configurations - Specify interface type, IP address and subnet mask Describe purpose of interface Issue no shutdown command If DCE serial interface issue clock rate command

should be completed:

• Verify basic configuration and router operations

• Save the changes on a router

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 Verify Basic Router Configuration

running-config startup-config command

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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Routing Table Structure

 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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 Adding a connected network to the routing table

table you must have directly connected networks

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 Static routes in the routing table

next hop router or exit interface

to connect remote networks before static or dynamic routing

can be used

 When to use static routes

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 Connected and Static routes

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 Dynamic routing protocols

 Automatic network discovery

table information

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 Maintaining routing tables

information with other router & to maintain and up date their

own routing table

 IP routing protocols - example of routing protocols

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 Routing Table Principles

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

how to get back

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 Effects of the 3 Routing Table Principles

another, on a hop by hop basis

“Y” (Asymmetric routing)

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Router Paths and Packet Switching

 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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 MAC Layer Frame Format

 MAC Frames are also divided into fields - they include:

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 A Metric is a numerical value used by routing protocols help determine the best path to a destination

– The smaller the metric value the better the 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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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

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pick the best path to a destination

 One of 3 path determinations results from searching for the best path

– Directly connected network

– Remote network

– No route determined

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router to switch a packet from an incoming interface to

an outgoing interface on the same router

• 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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 As a packet travels from one networking device to another

packet is forwarded from one router to the next

which point router discards packet (prevents packets from

endlessly traversing the network)

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 Path determination and switching function details PC1 wants to send something to PC 2

– Step 1 - PC1 encapsulates packet into a frame - frame

contains R1’s destination MAC address

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next hop IP address

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next hop IP address

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 Step 4 - Packet arrives at R3

connected to destination via its fast Ethernet interface

 Step 5 - IP packet arrives at PC2 - frame is decapsulated and

processed by upper layer protocols

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Summary

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so that router can examine the destination IP address

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

destination IP addresses do not change

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Version 4.0

Static Routing

Routing Protocols and

Concepts – Chapter 2

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Objectives

 Define the general role a router plays in networks

 Describe the directly connected networks, different

router interfaces

 Examine directly connected networks in the routing

table and use the CDP protocol

 Describe static routes with exit interfaces

 Describe summary and default route

 Examine how packets get forwarded when using

static routes

 Identify how to manage and troubleshoot static

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General Role of the Router

 Functions of a Router

– Forwarding packets to destination

 Introducing the Topology

– 3 1800 series routers connected via WAN links

– Each router connected to a LAN represented by a switch and a PC

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General Role of the Router

 Connections of a Router for WAN

standards

 Connections of a Router for Ethernet

Cross-over

Hub-to-PC, Hub-to-Server

Hub-to-Hub, Router-to-Router

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Interfaces

– Show IP router command – used to view routing table

– Show Interfaces command – used to show status of an

interface

– Show IP Interface brief command – used to show a portion of

the interface information

– Show running-config command – used to show configuration

file in RAM

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Interfaces

– By default all serial and Ethernet interfaces are down

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Interfaces

– Show interfaces for fastEthernet 0/0 – command used to show status of fast Ethernet port

– Show ip interface brief

– Show running-config

 Ethernet interfaces participate in ARP

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Interfaces

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Interfaces

service provider CSU/DSU is a DCE device

the DTE device

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Interfaces

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Routing Table and CDP Protocol

adding or removing routes

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 When a router only has its interfaces configured & no

other routing protocols are configured then:

– The routing table contains only the directly connected networks

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15

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• Layer 3 neighbors

• Layer 2 neighbors

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– Show cdp neighbors command

– Show cdp neighbors detail command

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Static Routes with Exit Interfaces

to a stub network

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the "next-hop" to this network

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 Zinin’s 3 routing principles

– Principle 1: “Every router makes its decision alone, based on

the information it has in its own routing table.”

– Principle 2: “The fact that one router has certain information in its routing table does not mean that other routers have the

same information.”

– Principle 3: “Routing information about a path from one

network to another does not provide routing information about

the reverse, or return path.”

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 Using Zinin’s 3 routing principles, how would you

answer the following?

– Would packets from PC1 reach their destination?

192.168.1.0/24 networks would reach their destination

– Does this mean that any packets from these networks destined for 172.16.3.0/24 network will reach their destination?

172.16.3.0/24 network

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– Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet

A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup)

address with the Next hop address

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 Configuring a Static route with an Exit Interface

– Static routes configured with an exit interface are more efficient

because the routing

search instead of 2 searches

exit interface

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– Existing static routes cannot be modified The old static route

• no ip route 192.168.2.0 255.255.255.0 172.16.2.2

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destination and that Return path is working

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– If a static route is configured on an Ethernet link and packet is sent to the next-hop router then…

• The destination MAC address will be the address of the

next hop’s Ethernet interface

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Summary and Default Route

table

number of static routes into a single static route.

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– This is a route that will match all packets Stub routers that

have a number of static routes all exiting the same interface

are good candidates for a default route

the routing table

address and subnet mask are all zeros

ip-address ]

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match when comparing destination IP address and subnet

mask

all packets will match

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Static Routes and Packet Forwarding

 Packet forwarding with static routes (recall Zinin’s 3

routing principles)

 Router 1

192.168.2.0/24

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Static Routes and Packet Forwarding

 Packet forwarding with static routes (recall Zinin’s 3

routing principles)

 Router 2

Tiêu đề	Routing Protocols and Concepts
Chuyên ngành	Networking
Thể loại	Lecture notes
Năm xuất bản	2007

Định dạng
Số trang	395
Dung lượng	12,7 MB