Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

arp -a

Interface: 172.16.10.2 --- 0x3

Internet Address Physical Address Type

172.16.10.1 00-15-05-06-31-b0 dynamic

172.16.20.1 00-15-05-06-31-b0 dynamic

Did you notice that the hardware (MAC) address that Host_A uses to get

to Host_B is the Lab_A E0 interface? Hardware addresses are always

local, and they never pass through a router’s interface. Understanding

this process is as important as air to you, so carve this into your memory!

The Cisco Router Internal Process

One more thing before we get to testing your understanding of my 36

steps of IP routing. I think it’s important to explain how a router forwards

packets internally. For IP to look up a destination address in a routing

table on a router, processing in the router must take place, and if there

are tens of thousands of routes in that table, the amount of CPU time

would be enormous. It results in a potentially overwhelming amount of

overhead—think about a router at your ISP that has to calculate millions

of packets per second and even subnet to find the correct exit interface!

Even with the little network I’m using in this book, lots of processing

would need to be done if there were actual hosts connected and sending

data.

Cisco uses three types of packet-forwarding techniques.

Process switching This is actually how many people see routers to this

day, because it’s true that routers actually did perform this type of bare-

bones packet switching back in 1990 when Cisco released their very first

router. But those days when traffic demands were unimaginably light are

long gone—not in today’s networks! This process is now extremely

complex and involves looking up every destination in the routing table

and finding the exit interface for every packet. This is pretty much how I

just explained the process in my 36 steps. But even though what I wrote

was absolutely true in concept, the internal process requires much more

than packet-switching technology today because of the millions of

packets per second that must now be processed. So Cisco came up with

some other technologies to help with the “big process problem.”

Fast switching This solution was created to make the slow performance

of process switching faster and more efficient. Fast switching uses a cache

to store the most recently used destinations so that lookups are not

required for every packet. By caching the exit interface of the destination

device, as well as the layer 2 header, performance was dramatically

improved, but as our networks evolved with the need for even more

speed, Cisco created yet another technology!

Cisco Express Forwarding (CEF) This is Cisco’s newer creation, and

it’s the default packet-forwarding method used on all the latest Cisco

routers. CEF makes many different cache tables to help improve

performance and is change triggered, not packet triggered. Translated,

this means that when the network topology changes, the cache changes

along with it.

To see which packet switching method your router interface is

using, use the command

show ip interface

.

Testing Your IP Routing Understanding

Since understanding IP routing is super-important, it’s time for that little

test I talked about earlier on how well you’ve got the IP routing process

down so far. I’m going to do that by having you look at a couple of figures

and answer some very basic IP routing questions based upon them.

Figure 9.4

shows a LAN connected to RouterA that’s connected via a

WAN link to RouterB. RouterB has a LAN connected with an HTTP

server attached.

FIGURE 9.4

IP routing example 1

The critical information you want to obtain by looking at this figure is

exactly how IP routing will occur in this example. Let’s determine the

characteristics of a frame as it leaves HostA. Okay—we’ll cheat a bit. I’ll

give you the answer, but then you should go back over the figure and see

if you can answer example 2 without looking at my three-step answer!

1.  The destination address of a frame from HostA would be the MAC

address of Router A’s Fa0/0 interface.

2.  The destination address of a packet would be the IP address of the

HTTP server’s network interface card (NIC).

3.  The destination port number in the segment header would be 80.

That was a pretty simple, straightforward scenario. One thing to

remember is that when multiple hosts are communicating to a server

using HTTP, they must all use a different source port number. The source

and destination IP addresses and port numbers are how the server keeps

the data separated at the Transport layer.

Let’s complicate matters by adding another device into the network and

then see if you can find the answers.

Figure 9.5

shows a network with

only one router but two switches.

FIGURE 9.5

IP routing example 2

The key thing to understand about the IP routing process in this scenario

is what happens when HostA sends data to the HTTPS server? Here’s

your answer:

1.  The destination address of a frame from HostA would be the MAC

address of RouterA’s Fa0/0 interface.

2.  The destination address of a packet is the IP address of the HTTPS

server’s network interface card (NIC).

3.  The destination port number in the segment header will have a value

of 443.

Did you notice that the switches weren’t used as either a default gateway

or any other destination? That’s because switches have nothing to do with

routing. I wonder how many of you chose the switch as the default

gateway (destination) MAC address for HostA? If you did, don’t feel bad

—just take another look to see where you went wrong and why. It’s very

important to remember that the destination MAC address will always be

the router’s interface—if your packets are destined for outside the LAN,

as they were in these last two examples!

Before moving on into some of the more advanced aspects of IP routing,

let’s look at another issue. Take a look at the output of this router’s

routing table:

Corp#

sh ip route

[output cut]

R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

C 192.168.20.0 is directly connected, Serial0/0

L 192.168.20.1/32 is directly connected, Serial0/0

C 192.168.214.0 is directly connected, FastEthernet0/0

L 192.168.214.1/32 is directly connected, FastEthernet0/0

What do we see here? If I were to tell you that the corporate router

received an IP packet with a source IP address of 192.168.214.20 and a

destination address of 192.168.22.3, what do you think the Corp router

will do with this packet?

If you said, “The packet came in on the FastEthernet 0/0 interface, but

because the routing table doesn’t show a route to network 192.168.22.0

(or a default route), the router will discard the packet and send an ICMP

destination unreachable message back out to interface FastEthernet 0/0,”

you’re a genius! The reason that’s the correct answer is because that’s the

source LAN where the packet originated from.

Now, let’s check out the next figure and talk about the frames and packets

in detail. We’re not really going over anything new here; I’m just making

sure you totally, completely, thoroughly, fully understand basic IP

routing! It is the crux of this book, and the topic the exam objectives are

geared toward. It’s all about IP routing, which means you need to be all

over this stuff! We’ll use

Figure 9.6

for the next few scenarios.

FIGURE 9.6

Basic IP routing using MAC and IP addresses

Referring to

Figure 9.6

, here’s a list of all the answers to questions you

need inscribed in your brain:

1.  In order to begin communicating with the Sales server, Host 4 sends

out an ARP request. How will the devices exhibited in the topology

respond to this request?

2.  Host 4 has received an ARP reply. Host 4 will now build a packet, then

place this packet in the frame. What information will be placed in the

header of the packet that leaves Host 4 if Host 4 is going to

communicate to the Sales server?

3.  The Lab_A router has received the packet and will send it out Fa0/0

onto the LAN toward the server. What will the frame have in the

header as the source and destination addresses?

4.  Host 4 is displaying two web documents from the Sales server in two

browser windows at the same time. How did the data find its way to

the correct browser windows?

The following should probably be written in a teensy font and put upside

down in another part of the book so it would be really hard for you to

cheat and peek, but since I’m not that mean and you really need to have

this down, here are your answers in the same order that the scenarios

were just presented:

1.  In order to begin communicating with the server, Host 4 sends out an

ARP request. How will the devices exhibited in the topology respond

to this request? Since MAC addresses must stay on the local network,

the Lab_B router will respond with the MAC address of the Fa0/0

interface and Host 4 will send all frames to the MAC address of the

Lab_B Fa0/0 interface when sending packets to the Sales server.

2.  Host 4 has received an ARP reply. Host 4 will now build a packet, then

place this packet in the frame. What information will be placed in the

header of the packet that leaves Host 4 if Host 4 is going to

communicate to the Sales server? Since we’re now talking about

packets, not frames, the source address will be the IP address of Host

4 and the destination address will be the IP address of the Sales

server.

3.  Finally, the Lab_A router has received the packet and will send it out

Fa0/0 onto the LAN toward the server. What will the frame have in

the header as the source and ​destination addresses? The source MAC

address will be the Lab_A router’s Fa0/0 interface, and the

destination MAC address will be the Sales server’s MAC address

because all MAC addresses must be local on the LAN.

4.  Host 4 is displaying two web documents from the Sales server in two

different browser windows at the same time. How did the data find its

way to the correct browser windows? TCP port numbers are used to

direct the data to the correct application window.

Great! But we’re not quite done yet. I’ve got a few more questions for you

before you actually get to configure routing in a real network. Ready?

Figure 9.7

shows a basic network, and Host 4 needs to get email. Which

address will be placed in the destination address field of the frame when

it leaves Host 4?

FIGURE 9.7

Testing basic routing knowledge

The answer is that Host 4 will use the destination MAC address of the

Fa0/0 interface on the Lab_B router—you knew that, right? Look at

Figure 9.7

again: What if Host 4 needs to communicate with Host 1—not

the server, but with Host 1. Which OSI layer 3 source address will be

found in the packet header when it reaches Host 1?

Hopefully you’ve got this: At layer 3, the source IP address will be Host 4

and the destination address in the packet will be the IP address of Host 1.

Of course, the destination MAC address from Host 4 will always be the

Fa0/0 address of the Lab_B router, right? And since we have more than

one router, we’ll need a routing protocol that communicates between

both of them so that traffic can be forwarded in the right direction to

reach the network that Host 1 is connected to.

Okay—one more scenario and you’re on your way to being an IP routing

machine! Again, using

Figure 9.7

, Host 4 is transferring a file to the email

server connected to the Lab_A router. What would be the layer 2

destination address leaving Host 4? Yes, I’ve asked this question more

than once. But not this one: What will be the source MAC address when

the frame is received at the email server?

Hopefully, you answered that the layer 2 destination address leaving Host

4 is the MAC address of the Fa0/0 interface on the Lab_B router and that

the source layer 2 address that the email server will receive is the Fa0/0

interface of the Lab_A router.

If you did, you’re ready to discover how IP routing is handled in a larger

network environment!

Configuring IP Routing

It’s time to get serious and configure a real network.

Figure 9.8

shows

three routers: Corp, SF, and LA. Remember that, by default, these routers

only know about networks that are directly connected to them. I’ll

continue to use this figure and network throughout the rest of the

chapters in this book. As I progress through this book, I’ll add more

routers and switches as needed.

FIGURE 9.8

Configuring IP routing

As you might guess, I’ve got quite a nice collection of routers for us to

play with. But you don’t need a closet full of devices to perform most, if

not all, of the commands we’ll use in this book. You can get by nicely with

pretty much any router or even with a good router simulator.

Getting back to business, the Corp router has two serial interfaces, which

will provide a WAN connection to the SF and LA router and two Fast

Ethernet interfaces as well. The two remote routers have two serial

interfaces and two Fast Ethernet interfaces.

The first step for this project is to correctly configure each router with an

IP address on each interface. The following list shows the IP address

scheme I’m going to use to configure the network. After we go over how

the network is configured, I’ll cover how to configure IP routing. Pay

attention to the subnet masks—they’re important! The LANs all use a /24

mask, but the WANs are using a /30.

Corp

Serial 0/0: 172.16.10.1/30

Serial 0/1: 172.16.10.5/30

Fa0/0: 10.10.10.1/24

SF

S0/0/0: 172.16.10.2/30

Fa0/0: 192.168.10.1/24

LA

S0/0/0: 172.16.10.6/30

Fa0/0: 192.168.20.1/24

The router configuration is really a pretty straightforward process since

you just need to add IP addresses to your interfaces and then perform a

no shutdown

on those same interfaces. It gets a tad more complex later on,

but for right now, let’s configure the IP addresses in the network.

Corp Configuration

We need to configure three interfaces to configure the Corp router. And

configuring the hostnames of each router will make identification much

easier. While we’re at it, let’s set the interface descriptions, banner, and

router passwords too because it’s a really good idea to make a habit of

configuring these commands on every router!

To get started, I performed an

erase startup-config

on the router and

reloaded, so we’ll start in setup mode. I chose

no

when prompted to enter

setup mode, which will get us straight to the username prompt of the

console. I’m going to configure all my routers this same way.

Here’s how what I just did looks:

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

n

Press RETURN to get started!

Router>

en

Router#

config t

Router(config)#

hostname Corp

Corp(config)#

enable secret GlobalNet

Corp(config)#

no ip domain-lookup

Corp(config)#

int f0/0

Corp(config-if)#

desc Connection to LAN BackBone

Corp(config-if)#

ip address 10.10.10.1 255.255.255.0

Corp(config-if)#

no shut

Corp(config-if)#

int s0/0

Corp(config-if)#

desc WAN connection to SF

Corp(config-if)#

ip address 172.16.10.1 255.255.255.252

Corp(config-if)#

no shut

Corp(config-if)#

int s0/1

Corp(config-if)#

desc WAN connection to LA

Corp(config-if)#

ip address 172.16.10.5 255.255.255.252

Corp(config-if)#

no shut

Corp(config-if)#

line con 0

Corp(config-line)#

password console

Corp(config-line)#

logging

Corp(config-line)#

logging sync

Corp(config-line)#

exit

Corp(config)#

line vty 0 ?

<1-181> Last Line number

Corp(config)#

line vty 0 181

Corp(config-line)#

password telnet

Corp(config-line)#

login

Corp(config-line)#

exit

Corp(config)#

banner motd # This is my Corp Router #

Corp(config)#

^Z

Corp#

copy run start

Destination filename [startup-config]?

Building configuration...

[OK]

Corp# [OK]

Let’s talk about the configuration of the Corp router. First, I set the

hostname and enable secret, but what is that

no ip domain-lookup

command? That command stops the router from trying to resolve

hostnames, which is an annoying feature unless you’ve configured a host

table or DNS. Next, I configured the three interfaces with descriptions

and IP addresses and enabled them with the

no shutdown

command. The

console and VTY passwords came next, but what is that

logging sync

command under the console line? The logging synchronous command

stops console messages from writing over what you are typing in,

meaning it’s a sanity-saving command that you’ll come to love! Last, I set

my banner and then saved my configs.

If you’re having a hard time understanding this configuration

process, refer back to Chapter 6, “Cisco's Internetworking Operating

System (IOS).”

To view the IP routing tables created on a Cisco router, use the command

show ip route

. Here’s the command’s output:

Corp#

sh ip route

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B

- BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS

level-2

ia - IS-IS inter area, * - candidate default, U - per-user

static route

o - ODR, P - periodic downloaded static route, H - NHRP, l -

LISP

+ - replicated route, % - next hop override

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.10.10.0 is directly connected, FastEthernet0/0

L 10.10.10.1/32 is directly connected, FastEthernet0/0

Corp#

It’s important to remember that only configured, directly connected

networks are going to show up in the routing table. So why is it that only

the FastEthernet 0/0 interface shows up in the table? No worries—that’s

just because you won’t see the serial interfaces come up until the other

side of the links are operational. As soon as we configure our SF and LA

routers, those interfaces should pop right up!

But did you notice the

C

on the left side of the output of the routing table?

When you see that there, it means that the network is directly connected.

The codes for each type of connection are listed at the top of the

show ip

route

command, along with their descriptions.

For brevity, the codes at the top of the output will be cut in

the rest of this chapter.

SF Configuration

Now we’re ready to configure the next router—SF. To make that happen

correctly, keep in mind that we have two interfaces to deal with: Serial

0/0/0 and FastEthernet 0/0. So let’s make sure we don’t forget to add

the hostname, passwords, interface descriptions, and banners to the

router configuration. As I did with the Corp router, I erased the

configuration and reloaded since this router had already been configured

before.

Here’s the configuration I used:

R1#

erase start

% Incomplete command.

R1#

erase startup-config

Erasing the nvram filesystem will remove all configuration files!

Continue? [confirm]

[enter]

[OK]

Erase of nvram: complete

R1#

reload

Proceed with reload? [confirm]

[enter]

[output cut]

%Error opening tftp://255.255.255.255/network-confg (Timed out)

%Error opening tftp://255.255.255.255/cisconet.cfg (Timed out)

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

n

Before we move on, let’s talk about this output for a second. First, notice

that beginning with IOS 12.4, ISR routers will no longer take the

command

erase start

. The router has only one command after

erase

that starts with s, as shown here:

Router#

erase s?

startup-config

I know, you’d think that the IOS would continue to accept the command,

but nope—sorry! The second thing I want to point out is that the output

tells us the router is looking for a TFTP host to see if it can download a

configuration. When that fails, it goes straight into setup mode. This gives

you a great picture of the Cisco router default boot sequence we talked

about in Chapter 7, “Managing a Cisco Internetwork.”

Let’s get back to configuring our router:

Press RETURN to get started!

Router#