Overview Now that you have learned about routing protocols, you ...

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Overview

Now that you have learned about routing protocols, you are ready to configure IP routing
protocols. As you know, routers can be configured to use one or more IP routing protocols. In
this chapter, you will learn about the initial configurati
on of the router to enable the IP routing
protocols of Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).
In addition, you will learn how to monitor IP routing protocols.


12.1

Initial Router Configuration


12.1.1

Setup mode

After testing the hardware and loading the Cisco IOS system image, the router finds and
applies the configuration statements. These entries provide the router with details about
router
-
specific attributes, protocol functions, and interface addresses. Howe
ver, if the router is
unable to locate a valid startup
-
config file, it enters an initial router configuration mode called
setup mode
.



With the setup mode command facility, you can answer questions in the system configuration
dialog. This facility prompts

you for basic configuration information. The answers you enter
allow the router to use a sufficient, but minimal
-
feature, router configuration that includes the
following:




an inventory of interfaces




an opportunity to enter global parameters




an opportun
ity to enter interface parameters




a setup script review




an opportunity to indicate whether you want the router to use this configuration


After you approve setup mode entries, the router uses the entries as a running configuration.
The router also stores

the configuration in NVRAM as a new startup
-
config, and you can start
using the router. For additional protocol and interface changes, you can use the enable mode
and enter the command
configure
.


12.1.2

Initial IP routing table

Initially, a router must

refer to entries about networks or subnets that are directly connected to
it. Each interface must be configured with an IP address and a mask. The Cisco IOS software
learns about this IP address and mask information from a configuration that has been inpu
t
from some source. The initial source of addressing is a user who types it into a configuration
file.



In the lab that follows, you will start up your router in a just
-
received condition, a state that
lacks another source for the startup configuration. T
his condition on the router will permit you
to use the setup
-
mode command facility and answer prompts for basic configuration
information. The answers you enter will include address
-
to
-
port commands to set up router
interfaces for IP.


12.1.3

How a router

learns about destinations

By default, routers learn paths to destinations three different ways
:




static routes

--

manually defined by the system administr
ator as the next hop to a
destination; useful for security and traffic reduction




default routes

--

manually defined by the system administrator as the path

to take
when there is no known route to the destination




dynamic routing

--

the router learns of paths to destinations by receiving periodic
updates from o
ther routers.

12.1.4

The

ip route command

The
ip route

command sets up a static route.
-




The
administrative distance

is a rating of the trustworthiness of a routing information source,
expressed as a numeric value from 0 to 255. The higher the number, the lower the
trustworthiness rating.

A static route allow
s manual configuration of the routing table. No dynamic changes to this
table entry will occur as long as the path is active. A static route may reflect some special
knowledge of the networking situation known to the network administrator. Manually
-
entered

administrative distance values for static routes are usually low numbers (1 is the default).
Routing updates are not sent on a link if they are only defined by a static route, therefore, they
conserve bandwidth.

12.1.5

Using the
ip route

command

The assi
gnment of a static route to reach the stub network 172.16.1.0 is proper for Cisco A
because there is only one way to reach that network. The assignment of a static route from
Cisco B to the cloud networks is also possible. However, a static route assignmen
t is required
for each destination network, in which case a default route may be more appropriate.
-





Lab Activity



In t
his lab you will configure a static route between neighboring routers.





12.1.6

The ip default
-
network command

The
ip default
-
network
command establishes a default route in networks using dynami c
routing protocols..
-




Default routes keep routing tables shorter. When an entry for a destination network do
es not
exist in a routing table, the packet is sent to the default network. Because a router does not
have complete knowledge about all destination networks, it can use a default network number
to indicate the direction to take for unknown network numbers.

Use the default network
number when you need to locate a route but have only partial information about the destination
network. The
ip default
-
network

command must be added to all routers in the network or
used with the additional command
redistribute sta
tic

so all networks have knowledge
of the candidate default network.

12.1.7

Using the
ip default
-
network

command

In the example, the global command
ip default
-
network 192.168.17.0

defines the
Class C network 192.168.17.0 as the destination path for packet
s that have no routing table
entries. The Company X administrator does not want updates coming in from the public
network. Router A could need a firewall for routing updates. Router A may need a mechanism
to group those networks that will share Company X's

routing strategy. One such mechanism is
an autonomous system number.


12.2

Interior and Exterior Routing Protocols


12.2.1

Autonomous system

An autonomous system consists of routers, run by one or more operators, that present a
consistent vi ew of routi
ng to the external world. The Network Informati on Center (NIC) assigns
a unique autonomous system to enterprises. This autonomous system is a 16 bit number. A
routing protocol such as Cisco's IGRP requires that you specify this unique, assigned
autonomous
system number in your configuration.



12.2.2

Interior versus exterior routing protocols

Exterior routing protocols are used for communications bet ween autonomous systems. Interi or
routing protocols are used within a single autonomous system.


12.2.3

In
terior IP routing protocols

At the Internet layer of the TCP/IP suite of protocols, a router can use an IP routing protocol to
accomplish routing through the implementation of a specific routing algorithm. Examples of IP
routing protocols include:




RIP

--

a distance
-
vector routing protocol




IGRP

--

Cisco's distance
-
vector routing protocol




OSPF

--

a link
-
state routing protocol





EIGRP

--

a balanced hybrid routing protocol


The following sections show you how to configure the first two of these protocols
.


12.2.4

IP routing configuration tasks

The selection of an IP routing protocol involves the setting of both global and interface
parameters. Global tasks include selecting a routing protocol, either RIP or IGRP, and
indicating IP network numbers with spec
ifying subnet values. The interface task is to assign
network/subnet addresses and the appropriate subnet mask. Dynamic routing uses
broadcasts and multicasts to communicate with other routers. The routing metric helps routers
find the best path to each ne
twork or subnet.


12.2.5

Using the router and network commands

The
router

command starts a routing process.



The
network

command is required because it enables the routing process to determine
which interfaces will participate in the sending and receiving of routing updates.


The network numbers must be based on the network class addresses, not subnet addresses
or

i
ndi vidual host addresses. Major network addresses are limited to Class A, B and C network
numbers.

12.3

RIP


12.3.1

Key elements of RIP

RIP was originally specified in RFC 1058. Its key characteristics include the following:




It is a distance
-
vector rou
ting protocol.




Hop count is used as the metric for path selection.




If the hop count is greater than 15, the packet will be discarded.




By default, routing updates are broadcast every 30 seconds.


12.3.2

Using
router rip

and
network

commands to enable RI
P

The
router rip

command selects RIP as the routing protocol. The network command
assigns a
network class

address to which a router will be directly connected. The routing
process associates interfaces with the network addresses and begins using RIP on th
e
specified networks. Note: In RIP all subnet masks must be the same. RIP does not share
subnetting information in routing updates.



12.3.3

Enabling RIP on an IP
-
addressed network

In the example, the descriptions for the commands are as follows:




router

rip

--

selects RIP as the routing protocol




network 1.0.0.0

--

specifies a directly connected network




network 2.0.0.0

--

specifies a directly connected network


The Cisco A router interfaces that are connected to networks 1.0.0.0 and 2.0.0.0 send and
rec
eive RIP updates. These routing updates allow the router to learn the network topology.


12.3.4

Monitoring of IP packet flow using the show ip protocol command

The
show ip protocol

command displays values, about routing timers and network
information, t
hat are associated with the entire router. Use this information to identify a router
that you suspect of delivering bad routing information.


The router in the example sends updated routing table information every 30 seconds
(configured interval ). Seventee
n seconds have elapsed since it sent its last update; it will send
the next one in 13 seconds. Following the "Routing for Networks" line, the router specifies
routes for the listed networks. The last line shows that the RIP administrative distance is 120.


12.3.5

The show

ip route

command


The
show ip route

command displays the contents of the IP routing table, which contains
entries for all known networks and subnetworks, along with a code that indicates how that
information was learned.


Lab Activity



In this lab you will configure RIP as the routing protocol.







12.4

IGRP


12.4.1

Key characteristics of IGRP


IGRP is a distance
-
vector routing protocol devel oped by Cisco. IGRP sends routing updates at
90 second intervals, advertising networks for a particular autonomous system. Some of the
IGRP key design ch
aracteristics emphasize the following:



versatility that enables it to automatically handle indefinite, complex topologies




flexibility for segments that have different bandwidth and delay characteristics




scalability for functioning in very large networks


The IGRP routing protocol by default uses two metrics, bandwidth and delay. IGRP can be
confi gured to use a combi nation of vari ables to determine a composite metric. Those vari ables
include:



bandwidth



delay



load




reliability



12.4.2

Using router igrp

and
network

commands to enable IGRP


The
router igrp
command selects IGRP as a routing protocol.



The
network

command specifies any directly connected networks that are to be included.
Note: Like RIP, all subnet masks must be the same. IGRP does not share subnetting
information in routing updates.


12.4.3

Enabling IGRP on an IP
-
addressed network


IGRP is selected as

the routing protocol for autonomous system 109. All interfaces connected
to networks 1.0.0.0 and 2.0.0.0 will be used to send and receive IGRP routing updates. In the
example:



router igrp 109

--

selects IGRP as the routing protocol for autonomous system
109



network 1.0.0.0
--

specifies a directly connected network



network 2.0.0.0

--

specifies a directly connected network


12.4.4

Monitoring IP packet flow using the show
ip protocol

command


The
show ip protocol

command displays parameters, filters, and

network information
about all of the routing protocol (s) (i.e. RIP, IGRP, etc.) in use on the router. The algorithm used
to calculate the routing metric for IGRP is shown in this display. It defines the value of the
K1
-
K5 metrics and the maximum hop count
. The metric K1 represents bandwidth and the
metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1. K2,K4
and K5 metric values are set to 0.


12.4.5

The

show ip interfaces command


The
show ip interfaces

command displays

the status and global parameters associated
with all IP interfaces. The Cisco IOS software automatically enters a directly
-
connected route
in the routing table if the interface is one through which soft ware can send and recei ve packets.
Such an interface
is marked
up
. If the interface is unusable, it is removed from the routing
table. Removing the entry allows the use of backup routes, if they exist.



12.4.6

The show ip route

command


The
show ip route

command displays the contents of an IP routing tabl
e. The tabl e contains
a list of all known networks and subnets and the metrics associated with each entry. Note that
in this example the information was derived from IGRP (I), or from direct connections (C).



12.4.7

The debug ip rip command


The
debug i
p rip

command displays RIP routing updates as they are sent and recei ved. In
this example, the update is sent by 183.8.128.130. It reported on three routers, one of which is
inaccessible because its hop count is greater than 15. Updates were then broadcast

through
183.8.128.2.



Use caution when using debug commands. Debug commands are processor intensive and
can decrease network performance or cause loss of connectivity. Use only during times of low
network usage. Disable the command when finished by using

the command,
no debug ip
rip
or

no debug all
.


12.5

Challenge Labs


12.5.1

Rip convergence challenge



Lab Activity



As a system administrator, there will be times where configuring static
routes can be very useful. Static routes are useful for stub networks
because there is only one way to get to that network. Secu
rity is another
reason to use static routes. For example, if you have a network or
networks that you don't want the rest of the network to be able to "see"
you would not want RIP or other routing protocols sending periodic
updates to other routers. With si
mple networks (few routers) it is
sometimes more efficient to use static routes since it conserves
bandwidth on WAN links. In this lab you will use static routes for
troubl eshooting purposes and to see their rel ationshi p to dynamic routes
and routing proto
cols.




12.5.2

Routing loops setup challenge


Lab Activity



In this lab you will setup a WAN connection between Lab
-
A and Lab
-
E to
create alternate paths in the standard router lab setup. Using a set of WAN
serial cables, connect Lab
-
A Serial 1 to Lab
-
E Serial 0. Remember to set
the clock rate on the DCE side of th
e cable (Lab
-
E's Serial 0 interface).




12.5.3

Preventing routing loops


Lab Activity



In the previous challenge lab, you saw how long it took to converge when
a link went down. In this lab, your task is to find out how to prevent and
control routing loops. The use of hold
-
down timers, defining a maximum
hop count,

counting to infinity, poison reverse and split
-
horizon are all
methods of controlling routing loops. You will use the RIP hop count metric
to control routing loops in this lab.





Summary



Initially, a router must refer to entries about networks or s
ubnets that are directly
connected.




Default routers learn paths to destinations three different ways:




Static routes




Default routes




Dynamic routes




The
ip route

command sets up a static route.




The
ip default
-
network

command establishes a default route.




Routers can be configured to use one or more IP routing protocols, such as RIP and
IGRP.