Chapter 17 – Operating Cisco Routers

Differences between routers and switches: 

The configuration of IP addresses differs in some ways, with switches using a VLAN 
interface and routers using an IP address configured on each working interface. 
• Many Cisco router models have an auxiliary (Aux) port, intended to be connected to an 
external modem and phone line to allow remote users to dial in to the router, and access 
the CLI, by making a phone call. Cisco switches do not have auxiliary ports. 
Router IOS defaults to disallow both Telnet and SSH into the router because of 
the default setting of transport input none in vty configuration mode. Chapter 8, 
"Configuring Basic Switch Management," already discussed the various options on this 
command to enable Telnet (transport input telnet), SSH (transport input ssh), or both 
(transport input all or transport input telnet ssh).
  • One minor difference between Cisco switches and routers is that routers support a much wider variety of interfaces.  
  • Today, LAN switches support Ethernet LAN interfaces of various speeds.  
  • Routers support a variety of other types of interfaces, including serial interfaces, cable TV, DSL, 3G/4G wireless, and others not mentioned in this book. 
  • Some Cisco routers have serial interfaces.  
  • As you might recall from Chapter 3, Cisco routers use serial interfaces to connect to a serial link.  
  • Each point-to-point serial link can then use High-Level Data Link Control (HDLC, the default) or Point-to-Point Protocol (PPP). 
  • Types of router interfaces: 
    • interface ethernet 0  
    • interface fastEthernet 0/1  
    • interface gigabitethernet 0/0  
    • interface serial 1/0/1 
  • Two of the most common commands to display the interfaces, and their status, are the show ip interface brief and show interfaces commands. 
Example 17-1 Listing the Interfaces in a Router 
show Ip Intertace brlet 
Interface 
I p -Address 
Embedded-Service -EnglneO/O unassigned 
gagab1tEthernet0/é 
GIgab1tEthernetO/1 
gerlalo/o/o 
$erlalo/o/@ 
serlalo/l/o 
serialo/l/l 
172.16. 1.1 
unass Igned 
172.16.4.1 
172.16.5.1 
unass Igned 
unass Igned 
0K? 
YES 
YES 
YES 
Y Eg 
Y Eg 
Y Eg 
Y Eg 
Method 
NVRAM 
NVRAM 
manual 
NVRAM 
NVRAM 
NVRAM 
NVRAM 
Status 
admin Is tratlvely 
down 
administratively 
up 
up 
up 
administratively 
down 
down 
down 
protocol 
down 
down 
down 
up 
up 
up 
down 
show Interfaces serial 0/0/0 
geria10/O/O lg up, line protocol lg up 
Hardware Is WIC MBRD gerlal 
Degerlptlon: Link In lab to R2•g go/0/@ 
Internet address Is 172.16.4.1/24 
wrt.J 1500 bytes, 1544 Kblt/seg, DLY 20000 usec, 
rellablllty 255/255, txload 1/255, rxload 1/255 
encapsulation HDL$, loopback not set 
Keepallve set (10 sec) 
Last Input 00: 00:03, output 00 output hang never 
Last clearing or "show Interface" counters never
  • Each interface has two interface status codes. 
  • To be usable, the two interface status codes must be in an “up” state.  
  • The first status code refers essentially to whether Layer 1 is working, and the second status code mainly (but not always) refers to whether the data link layer protocol is working. 
Table 17- 
Key 
Topic 
Name 
Line 
status 
Protocol 
status 
2 Interface Status Codes and Their Meanings 
Location General Meaning 
First status Refers to the Layer 1 status. (For example, is the cable installed, is it 
code 
Second 
status 
code 
the ri ht/wro cable, is the device on the other end owered on?) 
Refers generally to the Layer 2 status. It is always down if the line 
status is down. If the line status is up, a protocol status of down is 
usually caused by a mismatched data link layer configuration.
Key 
Topic 
Table 17-3 
Line Status 
Typical Combinations of Interface Status Codes 
Protocol Typical Reasons 
Status 
Administratively Down 
down 
Down 
Up 
Down 
Down 
The interface has a shutdown command configured on it. 
The interface is not shutdown, but the physical layer has 
a problem. For example, no cable has been attached to the 
interface, or with Ethernet, the switch interface on the other 
end of the cable is shut down or the switch is powered off. 
Almost always refers to data link layer problems, most often 
configuration problems. For example, serial links have this 
combination when one router was configured to use PPP and 
the other defaults to use HDLC. 
r 1 and er 2 of this interface are function •
172181. 
.11 
so,'0/1 
GO/O 
172162. 
172_16.3. 
Ra 
_101 
_102 
_101 
_102 
Figure 17-6 IPv4Addresses Used in Example 17-2 
Example 17-2 Configuring IP Addresses on Cisco Routers 
configure terminal 
Enter configuration commands, one per line. End wlth CNTL/Z. 
Interface GO/O 
RI (conflg 
RI (conflg 
RI (conflg 
RI (conflg 
RI (conflg 
RI (conflg 
RI (contlg-lt) 
RI (contlg-lt) 
RI (contlg-lt) 
Ip address 172.16.1.1 2SS.2SS.2SS.O 
no tdown 
Interface SO/O/O 
Ip address 172.16.4.1 2SS.2SS.2SS.O 
no tdown 
Interface so/o/l 
Ip address 172.16.5.1 255.255.255.0 
no shutdown
Key 
Topic 
Table 17-4 
Cornmand 
Key Commands to List Router Interface Status 
show i interface brief 
show rotocols 
e number 
show interfaces [ty numberl 
Lines of Output 
per Interface 
1 
1 or 2 
Many 
IP Configuration 
Listed 
Address 
Address/mask 
Address/mask 
Interface Status 
Listed? 
Yes 
Yes 
Yes
Example 17-3 Verifying IP Addresses on Cisco Routers 
RI # show protocols 
Global values: 
Internet Protocol routing Is enabled 
Embedded-service-Englneo/o Is admlnlstratlvely down, 
llne protocol Is down 
Glgabltßtherneto/o 
Internet address 
Glgabl / 1 
ser1a10/0/0 Is up, 
Internet address 
ser1a10/0/1 Is up, 
Internet address 
Is up, llne protocol Is up 
Is 172.16.1.1/24 
Is admlnlstratlvely down, line protocol Is down 
11ne protocol Is up 
Is 172.16.4.1/24 
11ne protocol Is up 
Is 172.16.5.1/24 
ser1a10/1/0 Is admlnlstratlvely down, 
ser1a10/1/1 Is administratively down, 
11ne protocol Is down 
11ne protocol Is down
Key 
Topic 
Table 17-4 
Cornmand 
Key Commands to List Router Interface Status 
show i interface brief 
show rotocols 
e number 
show interfaces [ty numberl 
Lines of Output 
per Interface 
1 
1 or 2 
Many 
IP Configuration 
Listed 
Address 
Address/mask 
Address/mask 
Interface Status 
Listed? 
Yes 
Yes 
Yes

Bandwidth and clock rate on serial interfaces: 

  • WAN serial links can run at a wide variety of speeds.  
  • To deal with the wide range of speeds, routers physically slave themselves to the speed as dictated by the CSU/DSU through a process called clocking.  
  • As a result, routers can use serial links without the need for additional configuration or auto-negotiation to sense the serial link’s speed.  
  • The CSU/DSU knows the speed, the CSU/DSU sends clock pulses over the cable to the router, and the router reacts to the clocking signal. 
clock rate Command Goes Here 
DCE 
Router 2 
Serial 
Cable 
Router 1 
Figure 17-7 
DTE 
Serial 
Cable 
Rx 
DTE Cable 
Serial Link in Lab 
Rx 
DCE Cable
  • Using the correct cabling works, as long as you add one command: the clock rate interface subcommand.  
  • This command tells that router the speed at which to transmit bits on a serial link like the one shown in Figure 17-7.  
  • The clock rate command is not needed on real serial links, because the CSU/DSU provides the clocking.  
  • When you create a serial link in the lab using cables, without any real CSU/DSUs on the link, the router with the DCE cable must supply that clocking function, and the clock rate command tells the router to provide it. 
Example 17-4 Router RI Configuration with the clock rate Command 
show running-config 
! llnes omltted tor brevity 
Interface serial 0/0/0 
Ip address 172.16.4.1 255.255.255.0 
clock rate 2000000 
Interface serial 0/0/1 
Ip address 172.16.5.1 255.255.255.0 
clock rate 
lines omitted tor brevity 
RIå show controllers serial 0/0/1 
Interface Ser1a10 
Hardware Is PowerQU1cc MPC860 
DCE v.35, clock rate 128000 
Idb at ox8169BB20, driver data structure at OxB16A35E4 
Lines omitted for brevity
  • The clock rate command does not allow just any speed to be configured.  
  • However, the list of speeds does vary from router to router. 
  • The clock rate command sets the actual Layer 1 speed used on the link, if no CSU/DSU is used, as just described. 
  • clock rate 128000 command sets the clock rate to 128 kbps 
To see the clock rate, look for the clock rate interface subcommand in the configuration, 
or use the show controllers serial type number command (as shown in Example 17-4.) 
To see the bandwidth setting on an interface, look for the bandwidth interface subcom- 
mand in the configuration, or use the show interfaces lope numberl command (as shown 
in Example 17-5).

Router auxiliary port

  • Both routers and switches have a console port to allow administrative access, but most Cisco routers have an extra physical port called an auxiliary (Aux) port.  
  • The Aux port typically serves as a means to make a phone call to connect into the router to issue commands from the CLI. 
Table 17-7 Chapter 17 Configuration Command Reference 
command 
interface type number 
ip address address mask 
(nol shutdown 
duplex (full I half I auto) 
speed (10 1 100 1 1000) 
clock rate rate-in-bps 
description text 
bandwidth rate-in-kbps 
Description 
Global command that moves the user into configuration mode of 
the named interface. 
Interface subcommand that sets the router's IPv4 address and 
mask. 
Interface subcommand that enables (no shutdown) or disables 
(shutdown) the interface. 
Interface command that sets the duplex, or sets the use of IEEE 
autonegotiation, for router LAN interfaces that support multiple 
speeds. 
Interface command for router Gigabit (10/100/1000) interfaces 
that sets the speed at which the router interface sends and receives 
data. 
Interface command that sets the speed at which the router 
supplies a clocking signal, applicable only when the router has a 
DC„E cable installed. The unit is bits/second. 
An interface subcommand with which you can type a string of 
text to document information about that 
ticular interface. 
Interface command that sets the speed at which the router 
considers the interface to operate, but does not dictate or control 
the actual speed. IOS then uses this setting for features that need 
some information about the speed of the interface (for example, 
some routing protocols use the information when calculating 
metrics). The unit is kilobits/second.
Table 17-8 Chapter 17 EXEC Command Reference 
Command 
show interfaces ltype numberl 
show ip interface brief 
show protocols [type numberl 
show controllers [type 
numberl 
Purpose 
Lists a large set of informational messages about each 
interface, or about the one specifically listed interface. 
Lists a single line of information about each interface, 
including the IP address, line and protocol status, and the 
method with which the address was configured (manual or 
D namic Host Conf uration Protocol DHCP 
Lists information about the listed interface (or all interfaces 
if the interface is omitted), including the IP address, mask, 
and line/ rotocol status. 
Lists many lines of information per interface, or for one 
interface, for the hardware controller of the interface. On 
serial interfaces, this command identifies the cable as either 
a DCE or DTE cable.

Leave a Reply