Continue the series of Abstract CCNA study guide book .
Router Interfaces
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
Then we can choose serial port 5, for example:
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)#
As I showed you above, the 2500 router is a fixed-configuration
router. This means that when you buy that model, you’re stuck with that
physical configuration.
To configure an interface, we used the interface type
number sequence, but with the 2600 and 2800 series routers (actually, any
ISR router for that matter), there’s a physical slot in the router, with a port
number on the module plugged into that slot. So on a modular 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
For the ISR series, it’s basically the same, only you get
even more options. For example, the built-in FastEthernet interfaces work with
the same configuration we used with the 2600 series:
Todd(config)#int fastEthernet 0/?
<0-1> FastEthernet interface number
Todd(config)#int fastEthernet 0/0
Todd(config-if)#
But the rest of the modules are different—they use three
numbers instead of two. The first 0 is the router itself, and then you choose
the slot, and then the port. Here’s an example of a serial interface on my
2811:
Todd(config)#interface serial ?
<0-2> Serial interface number
Todd(config)#interface serial 0/0/?
<0-1> Serial interface number
Todd(config)#interface serial 0/0/0
Todd(config-if)#
you should always view a running-config output first so you
know what interfaces you have to deal with.
Just understand that if you type interface e0 on a
2500, interface fastethernet 0/0 on a 2600, or interface serial 0/1/0
on a 2800, all you’re doing is choosing an interface to configure, and
basically, they’re all configured the same way after that.
Bringing Up an Interface
You can disable an
interface with the command shutdown and enable it with the no
shutdown command.
If an interface is shut down, it’ll display
administratively down when you use the show interfaces command (sh int for
short) or show running-config :
Todd#sh int f0/1
FastEthernet0/1 is administratively down, line protocol is
down
[output cut]
All interfaces are shut down by default.
You can bring up the interface with the no shutdown command
(no shut for short):
Todd#config t
Todd(config)#int f0/1
Todd(config-if)#no shutdown
Configuring an IP Address on an Interface
To configure IP addresses on an interface, use the ip
address command from interface configuration mode:
Todd(config)#int f0/1
Todd(config-if)#ip address 172.16.10.2 255.255.255.0
Don’t forget to enable the interface with the no shutdown
command.
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.
So let’s try it. To add a secondary IP address, just use
the secondary parameter:
Todd(config-if)#ip address 172.16.20.2 255.255.255.0 ?
secondary Make this IP address a secondary address
Todd(config-if)#ip address 172.16.20.2 255.255.255.0
secondary
I really wouldn’t recommend having multiple IP addresses on
an interface because it’s ugly and inefficient.
Using the Pipe
The pipe is used as
output modifier .This pipe ( | ) allows us to wade through all the
configurations or other long outputs and get straight to our goods fast. Here’s
an example:
Todd#sh run | ?
append Append
redirected output to URL (URLs supporting append operation only)
begin Begin
with the line that matches
exclude Exclude
lines that match
include Include
lines that match
redirect Redirect
output to URL
section Filter
a section of output
tee Copy
output to URL
Todd#sh run | begin interface
interface FastEthernet0/0
description Sales VLAN
ip address 10.10.10.1 255.255.255.248
interface FastEthernet0/1
ip address 172.16.20.2 255.255.255.0 secondary
ip address 172.16.10.2 255.255.255.0
I use it a lot when I am looking at a large routing table
to find out whether a certain route is in the
routing table. Here’s an example:
Todd#sh ip route | include 192.168.3.32
R 192.168.3.32 [120/2] via 10.10.10.8, 00:00:25,
FastEthernet0/0
Todd#
Serial Interface Commands
Before you configure a serial interface, you need some key
information— like knowing that the interface will usually be attached to a
CSU/DSU type of device that provides clocking for the line to the router, as
I’ve shown in Figure below.
DTE Here
you can see that the serial interface is used to connect to a DCE network via a
CSU/DSU that provides the clocking to the router interface. But if you have a
back-to-back configuration (for example, one that’s used in a lab environment
like I’ve shown you in Figure 4.5), 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, which means that you must configure 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:
Todd#config t
Todd(config)#int s0/0/0
Todd(config-if)#clock rate 1000000
The clock rate command is set in bits per second. Besides
looking at the cable end to check for a label of DCE or DTE, you can see if a
router’s serial interface has a DCE cable connected with the show controllers int
command:
Todd#sh controllers s0/0/0
Interface Serial0/0/0
Hardware is GT96K
DTE V.35idb at 0x4342FCB0, driver data structure at
0x434373D4
Here is an example of an output that shows a DCE
connection:
Todd#sh controllers s0/2/0
Interface Serial0/2/0
Hardware is GT96K
DCE V.35, clock rate 1000000
The next command you need to get acquainted with is the
bandwidth command.
Here’s an example of using the bandwidth command:
Todd#config t
Todd(config)#int s0/0/0
Todd(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
inherit Specify
that bandwidth is inherited
receive Specify
receive-side bandwidth
Todd(config-if)#bandwidth 1000
Did you notice that, the bandwidth command is configured in
kilobits?
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