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2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1

Chapter 4: RoutingConcepts

Routing & Switching


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Presentation_ID 2 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential

Chapter 4

4.0 Routing Concepts

4.1 Initial Configuration of a Router

4.2 Routing Decisions

4.3 Router Operation

4.4 Summary


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Chapter 4: Objectives

Configure a router to route between multiple directly connectednetworks

Describe the primary functions and features of a router.

Explain how routers use information in data packets to makeforwarding decisions in a small- to medium-sized business network.

Explain the encapsulation and de-encapsulation process used byrouters when switching packets between interfaces.

Compare ways in which a router builds a routing table whenoperating in a small- to medium-sized business network.

Explain routing table entries for directly connected networks.

Explain how a router builds a routing table of directly connectednetworks.


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Chapter 4: Objectives (cont.)

Explain how a router builds a routing table using static routes.

Explain how a router builds a routing table using a dynamic routingprotocol.


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Functions of a Router

Characteristicsof a Network


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Why Routing?

The router is responsible for the routing of trafficbetween networks.


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Routers are specialized computers containing thefollowing required components to operate:

Central processing unit (CPU)

Operating system (OS) - Routers use Cisco IOS

Memory and storage (RAM, ROM, NVRAM, Flash, harddrive)


Routers are Computers


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Routers use specialized ports and network interfacecards to interconnect to other networks.


Routers are Computers


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Routers can connect multiple networks.

Routers have multiple interfaces, each on a differentIP network.


Routers Interconnect Networks


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Routers Choose Best Paths

Routers use static routes and dynamic routing protocolsto learn about remote networks and build their routingtables.

Routers use routing tables to determine the best path tosend packets.

Routers encapsulate the packet and forward it to theinterface indicated in routing table.


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Routers Choose Best Paths


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Process switching An olderpacket forwarding mechanismstill available for Cisco routers.

Fast switching A commonpacket forwarding mechanismwhich uses a fast-switchingcache to store next hop

information.

Cisco Express Forwarding(CEF) The most recent,fastest, and preferred Cisco

IOS packet-forwardingmechanism. Table entries arenot packet-triggered like fastswitching but change-triggered.


Packet Forwarding Methods


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Connect Devices

Connect to a Network


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To enable network accessdevices must be configuredwith the following IP addressinformation

IP address- Identifies aunique host on a localnetwork.

Subnet mask - Identifiesthe hosts networksubnet.

Default gateway -Identifies the router a

packet is sent to to whenthe destination is not onthe same local networksubnet.

Connect Devices

Default Gateways


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Network Documentation should include at least the following in a topologydiagram and addressing table:

Device names

Interfaces

IP addresses and

subnet mask

Default gateways

Connect Devices

Document Network Addressing


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Connect Devices

Enable IP on a Host

Statically Assigned IP address The host is manually assignedan IP address, subnet mask and default gateway. A DNS server IP

address can also be assigned. Used to identify specific network resources such as

network servers and printers.

Can be used in very small networks with few hosts.

Dynamically Assigned IP Address IP Address information isdynamically assigned by a server using Dynamic HostConfiguration Protocol (DHCP).

Most hosts acquire their IP address information throughDHCP.

DHCP services can be provided by Cisco routers.


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Connect Devices

Device LEDs


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Console access requires:

Console cable RJ-45-to-DB-9 console cable

Terminal emulation software Tera Term, PuTTY,HyperTerminal

Connect Devices

Console Access


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Network infrastructure devices require IP addresses to enableremote management.

On a switch, the management IP address is assigned on avirtual interface.

Connect Devices

Enable IP on a Switch


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Basics tasks that should be first configured on a Cisco Router andCisco Switch:

Name the device Distinguishes it from other routers

Secure management access Secures privileged EXEC, userEXEC, and Telnet access, and encrypts passwords to theirhighest level

Configure a banner Provides legal notification ofunauthorized access.

Save the Configuration

Basic Settings on a Router

Configure Basic Router Settings


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To be available, a routerinterface must be:

Configured with an address andsubnet mask .

Must be activated using noshutdown command. By defaultLAN and WAN interfaces arenot activated.

Serial cable end labeled DCEmust be configured with theclock rate command.

Optional description can beincluded.


Configure an IPv4 Router Interface


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To configure interface with IPv6address and subnet mask:

Use the ipv6 address ipv6-address/ipv6-length [link-local | eui-64]interface configuration command.

Activate using the no shutdowncommand.

IPv6 interfaces can support more thanone address:

Configure a specified global unicast -ipv6-address/ipv6-length

Configure a global IPv6 address withan interface identifier (ID) in the low-order 64 bits - ipv6-address/ipv6-length eui-64

Configure a link-local address - ipv6-address/ipv6-length link-local


Configure an IPv6 Router Interface


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Configure a Loopback InterfaceA loopback interface is alogical interface that isinternal to the router:

It is not assigned to aphysical port, it isconsidered a softwareinterface that is

automatically in an UPstate.

A loopback interface isuseful for testing.

It is important in the OSPFrouting process.


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Verify Connectivity of Directly Connected Networks

Verify Interface Settings

Show commands are usedto verify operation and

configuration of interface:

show ip interfaces brief

show ip route

show running-config

Show commands are usedto gather more detailedinterface information:

show interfaces

show ip interfaces


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Verify Interface SettingsSome of the commoncommands to verify the IPv6interface configuration are:

show ipv6 interface brief -displays a summary for eachof the interfaces.

show ipv6 interface

gigabitethernet 0/0 -displays the interface statusand all the IPv6 addresses forthis interface.

show ipv6 route - verifies

that IPv6 networks andspecific IPv6 interfaceaddresses have beeninstalled in the IPv6 routingtable.


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Show command output can be managed using the following command andfilters:

Use theterminal length number command to specify the number of linesto be displayed. A value of 0 (zero) prevents the router from pausingbetween screens of output.

To filter specific output of commands use the (|)pipe character after showcommand. Parameters that can be used after pipe include:

section, include, exclude, begin


Filter Show Command Output


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Command History Feature

The command history feature temporarily stores a list of executed

commands for access:

To recall commands pressCtrl+P or the UP Arrow.

To return to more recent commands press Ctrl+N or theDownArrow.

By default, command history is enabled and the system capturesthe last 10 commands in the buffer. Use the show historyprivileged EXEC command to display the buffer contents.

Use the terminal history size user EXEC command to increase ordecrease size of the buffer.


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Switching Packets between Networks

Router Switching Functions


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Send a Packet


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Forward to the Next HopJG1


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Slide 30

JG1 this graphic is from page 4.2.1.5, please replace with the graphic from 4.2.1.3Jane Gibbons, 10/9/2013


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Packet Routing


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Reach the Destination


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Path Determination

Routing Decisions


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Path Determination

Load BalancingWhen a router has two or more paths to a destination withequal cost metrics, then the router forwards the packets

using both paths equally:

Equal cost load balancing can improve networkperformance.

Equal cost load balancing can be configured to useboth dynamic routing protocols and static routes.

RIP, OSPF and EIGRP support equal cost load

balancing.


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The Routing Table

The Routing TableA routing table is a file stored in RAM that containsinformation about:

Directly connected routes

Remote routes

Network or next hop associations


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The Routing Table

Routing Table Sources


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The Routing Table

Remote Network Routing EntriesInterpreting the entries in the routing table.


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Directly Connected Routes

Directly Connected InterfacesA newly deployed router, without any configured interfaces, has anempty routing table. An active, configured, directly connected interfacecreates two routing table entries:

Link Local (L)

Directly Connected (C)


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Directly Connected Routes

Directly Connected IPv6 ExampleThe show ipv6 routecommand shows the ipv6 networks and routesinstalled in the routing table.


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Statically Learned Routes

Static RoutesStatic routes and default static routes can be implemented afterdirectly connected interfaces are added to the routing table:

Static routes are manually configured

They define an explicit path between two networking devices.

Static routes must be manually updated if the topology changes.

Their benefits include improved security and control of resources. Configure a static route to a specific network using the ip route

network mask{next-hop-ip| exit-intf} command.

A default static route is used when the routing table does not contain

a path for a destination network.

Configure a default static route using the ip route 0.0.0.0

0.0.0.0 {exit-intf| next-hop-ip} command.


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Static Routes Example


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Static IPv6 Routes Example


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Dynamic Routing Protocols

Dynamic RoutingDynamic routing is used by routers to share information about thereachability and status of remote networks. It performs networkdiscovery and maintains routing tables.


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IPv4 Routing Protocols


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IPv6 Routing ProtocolsCisco ISR routers can support a variety of dynamic IPv6 routingprotocols including:

RIPng -RIP next generation OSPFv3

EIGRPfor IPv6

MP-BGP4 -Multicast Protocol-Border Gateway Protocol


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IPv6 Routing Protocols


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Chapter 4: Summary There are many key structures and performance-related

characteristics referred to when discussing networks: topology,speed, cost, security, availability, scalability, and reliability.

Cisco routers and Cisco switches have many similarities. Theysupport a similar modal operating system, similar commandstructures, and many of the same commands.

One distinguishing feature between switches and routers is the typeof interfaces supported by each.

The main purpose of a router is to connect multiple networks andforward packets from one network to the next. This means that arouter typically has multiple interfaces. Each interface is a memberor host on a different IP network.


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Chapter 4: Summary (cont.) The routing table is a list of networks known by the router.

A remote network is a network that can only be reached by

forwarding the packet to another router. Remote networks are added to the routing table in two ways: either

by the network administrator manually configuring static routes or byimplementing a dynamic routing protocol.

Static routes do not have as much overhead as dynamic routingprotocols; however, static routes can require more maintenance ifthe topology is constantly changing or is unstable.

Dynamic routing protocols automatically adjust to changes without

any intervention from the network administrator. Dynamic routingprotocols require more CPU processing and also use a certainamount of link capacity for routing updates and messages.


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Chapter 4: Summary (cont.) Routers make their primary forwarding decision at Layer 3, the

Network layer. However, router interfaces participate in Layers 1, 2,and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link

frame and encoded into bits at Layer 1.

Router interfaces participate in Layer 2 processes associated withtheir encapsulation. For example, an Ethernet interface on a routerparticipates in the ARP process like other hosts on that LAN.

Components of the IPv6 routing table are very similar to the IPv4routing table. For instance, it is populated using directly connectedinterfaces, static routes and dynamically learned routes.


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