Lesson 34 - Dynamic Routing Protocols Introduction

If you have read the previous post, you must have noticed that using a static routing method tends to be a bit cumbersome in larger implementations. Using one of the dynamic routing protocols feels like an easier solution in these scenarios.

In this post I will briefly explain the general concepts behind dynamic routing protocols. Then, we can jump to implementation fundamentals.

One way of classifying dynamic routing protocols is based on where they are used. This criterion allows us to distinguish between two major solutions:

1. Interior Gateway Protocols  (IGP) 
2. Exterior Gateway Protocols (EGP)

Common Interior Gateway Protocols are: 

• Routing Information Protocol (RIP), 
• Open Shortest Path First (OSPF), 
• Enhanced Interior Gateway Protocol (EIGRP, Cisco proprietary protocol), 
• Intermediate System to Intermediate System (IS-IS).

Exterior Gateway Protocols (currently there is only one in use)

• Border Gateway Protocol  (BGP)

IGPs are designed to work in private networks. EGPs are used to provide paths in the public network (Internet). 

We can also classify routing protocols based on the algorithm they use to distribute and maintain information (routing table). There are three major algorithms supported by Cisco routers:

1. Distance Vector (DV, aka Bellman-Ford) – example of protocol: RIP.
2. Link-State – example of protocols: OSPF, IS-IS.
3. Advanced Distance Vector – protocols: EIGRP (also BGP is partly distance vector protocol).

Understanding the algorithms helps us determine the proper solution for a given design. There is no one best routing protocol out there, but there could be the best one in a specific design.

In this post I am going to focus in on the first algorithm listed above.

Distance Vector Algorithm Characteristics

This method is sometimes referred to as ‘routing by rumor’. The main characteristics of this approach are:

• Routers do not know the topology of the network. They only know which is the outbound interface and the next-hop router’s IP address (vector) as well as the metric value which describes how far the destination is (distance).
• Routers advertise their full routing table periodically. This method of route distribution creates two problems: routing loops and counting to infinity. Special techniques were created to solve these issues (details later in the post).
• Routers perform automatic summarization if they are connected to different classful (A, B, C) networks.
• No VLSM support. All network masks must be identical if the subnets of a major class are used in the network (RIPv1). RIPv2 is classless (VLSM supported using ‘no auto-summary’ command).
• Routers are slow to converge. It takes a lot of time to invalidate lost routes and pick the new path if one is available as well as to synchronize their routing information.
• Routers use simple metric. The metric number tells a router how many routers the packet has to traverse in order to reach the destination. In modern networks bandwidth of the path is much more important than how many hops will be used.

The above characteristics do not encourage us to use this kind of solution in our modern networks. But knowing the DV rules help us appreciate protocols such as OSPF or EIGRP which are more likely to be used in our designs.

Let’s see how things work when DV algorithm is used. As an example, I will use RIP protocol hoping to explain the principles of operation and how the two design issues have been solved (routing loop and counting to infinity).

Distance Vector Principles of Operation

Consider this simple topology. Without getting into configuration (syntax) details let’s have a quick discussion on how information is distributed using DV algorithm. Initially, the routers recognize only connected subnets. They are populated in the routing table as soon as IP addresses and network masks are configured and they are activated (no shutdown).

Pic. 1 - Connected Subnets.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Let’s assume that we have enabled RIPv2 protocol in the topology presented above (pic. 1). This version of RIP allows the routers to announce both the subnet IP addresses and the network masks (we’ll put it into practice in the next post). 

The RIP process must be activated in the ‘config’ mode. Then we need to instruct it which interfaces should be activated in the RIP domain. This is configured in the ‘config-router’ mode (‘network’ statement).  The routers begin to ‘chat’ and advertise their routing tables every 30 seconds.

Pretend that R1’s timer of sending the advertisement has just kicked in (pic. 2). R1 is advertising its routing table out of the RIP-enabled interfaces (in my example all interfaces of all routers are in the RIP domain). This way, R2 learns about 10.1.1.0/24 subnet. So from R2’s perspective, R1 router becomes the gateway towards 10.1.1.0/24. 

Now, a word about the metric being advertised. 

Metric used in DV reflects how many routers the packet has to traverse to reach the destination network/subnet (so called 'hop-count'). R1’s routing table’s entries (subnets: 10.1.1.0/24 and 10.1.12.0/24) show the metric of ‘0’ hops (pic. 1) since they are directly connected to F0/0 and F0/1 interfaces respectively (they are local to R1). While advertising them to the neighbors (pic. 2), R1adds 1 hop (itself) to the existing metric found in the routing table.

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NOTICE!
Bear in mind, that algorithm prompts the router to send the full routing table. Current implementation changes that behavior (split-horizon) but more on this later in the post.

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Pic. 2 – R1’s RIP advertisements.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R2 accepts the advertisement about 10.1.1.0/24. It puts this information in the RIP’s database and then it creates the entry in the routing table (purple color). Pay a close attention to what has just happened (pic. 2). The update arrives on R2’s F0/0 interface (RIP-enabled), sourced by the IP address of 10.1.12.1. This way, R2 considers its F0/0 the egress (outbound) interface towards the subnet advertised by R1. The IP address of the sender (10.1.12.1) becomes the next-hop IP address towards the subnet 10.1.1.0/24

Next, let’s imagine R2’s timer has expired and it is sending its routing table out F0/0 and F0/1. Please take a closer look at the picture 3 which shows this process in the graphical form. Just like previously R1 router has done, R2 is sending its routing table adding itself as an additional hop added to the existing metric (existing metric +1).

Pic. 3 – R2’s RIP advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Now, R1 and R3 accept the advertisement from R2 and register the information sent in their RIP databases (the interfaces process the update as they RIP-enabled). Appropriate entries in the routing tables also show the egress interfaces and the metric expressed in the number of ‘hops’ (how many routers the packet will have to traverse to reach the destination subnet). Also, the IP address of the gateway (the sender IP address) is registered. Again, take a look at pic.3 which shows the new entries (in purple).

Now is the time for R3 to send its own advertisement. Using the same logic you should be able to tell what is going to happen. Take a look at pic. 4 to see what is going to be advertised and what is going to be learned.

The advertisement sent out R3’s F0/0 interface is useless in our topology because there is no other router listening to it. In my next post, I will show you how to prevent a router from doing it. Advertisement sent out F0/1 interface contains information about R3’s directly connected subnet 10.1.3.0/24. Since the existing metric in R3’s routing table for this subnet is ‘0’ (directly connected to F0/0), R3 will add itself as the hop and advertise it with the metric of ‘1’ (existing metric + 1). R2is going to learn it on its F0/1 interface which becomes the outbound interface to reach the subnet 10.1.3.0/24. It is the interface to reach the advertising router’s IP address 10.1.23.3 after all.

Pic. 4 – R3’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Picture 4 shows this process.

This whole process of advertising the routing table out of all RIP-enabled interfaces occurs every 30 seconds but in fact, there is a jitter time introduced so this may vary between 25-30 seconds. WhenR2 advertising timer expires, it will pass the information contained in the its routing table on to R1. By doing this, R1 learns about all subnets R2 can reach, including 10.1.3.0/24 now (pic. 5).

Pic. 5 – R2’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

The process of spreading the information explained using this method is referred to as the ‘routing by rumor’. The state in which all routers have stable information about all networks/subnets that can be reached is called the ‘convergence’. Do not confuse it with ‘convergent networks’ which allow all sort of packet transmissions (voice, video, and data).

Take a look at picture 6. It shows that all routers can reach all the subnets available in the RIP domain. Convergence has been accomplished since their routing table are synchronized and up-to-date.

Pic. 6 – Convergence Achieved.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

The method of distributing information presented is prone to introduce two problems:

• Routing Loops
• Counting to Infinity

Of course, they have been resolved by using different techniques which I am going to explain later in the post.

Let’s take a look at the downside of using distance vector algorithm.

Routing Loops
In the picture 7, R1’s F0/0 interface. As soon as the IOS detects this fact, the entry in the routing table about 10.1.1.0/24 is immediately flushed (removed from the routing table completely).

Pic. 7 – R1’s F0/0 Interface Goes Down.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

As per the DV algorithm R1 would still wait till its advertisement timer expires. So instead of sending this ‘update’ immediately after it has lost the subnet, it will wait till its timer says: ‘now you can advertise your routing table’. This behavior might create a loop between R1 and R2 as far as the 10.1.1.0/24 subnet is concerned. Consider this situation depicted below.

Pic 8 – R2’s Advertising Timer Expires.

Icons designed by: Andrzej Szoblik - http://www.newo.pl


R2 is advertising its full routing table out of all RIP-enabled interfaces. In this announcement, there is 10.1.1.0/24 subnet. The metric being advertised is: ‘2’ (the existing metric on R2 + 1). By now, you already know that the advertising router is going to add itself as the hop to the metric of the subnet/network it advertises.

Here is the issue. R1 is receiving 10.1.1/0/24 with the metric of 2 hops, the egress interface (the one the ad came on) is F0/1, and the next-hop-address is 10.1.12.2. Look at the pic. 8 and tell me (I can’t hear you though), what would you do if you were R1? Obviously, you would reject this information because by looking at the topology diagram, you already know that 10.1.1.0/24 is inaccessible (down) now, and the only way to reach it is through R1, right?

But the problem is, that routers using DV algorithm do NOT know the topology like explained in the characteristics section. In fact, R1 IS going to accept the information and treat R2 as the gateway towards 10.1.1.0/24 !!!

Wow! As ridiculous as it sounds, it is exactly what would happen according to the rules set by the designers of this algorithm. So R1’s routing table is going to look like shown in the picture 8. Take a look at it now again!

We have a loop between R1 and R2 regarding 10.1.1.0/24. If R2 receives the packets destined to 10.1.1.0/24 subnet, according to its knowledge (current routing table), it is going to send it out F0/0 interface towards R1. This one in turn, will use its F0/1 interface for the destination 10.1.1.0/24, sending it back to R2. The packets will be looped until their TTL values are decremented reaching the value of TTL=0. Then, a router must drop the packet.

Counting To Infinity
A routing loop is not going to be the only problem here. R1 is going to accept advertisements fromR2 regarding 10.1.1.0/24 with the number of hops equal '2’. When R1 advertises its own routing table, it is going to add itself (as the hop) to the metric that already exists in the routing table. Look what is going to happen (pic. 9)

Pic. 9 – R1’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Initially, R2 is going to ignore the information about 10.1.1.0/24 from R1 containing the metric of ‘3’ hops since it has much better entry in the routing table (lower metric). However, it was R1 that initially sent the metric of ‘1’ hop. Now, the same R1 router keeps sending the metric of ‘3’ hops. The previous metric of ‘1’ is no longer refreshed. Since it uses the aging timer of 180 seconds (how long the information is valid), it finally accepts the entry with the metric of ‘3’ hops instead.

Then R2 begins to advertise the metric of 4 regarding 10.1.1.0/24 subnet out F0/0 and F0/1. You can predict what is going to happen. Remember, that entries must be refreshed every 30 seconds. If they are not refreshed, the ‘Invalidation Timer’ (180 seconds), allows to accept the entry with worse metric than previously. Take a look at the sequence of events in the picture 10.

Pic. 10 – Larger and Larger Metric Propagation.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

It would last forever despite of the fact that 10.1.1.0/24 is not reachable at all!

The Distance Vector algorithm uses a few techniques to prevent these two problems from happening. Here they are:

• Triggered Update (aka flash update)
• Route Poisoning
• Maximum Metric (RIP considers 16 hops as inaccessible)
• Poison Reverse
• Hold-Down Timer
• Split-Horizon

These methods deserve a few words of explanation.

Triggered Update

IOS uses this method to send the update immediately rather than wait for the advertisement timer to expire. However, there is no guarantee that some router in the chain is not going to send its own information before it receives this update. This might still lead to a situation where the two problems occur. So this method, as the only solution here, is not enough to make it work. Other methods must be used as well in order to avoid routing loops and counting to infinity.

Route Poisoning

Upon losing subnet/network reachability, a router is sending a triggered update. This update is going to include the maximum metric value (RIP=16 hops) which is considered as ‘subnet/network inaccessible’ (cannot be reached).

Maximum Metric (RIP=16)

If a RIP router receives an update about a network/subnet with the metric of 16 hops it is considered as inaccessible. This way, the advertising router is excluded from the list of gateways for the subnet/network advertised with the maximum metric.

Poison Reverse

Once a router receives the advertisement including the maximum metric, if it does not have an alternate path towards the subnet/network lost, it is going to send the same subnet/network prefix with the maximum metric (RIP=16) informing the other routers about it. This will also be sent back to the sender of this information it does not have an alternate path (this might be seen as violation of split-horizon, but remember the metric is the maximum value). Poisoning the path back to the advertising router is the way of informing it that the receiver of this information has no alternate path available either.

Hold-Down Timer

Upon receiving information from a neighbor that a subnet/network is inaccessible, the receiving router is going to enable a hold-down timer for 180 seconds. During this time, the receiving router keeps sending packet to the destination being inaccessible for some time rather than withdrawing the entry from its routing table. Why?

In the past, the routers did not have that much power and the media were unreliable. Interfaces were prone to flaps more often than in today’s reliable networks. An ‘interface flap’ is the condition when it goes down and up subsequently in a very short space of time (1-2 seconds perhaps). Under such circumstances, a router would advertise network as inaccessible and then as accessible again. Since it takes some CPU power to withdraw the entry and put it back in, the designers preferred to wait a bit longer to be absolutely sure (180 seconds by default) that the entry was supposed to be removed from the routing table. In case of an interface flapping, not only would the packets still be delivered but the CPU would not waste its ‘precious’ cycles on removing and putting the entry back in the routing table. 

Split-Horizon

This method prevents the loops from occurring in the scenario we have talked about. This technique prevents a router from sending information it learned back out the interface it was received on. Consider our first example. R2 sent information about 10.1.1.0/24 before R1 had had a chance to send the maximum metric towards R2 (subnet down). Split-Horizon prevents R2 from sending information about 10.1.1.0/24 it learned on its F0/0 interface back out the same interface. As a result of that, R1 is never going to receive information it sent towards R2 (10.1.1.0/24) and believe R2 could be the gateway to 10.1.1.0/24. Thus, there is no loop

In my next post I’m going to show you how to enable RIP and how all these techniques work in practice.

Lesson 10 - Cisco Discovery Protocol

In the previous lesson we have explored how switches build their mac address table (aka Content Addressable Memory). It is critical to understand those concepts in order to perform troubleshooting related to connectivity issues.

In this lesson we'll continue studying layer 2 technologies. Today's theme is Cisco Discovery Protocol. This protocol comes in handy in many situations (trust boundary for Cisco IP Phones, auto qos and others).

What is Cisco Discovery Protocol?
CDP is Cisco proprietary layer 2 protocol. It is enabled by default on majority of Cisco devices including IP phones. It can work on any connections supporting SNAP (such as LANs, but also ATM and Frame-Relay). The only time you'll see the CDP turned off by default, is when you use frame-relay configured interfaces.

What does CDP do?
Every Cisco device using this protocol, reports information about itself by advertising special packets out of its all active interfaces. The important pieces of information it advertise include its:

• Hostname
• Platform
• Ports where CDP packets are advertised
• IOS version
• IP address

CDP can help administrator discover Cisco devices connected and create a  topology diagram or prepare an inventory of the gear used. It can also be an additional tool in troubleshooting problems in the network. Working as a support technician, I found it useful numerous times.

Let's take a quick tour through the CLI (command line interface) and see what major commands CDP allows us to use and what they show.

I'm connected to my Cisco switch SW1 and in the privileged mode type in the following command:

Pic. 1

Using our best friend '?' we can see the CDP options. The last line '' stands for: 'carriage return'. A good, old-fashioned terminal lingo for 'press enter'. Let's try this first.

Pic. 2

Based on the output (Pic. 2) we see that CDP version 2 is enabled, the information packets (though technically should be called frames) are sent every 60 seconds. We also learn, that SW1 will keep CDP information it receives from its Cisco neighboring devices for 180 seconds (holdtime). Let's look at another CDP command:

Pic. 3

In the Pic. 3 we can see the traffic statistics such as CDP packets sent, received, any CDP encapsulation problems etc.

The below command (output in Pic. 4) will tell you which interfaces CDP is running on.

SW1#show cdp interface  

You can disable CDP on a specific interface or group of interfaces. For instance, if you do not want to run CDP on Fas0/1 interface, you could use the following command:

SW1#configure terminal

SW1(config)#interface Fas0/1

SW1(config-if)#no cdp enable

If you want to disable CDP on a group of interfaces you can use 'interface range' command. For instance, disabling CDP on Fas0/1, Fas0/2, Fas0/5 and Fas0/8 would look like this:

SW1#configure terminal

SW1(config)#interface range fas0/1 - 2 , fas0/5 , fas0/8

SW1(config-if-range)#no cdp enable

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NOTICE

If you use the 'interface range' command, the consecutive ports can be specified with '-' but make sure your use 'space' before and after '-' (fas0/1 - 2). The same applies to non-consecutive ports (fas0/5 , fas0/8). There is 'space' before and after the comma character ','.

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As you will see later, CDP discloses some vital information (e.g. IOS version), so for the security reasons you may decide to turn off CDP altogether. Be careful before you do that though, as some application may rely on this protocol. Disabling CDP can cause cascading problems in your network. The command which disables CDP completely (on all interfaces) is:

SW1#configure terminal

SW1(config)#no cdp run

Pic. 4

Now, let's see what neighboring devices SW1 discovered by listening to their CDP packets (Pic. 5).

Pic. 5 

Dissecting the Pic. 5 output we learn the following:

• SW1 received CDP packets from the device named 'R1' (hostname).
• This CDP packet was sent from R1's Fas0/0 interface (the last column 'Port ID').
• SW1 received this CDP information packet on its Fas0/1 local interface ('Local Intrfce').
• This leads us to a conclusion that R1's Fas0/0 interface is directly connected to SW1'sFas0/1 interface.
• R1 neighbor is a router as the capability list shows 'R S I' (R=router, S=switching capability, I=IGMP support).
• R1 is 2611XM platform.

That's not all by any means. There is another command we can use to obtain more information about R1. Click at the Pic. 6

Pic. 6

It shows you information about IOS version running on R1, as well as its IP address 192.168.10.254 configured on Fas0/0 interface. Now you understand why you might consider turning off CDP on some interfaces. You do not want to show such details to a third party company (like your service provider), that connect to your devices.

Instead of using 'show cdp entry R1', you can also use the following command that displays detailed (similar to Pic. 6) output about ALL discovered neighbors :

SW1#show cdp neighbor detail

In lesson 11, you will learn the commands related to switch mac-address-table which was covered in theory only (lesson 9). Also, we'll hone all our skills we have obtained so far. Things will begin to fall into place. At least that's my hope.

How to : Basic Router Configuration

Initial Router Configurations: This post shows how to configure the router initially from router
 boot process to optimizing router for best user experience to securing routers console and telnet ports.
 I am using GNS3 and router 2691 image in gns3.

Initial Router Boot Process : Router boot Process consist of

1> POST : Power On Self Test

2> Booting IOS (operationg system of cisco devices) from routers flash memory.

3> Initial Configuration Setup : I basically prefer not to set up router from initial config

When you chose no to initial config dialog router will enter into user mode which is designated as

Router> This mode is known as user mode, you can do ?
For the commands you can enter in this mode. you can only use very general show commands
 from this mode.

Router>?

Exec commands:

access-enable Create a temporary Access-List entry

access-profile Apply user-profile to interface

clear Reset functions

connect Open a terminal connection

disable Turn off privileged commands

disconnect Disconnect an existing network connection

enable Turn on privileged commands

exit Exit from the EXEC

help Description of the interactive help system

lock Lock the terminal

login Log in as a particular user

logout Exit from the EXEC

modemui Start a modem-like user interface

mrinfo Request neighbor and version information from a multicast

router

mstat Show statistics after multiple multicast traceroutes

mtrace Trace reverse multicast path from destination to source

name-connection Name an existing network connection

pad Open a X.29 PAD connection

ping Send echo messages

ppp Start IETF Point-to-Point Protocol (PPP)

resume Resume an active network connection

rlogin Open an rlogin connection

show Show running system information

slip Start Serial-line IP (SLIP)

ssh Open a secure shell client connection

systat Display information about terminal lines

tclquit Quit Tool Command Language shell

telnet Open a telnet connection

terminal Set terminal line parameters

tn3270 Open a tn3270 connection

traceroute Trace route to destination

tunnel Open a tunnel connection

udptn Open an udptn connection

where List active connections

x28 Become an X.28 PAD

x3 Set X.3 parameters on PAD

To Enter into privilege mode do

Router>en

Router# In this mode you can do some show commands, copy commands to save your
 running-configuration to your routers startup configuration

To enter to the main configuration mode do

Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)# (This is the main mode for configuring basically everything in the router)

So lets start configuring the router

How to change the hostname of router

Router(config)#hostname testrouter

testrouter(config)#

here above you can see the change in hostname from default hostname Router to testrouter

How to configure log on banner on router

testrouter(config)#banner motd *

Enter TEXT message. End with the character '*'.

Personal Router Unauthorized access Restricted

*

Above motd means message of the day and this banner is displayed at first log on.
 You have to enter the same Character at the start and end of each motd in this case it is *.

Below you can see how the banner is displayed

testrouter con0 is now available

Press RETURN to get started.

*Mar 1 00:24:00.555: %SYS-5-CONFIG_I: Configured from console by console
Personal Router Unauthorized access Restricted

How to Secure the console login of router : ( When you connect your router to P.C with console cable,
 this console password is required)

testrouter(config)#line console 0

testrouter(config-line)#password cisco

testrouter(config-line)#login (The opposite of login i.e no login will not ask for password when loggin
through console of the router , which can be very risky)

How to secure telnet login of the router : This will secure the login when telneting from the remote P.C

testrouter(config)#line vty 0 ?

<1-935> Last Line number



The above command shows that this router supports 936 vty ports (virtual tele terminal i.e 936 connections )

testrouter(config)#line vty 0

testrouter(config-line)#line vty 0 4

testrouter(config-line)#login

% Login disabled on line 66, until 'password' is set

% Login disabled on line 67, until 'password' is set

% Login disabled on line 68, until 'password' is set

% Login disabled on line 69, until 'password' is set

% Login disabled on line 70, until 'password' is set

testrouter(config-line)#password test

The commands allows the telnet login for vty 0 4 and requires password test to enter
 to the router through telnet.

How to secure privelege/enable mode on router : This configuration will ask for the password 
when entering from the user mode to privilege mode

testrouter(config)#enable secret test

testrouter(config)#exit

testrouter#exit

Below shows how this router asks for the password

testrouter con0 is now available

Press RETURN to get started.

*Mar 1 00:37:34.707: %SYS-5-CONFIG_I: Configured from console by console

Personal Router Unauthorized access Restricted

User Access Verification

Password:

testrouter>en

Password:

testrouter#

How to optimize router for best user experience:

Below command should be entered both for line console and for line vty, I will be showing in console,
 you can do the same in line vty

testrouter(config)#line co

testrouter(config)#line console 0

testrouter(config-line)#log

testrouter(config-line)#logg

testrouter(config-line)#logging sy

testrouter(config-line)#logging synchronous

loggin synchronous will keep the status messages from interrupting what you are typing

testrouter(config-line)#exec-timeout 20 0 : exec timeout will give the timeout interval i.e here if
 no command is typed for 20 minutes the router will throw the user to user mode

Thank You,

In my next post of cisco basics I'll be configuring interfaces. Hope this will be informative for you

How to Configure Interface on Routers :

There may be various kinds of interfaces on the router like fastethernet, serial etc, but for all the 
interface, configuration are basically similar. I am going to show you here the commands
 for configuring the most popular interfaces i.e fastethernet and serial

testrouter#show ip interface brief (This command will show you all the interface, configured ip address,
 interface status, protocol status etc)

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES unset administratively down down

FastEthernet0/1 unassigned YES unset administratively down down

If you want more detailed description then you can do

testrouter#show interfaces ( This will show you detailed view of the interfaces like MTU size,
 Encapsulation on the interface, Erros etc which will be discussed later on other topics.)

you can also do

testrouter#show interfaces fastEthernet 0/0

for detailed configuration of specific interfaces.

To configure interface goto the interface in your routers

testrouter#config t

testrouter(config)#interface fastEthernet 0/0

testrouter(config-if)#ip address 192.168.1.20 255.255.255.0

testrouter(config-if)#no shutdown

*Mar 1 00:07:17.667: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Mar 1 00:07:18.667: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0
changed state to up

testrouter(config-if)#do show ip interface brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 192.168.1.20 YES manual up up

FastEthernet0/1 unassigned YES unset administratively down down

As you can see from the above that now FastEthernet0/0 is configured with ip address 192.168.1.20 and its status and protocol are both up. Any time you see status up but protocol down then it means the interface is administratively down and you have to issue no shutdown commands on the interface. If status is down then you have to check your physical connection as Status deals with physical layer. By the way , you can use show commands from any mode, but you have to use it with do.