Encrypting Your Passwords (Continued)
Saturday, July 12, 2008Encrypting Your Passwords (Continued)
Because only the enable secret password is encrypted by default, you’ll need to manually configure the user-mode and enable passwords for encryption. Notice that you can see all the passwords except the enable secret when performing a show running-config on a router:
Router#sh running-config
[output cut]
!
enable secret 5 $1$rFbM$8.aXocHg6yHrM/zzeNkAT.
enable password Brain1
!
[output cut]
line con 0
password Brain1
login
line aux 0
password Brain
login
line vty 0 4
password Brain2
login
!
end
Router#
To manually encrypt your passwords, use the service password-encryption command. Here’s an example of how to do it:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#service password-encryption
Router(config)#^Z
Router#sh run
Building configuration...
[output cut]
!
enable secret 5 $1$rFbM$8.aXocHg6yHrM/zzeNkAT.
enable password 7 0835434A0D
!
[output cut]
!
line con 0
password 7 111D160113
login
line aux 0
password 7 071B2E484A
login
line vty 0 4
password 7 0835434A0D
login
line vty 5 197
password 7 09463724B
login
!
end
Router#config t
Router(config)#no service password-encryption
Router(config)#^Z
There you have it! The passwords will now be encrypted. You just encrypt the passwords, perform a show run , and then turn off the command. You can see that the enable password and the line passwords are all encrypted.
Banners
A good reason for having a banner is to add a security notice to users dialing or telnetting into your internetwork. You can set a banner on a Cisco router so that when either a user logs into the router or an administrator telnets into the router, the banner will give them the information you want them to have. There are four different banners available that you need to be aware of:
Router(config)#banner ?
LINE c banner-text c, where 'c' is a delimiting
character
exec Set EXEC process creation banner
incoming Set incoming terminal line banner
login Set login banner
motd Set Message of the Day banner
Message of the day (MOTD) is the most extensively used banner. It gives a message to every person dialing into or connecting to the router via Telnet or auxiliary port, or through a console port as seen here:
Router(config)#banner motd ?
LINE c banner-text c, where 'c' is a delimiting character
Router(config)#banner motd #
Enter TEXT message. End with the character '#'.
$ Acme.com network, then you must disconnect immediately.
#
Router(config)#^Z
Router#
00:25:12 : %SYS-5-CONFIG_I: Configured from console by
console
Router#exit
Router con0 is now available
Press RETURN to get started.
If you are not authorized to be in Acme.com network, then you must disconnect immediately.
Router>
The preceding MOTD banner essentially tells anyone connecting to the router that if they’re not on the guest list, get lost! The part to understand is the delimiting character— the thing that’s used to tell the router when the message is done. You can use any character you want for it, but you can’t use the delimiting character in the message itself. Also, once the message is complete, press Enter, then the delimiting character, then Enter again. It’ll still work if you don’t do that, but if you have more than one banner, they’ll be combined as one message and put on a single line.
These are the other banners:
Exec banner You can configure a line-activation (exec) banner to be displayed when an EXEC process (such as a line-activation or incoming connection to a VTY line) is created. By simply starting a user exec session through a console port, you will activate the exec banner.
Incoming banner You can configure a banner to be displayed on terminals connected to reverse Telnet lines. This banner is useful for providing instructions to users who use reverse Telnet.
Login banner You can configure a login banner to be displayed on all connected terminals. This banner is displayed after the MOTD banner, but before the login prompts. The login banner can’t be disabled on a per-line basis, so to globally disable it, you’ve got to delete it with the no banner login command.
Router Interfaces Interface configuration is one of the most important router configurations, because without interfaces, a router is a totally useless thing. Plus, interface configurations must be exact to enable communication with other devices. Some of the configurations used to configure an interface are Network layer addresses, media type, bandwidth, and other administrator commands.
Different routers use different methods to choose the interfaces used on them. For instance, the following command shows a Cisco 2522 router with 10 serial interfaces, labeled 0 through 9:
Router(config)#int serial ?
<0-9> Serial interface number
Now it’s time to choose the interface you want to configure. Once you do that, you will be in interface configuration for that specific interface. The command to choose serial port 5, for example, would be:
Router(config)#int serial 5
Router(config)-if)#
The 2522 router has one Ethernet 10BaseT port, and typing interface ethernet 0 can configure that interface, as seen here:
Router(config)#int ethernet ?
<0-0> Ethernet interface number
Router(config)#int ethernet 0
Router(config-if)#
The 2500 router, as previously demonstrated, is a fixed configuration router, which means that when you buy that model, you’re stuck with that physical configuration. To configure an interface, you always use the interface type number sequence, but the 2600, 3600, 4000, and 7000 series routers use a physical slot in the router, with a port number on the module plugged into that slot. So on a 2600 router, the configuration would be interface type slot/port , as seen here:
Router(config)#int fastethernet ?
<0-1> FastEthernet interface number
Router(config)#int fastethernet 0
% Incomplete command.
Router(config)#int fastethernet 0?
/
Router(config)#int fastethernet 0/?
<0-1> FastEthernet interface number
And make note of the fact that you can’t just type int fastethernet 0 . You must type the full command: type slot/port , or int fastethernet 0/0 , or int fa 0/0 . To set the type of connector used, use the media-type command (this is usually auto-detected):
Router(config)#int fa 0/0
Router(config-if)#media-type ?
100BaseX Use RJ45 for -TX; SC FO for -FX
MII Use MII connector
In the following sections, I will continue with the router interface discussion, including how to bring up the interface and set an IP address on a router interface.
Bringing Up an Interface
You can turn an interface off with the interface command shutdown , and turn it on with the no shutdown command. If an interface is shut down, it’ll display administratively down when using the show interfaces (sh int for short) command:
Router#sh int ethernet0
Ethernet0 is administratively down, line protocol is down
[output cut]
Another way to check an interface’s status is via the show running-config command. All interfaces are shut down by default. You can bring up the interface with the no shutdown command ( no shut for short):
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int ethernet0
Router(config-if)#no shutdown
Router(config-if)#^Z
00:57:08 : %LINK-3-UPDOWN: Interface Ethernet0, changed
state to up
00:57:09 : %LINEPROTO-5-UPDOWN: Line protocol on Interface
Ethernet0, changed state to up
Router#sh int ethernet0
Ethernet0 is up, line protocol is up
[output cut]
Configuring an IP Address on an Interface
Even though you don’t have to use IP on your routers, it’s most often what people use. To configure IP addresses on an interface, use the ip address command from interface configuration mode:
Router(config)#int e0
Router(config-if)#ip address 172.16.10.2 255.255.255.0
Router(config-if)#no shut
Don’t forget to turn on an interface with the no shutdown command. Remember to look at the command show interface e0 to see if it’s administratively shut down or not. Show running-config will also give you this information. If you want to add a second subnet address to an interface, you have to use the secondary parameter. If you type another IP address and press Enter, it will replace the existing IP address and mask. This is definitely a most excellent feature of the Cisco IOS. So, let’s try it. To add a secondary IP address, just use the secondary parameter:
Router(config-if)#ip address 172.16.20.2 255.255.255.0
secondary
Router(config-if)#^Z
You can verify that both addresses are configured on the interface with the show runningconfig command ( sh run for short):
Router#sh run
Building configuration...
Current configuration:
[output cut]
!
interface Ethernet0
ip address 172.16.20.2 255.255.255.0 secondary
ip address 172.16.10.2 255.255.255.0
!
I really wouldn’t recommend having multiple IP addresses on an interface because it’s inefficient, but I showed you anyway just in case you someday find yourself dealing with an MIS manager who’s in love with really bad network design and makes you administer it! And who knows? Maybe someone will ask you about it some day and you’ll get to seem really smart because you know!
Serial Interface Commands
Before you jump in and configure a serial interface, there are a couple of things you need to know. First, the interface will usually be attached to a CSU/DSU type of device that provides clocking for the line to the router. But if you have a back-to-back configuration (for example, one that’s used in a lab environment), one end—the data communication equipment (DCE) end of the cable—must provide clocking. By default, Cisco routers are all data terminal equipment (DTE) devices, so you must tell an interface to provide clocking if you need it to act like a DCE device.
You configure a DCE serial interface with the clock rate command:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int s0
Router(config-if)#clock rate ?
Speed (bits per second)
1200
2400
4800
9600
19200
38400
56000
64000
72000
125000
148000
250000
500000
800000
1000000
1300000
2000000
4000000
<300-4000000> Choose clockrate from list above
Router(config-if)#clock rate 64000
%Error: This command applies only to DCE interfaces
Router(config-if)#int s1
Router(config-if)#clock rate 64000
It doesn’t hurt anything to try to put a clock rate on an interface. Notice that the clock rate command is in bits per second. You can see if a router’s serial interface has a DCE cable connected with the show controllers int command.
Router>sh controllers s 0
HD unit 0, idb = 0x297DE8, driver structure at 0x29F3A0
buffer size 1524 HD unit 0, V.35 DCE cable
The next command you need to get acquainted with is the bandwidth command. Every Cisco router ships with a default serial link bandwidth of T-1 (1.544Mbps). But this has nothing to do with how data is transferred over a link. The bandwidth of a serial link is used by routing protocols such as IGRP, EIGRP, and OSPF to calculate the best cost (path) to a remote network. So if you’re using RIP routing, then the bandwidth setting of a serial link is irrelevant, since RIP uses only hop count to determine that. (Routing protocols and metrics are discussed in Next Chapter) .Here is an example of using the bandwidth command:
Router(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
Router(config-if)#bandwidth 64
Did you notice that, unlike the clock rate command, the bandwidth command is configured in kilobits?
Hostnames
You can set the identity of the router with the hostname command. This is only locally significant, which means it has no bearing on how the router performs name lookups or how the router works on the internetwork. Here is an example:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname Brain
Brain(config)#hostname Brainmatics
Brainmatics(config)#
Even though it’s pretty tempting to configure the hostname after your own name, it’s a better idea to name the router something pertinent to the location.
Descriptions
Setting descriptions on an interface is helpful to the administrator and, like the hostname, only locally significant. The description command is a helpful command because you can, for instance, use it to keep track of circuit numbers. Here is an example:
Brainmatics (config)#int e0
Brainmatics (config-if)#description Sales Lan
Brainmatics (config-if)#int s0
Brainmatics (config-if)#desc Wan to Miami circuit:6fdda4321
You can view the description of an interface either with the show running-config command or the show interface command:
Brainmatics #sh run
[cut]
interface Ethernet0
description Sales Lan
ip address 172.16.10.30 255.255.255.0
no ip directed-broadcast
!
interface Serial0
description Wan to Miami circuit:6fdda4321
no ip address
no ip directed-broadcast
no ip mroute-cache
Brainmatics #sh int e0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0010.7be8.25db (bia
0010.7be8.25db)
Description: Sales Lan
[output cut]
Brainmatics #sh int s0
Serial0 is up, line protocol is up
Hardware is HD64570
Description: Wan to Miami circuit:6fdda4321
[output cut]
Brainmatics #
| Description: The Helpful Command Bob, a Senior Network Administrator at Acme Corporation in San Francisco, has over 50 WAN links to various branches throughout the U.S. and Canada. Whenever an interface goes down, Bob spends a lot of time trying to figure out the circuit number as well as the phone number of the responsible provider of the WAN link. The interface description command would be very helpful to Bob because not only can he use this command on his LAN links so he knows where every router interface is connected to, but he would benefit the most by adding circuit numbers to each and every WAN interface, as well as the phone number of the responsible provider. By spending the few hours it would take to add this information to each and every router interface, Bob can save a lot of precious time when his WAN links go down (and they will!), and time is of the essence. |
Viewing and Saving Configurations
If you run through setup mode, you’ll be asked if you want to use the configuration you just created. If you say Yes, then it will copy the configuration running in DRAM, (known as the running-config), into NVRAM, and name the file startup-config . You can manually save the file from DRAM to NVRAM by using the copy runningconfig startup-config command (you can use the shortcut copy run start also:
Brainmatics #copy run start
Destination filename [startup-config]?[Enter]
Warning: Attempting to overwrite an NVRAM configuration
previously written by a different version of the system
image.
Overwrite the previous NVRAM configuration?[confirm][Enter]
Building configuration...
Notice that the message we received tells us we’re trying to write over the older startupconfig. The IOS had just been upgraded to version 12.2, and the last time the file was saved, 11.3 was running. Sometimes, when you see a question with an answer in [ ] , it means that if you just press Enter, you’re choosing the default answer.
Also, when the command asked for the destination filename, the default answer was startupconfig. The “feature” aspect of this command output is that you can’t even type anything else in or you’ll get an error, as seen here:
Brainmatics #copy run start
Destination filename [startup-config]?brain
%Error opening nvram:Brain (No such file or directory)
Brainmatics #
You can view the files by typing show running-config or show startup-config from privileged mode. The sh run command, which is a shortcut for show running-config , tells us that we are viewing the current configuration:
Brainmatics #sh run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Brainmatics
ip subnet-zero
frame-relay switching
!
[output cut]
The sh start command —one of the shortcuts for the show startup-config command— shows us the configuration that will be used the next time the router is reloaded. It also tells us how much NVRAM is being used to store the startup-config file. Here is an example:
Brainmatics #sh start
Using 4850 out of 32762 bytes
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Brainmatics
!
!
ip subnet-zero
frame-relay switching
!
[output cut]
You can delete the startup-config file by using the erase startup-config command, after which you’ll receive an error if you ever try to view the startup-config file:
Brainmatics #erase startup-config
Erasing the nvram filesystem will remove all files!
Continue? [confirm] [Enter]
[OK]
Erase of nvram: complete
Brainmatics #sh start
%% Non-volatile configuration memory is not present
Brainmatics #reload
If you reload or power down and up the router after using the erase startup-config command, you’ll be offered setup mode because there’s no configuration saved in NVRAM. You can press Ctrl+C to exit setup mode at any time (the reload command can only be used from privileged mode).
At this point, you shouldn’t use setup mode to configure your router. Setup mode was designed to help people who do not know how to use the Cash Line Interface (CLI), and this no longer applies to you!
Verifying Your Configuration
Obviously, show running-config would be the best way to verify your configuration, and show startup-config would be the best way to verify the configuration that’ll be used the next time the router is reloaded—right?
Well, once you take a look at the running-config, if all appears well, you can verify your configuration with utilities such as Ping and Telnet. Ping is Packet Internet Groper, a program that uses ICMP echo requests and replies. (ICMP is discussed in Chapter 2, “Internet Protocols.”) Ping sends a packet to a remote host, and if that host responds, you know that the host is alive. But you don’t know if it’s alive and also well —just because you can ping an NT server does not mean you can log in! Even so, Ping is an awesome starting point for troubleshooting an internetwork. Did you know that you can ping with different protocols? You can, and you can test this by typing ping ? at either the router user-mode or privileged mode prompt:
Router#ping ?
WORD Ping destination address or hostname
appletalk Appletalk echo
decnet DECnet echo
ip IP echo
ipx Novell/IPX echo
srb srb echo
If you want to find a neighbor’s Network layer address, either you need to go to the router or switch itself, or you can type show cdp entry * protocol to get the Network layer addresses you need for pinging. (Cisco Discovery Protocol [CDP] is covered in Chapter 9.) Traceroute uses ICMP with IP Time To Live (TTL) timeouts to track the path a packet takes through an internetwork, in contrast to Ping, which just finds the host and responds. And Traceroute can also be used with multiple protocols.
Router#traceroute ?
WORD Trace route to destination address or hostname
appletalk AppleTalk Trace
clns ISO CLNS Trace
ip IP Trace
oldvines Vines Trace (Cisco)
vines Vines Trace (Banyan)
Telnet is the best tool since it uses IP at the Network layer and TCP at the Transport layer to create a session with a remote host. If you can telnet into a device, your IP connectivity just has to be good. You can only telnet to devices that use IP addresses, and you can use Windows hosts or router prompts to telnet to a remote device:
Router#telnet ?
WORD IP address or hostname of a remote system
From the router prompt, you just type a hostname or IP address and it will assume you want to telnet—you don’t need to type the actual command, telnet . In the following sections, I am going to show you how to verify the interface statistics.
Verifying with the show interface Command
Another way to verify your configuration is by typing show interface commands, the first of which is show interface ? . That will reveal all the available interfaces to configure. The following output is from my 2600 routers:
Router#sh int ?
Async Async interface
BVI Bridge-Group Virtual Interface
CTunnel CTunnel interface
Dialer Dialer interface
FastEthernet FastEthernet IEEE 802.3
Loopback Loopback interface
MFR Multilink Frame Relay bundle interface
Multilink Multilink-group interface
Null Null interface
Serial Serial
Tunnel Tunnel interface
Vif PGM Multicast Host interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
accounting Show interface accounting
crb Show interface routing/bridging info
dampening Show interface dampening info
description Show interface description
irb Show interface routing/bridging info
mac-accounting Show interface MAC accounting info
mpls-exp Show interface MPLS experimental accounting info
precedence Show interface precedence accounting info
rate-limit Show interface rate-limit info
summary Show interface summary
switching Show interface switching
| Output modifiers
The only “real” physical interfaces are FastEthernet, Serial, and async; the rest are all logical interfaces. In addition, the newer IOS shows the “possible” show commands that can be used to verify your router interfaces—a very new feature from Cisco. The next command is show interface fastethernet 0/0 . It reveals to us the hardware address, logical address, and encapsulation method, as well as statistics on collisions, as seen here:
Router#sh int fastethernet 0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is 00b0.6483.2320 (bia 00b0.6483.2320)
Description: connection to LAN 40
Internet address is 192.168.1.33/27
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04: 00:00
Last input never, output 00:00:04, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
84639 packets output, 8551135 bytes, 0 underruns
0 output errors, 0 collisions, 16 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
The most important statistic of the show interface command is the output of the line and
data-link protocol status.
If the output reveals that FastEthernet 0/0 is up and the line protocol is up, then the interface
is up and running:
Router#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
The first parameter refers to the Physical layer, and it’s up when it receives carrier detect. The
second parameter refers to the Data Link layer, and it looks for keepalives from the connecting
end. (Keepalives are used between devices to make sure connectivity has not dropped.)
Here is an example:
Router#sh int s0/0
Serial0/0 is up, line protocol is down
If you see the line is up but the protocol is down, as shown above, you are experiencing a
clocking (keepalive) or framing problem. Check the keepalives on both ends to make sure that
they match, the clock rate is set, if needed, and the encapsulation type is the same on both ends.
The output above would be considered a Data Link layer problem.
Router#sh int s0/0
Serial0/0 is down, line protocol is down
If you discover that both the line interface and the protocol are down, it’s a cable or interface problem. The output above would be considered a Physical layer problem. If one end is administratively shut down (as shown next), the remote end would present as down and down:
Router#sh int s0/0
Serial0/0 is administratively down, line protocol is down
To enable the interface, use the command no shutdown from interface configuration mode. The next show interface serial 0/0 command demonstrates the serial line and the maximum transmission unit (MTU)—1500 bytes by default. It also shows the default bandwidth (BW) on all Cisco serial links: 1.544Kbps. This is used to determine the bandwidth of the line for routing protocols such as IGRP, EIGRP, and OSPF. Another important configuration to notice is the keepalive, which is 10 seconds by default. Each router sends a keepalive message to its neighbor every 10 seconds, and if both routers aren’t configured for the same keepalive time, it won’t work. You can clear the counters on the interface by typing the command clear counters :
Router#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set, keepalive set
(10 sec)
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,
0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 16 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions
DCD=down DSR=down DTR=down RTS=down CTS=down
Router#clear counters ?
Async Async interface
BVI Bridge-Group Virtual Interface
CTunnel CTunnel interface
Dialer Dialer interface
FastEthernet FastEthernet IEEE 802.3
Group-Async Async Group interface
Line Terminal line
Loopback Loopback interface
MFR Multilink Frame Relay bundle interface
Multilink Multilink-group interface
Null Null interface
Serial Serial
Tunnel Tunnel interface
Vif PGM Multicast Host interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
Router#clear counters s0/0
Clear "show interface" counters on this interface
[confirm][Enter]
Router#
00:17:35 : %CLEAR-5-COUNTERS: Clear counter on interface
Serial0 by console
Router#
Verifying with the show ip interface Command
The show ip interface command will provide you with information regarding the layer 3 configurations of a router’s interfaces:
Router#sh ip interface
FastEthernet0/0 is up, line protocol is up
Internet address is 1.1.1.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
[output cut]
The status of the interface, the IP address and mask, information on whether an access list is set on the interface, and basic IP information are included in this output.
Using the show ip interface brief Command
The show ip interface brief command is probably one of the most helpful commands that you can ever use on a Cisco router. This command provides a quick overview of the router’s interfaces including the logical address and status:
Router#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.33 YES manual up up
FastEthernet0/1 10.3.1.88 YES manual up up
Serial0/0 10.1.1.1 YES manual up up
Serial0/1 unassigned YES NVRAM administratively down down
Using the show controllers Command
The show controllers command displays information about the physical interface itself. It’ll also give you the type of serial cable plugged into a serial port. Usually, this will only be a DTE cable that plugs into a type of data service unit (DSU).
Router#sh controllers serial 0/0
HD unit 0, idb = 0x1229E4, driver structure at 0x127E70
buffer size 1524 HD unit 0, V.35 DTE cable
cpb = 0xE2, eda = 0x4140, cda = 0x4000
Router#sh controllers serial 0/1
HD unit 1, idb = 0x12C174, driver structure at 0x131600
buffer size 1524 HD unit 1, V.35 DCE cable
cpb = 0xE3, eda = 0x2940, cda = 0x2800
Notice that serial 0/0 has a DTE cable, whereas the serial 0/1 connection has a DCE cable. Serial 0/1 would have to provide clocking with the clock rate command. Serial 0/0 would get its clocking from the DSU.
This was a fun chapter! I really showed you a lot about the Cisco IOS and I really hope you gained a lot of insight into the Cisco router world. This chapter started off by explaining the Cisco Internetwork Operating System (IOS) and how you can use the IOS to run and configure Cisco routers. You learned how to bring a router up and what setup mode does. Oh, and by the way, since you can now basically configure Cisco routers, you should never use setup mode, right? After I discussed how to connect to a router with a console and LAN connection, I covered the Cisco help features, and how to use the CLI to find commands and command parameters. In addition, I discussed some basic show commands to help you verify your configurations. Setting router passwords is one of the most important configurations you can perform on your routers. I showed you the five passwords to set. In addition, I used the hostname, interface description, and banners to help you administer your router. Well, that concludes your introduction to the Cisco Internetwork Operating System (IOS)! And, as usual, it’s super-important for you to have the basics that we went over in this chapter before you move on to the following chapters.
