Interface configuration is one of the most important router configurations, because without interfaces, a router is pretty much a completely useless object. Plus, interface configurations must be totally precise 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.
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 0/0, for example, would be:
Router#
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#
Router(config)#int se 0/0
Router(config-if)#
Serial 0/0 means that this is the first interface from a module, attached in slot 0 into the cassis of the router. The 3745 router is a modular router, which means that you can buy this router and after that to attach additional modules with interfaces.
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. I prefer to use command sh ip int brief, which is much more generalized and users friendly:
Router#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset administratively down down
Serial0/0 unassigned YES unset administratively down down
FastEthernet0/1 unassigned YES unset administratively down down
Router#
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int fa 0/0
Router(config-if)#no shu
Router(config-if)#end
Router#
*Mar 1 00:12:34.587: %SYS-5-CONFIG_I: Configured from console by console
Router#
*Mar 1 00:12:35.915: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 1 00:12:36.915: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
Router#
Router#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
Serial0/0 unassigned YES unset administratively down down
FastEthernet0/1 unassigned YES unset administratively down down
Router#
Configuring an IP Address on an Interface
To configure IP addresses on an interface, use the ip address command from interface configuration mode:
Router#
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int fa 0/0
Router(config-if)#ip addr 192.168.10.1 255.255.255.0
Router(config-if)#
Don’t forget to turn on an interface with the no shutdown command. If you want to add a second subnet address to an interface, you have to use the secondaryparameter. 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:
Router(config-if)#ip addr 172.16.248.254 255.255.255.128 secondary
Router(config-if)#
Router(config-if)#end
Router#
Router#sh run int fa 0/0
Building configuration...Current configuration : 150 bytes
!
interface FastEthernet0/0
ip address 172.16.248.254 255.255.255.128 secondary
ip address 192.168.10.1 255.255.255.0
duplex auto
speed auto
end
Serial Interfaces
The serial 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.
Let’s set a DCE serial interface:
Router(config)#int se 0/0
Router(config-if)#clock rate ?
Speed (bits per second)
1200
2400
4800
9600
14400
19200
28800
32000
38400
48000
56000
57600
64000
72000
115200
125000
128000
148000
192000
250000
256000
384000
500000
512000
768000
800000
1000000
2000000
4000000
8000000<300-8000000> Choose clockrate from list above
Router(config-if)#clock rate 64000
Router(config-if)#
You can see if a router’s serial interface has a DCE cable connected with the show controllers int command.
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.
Router(config)#int se 0/0
Router(config-if)#band
Router(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
inherit Specify how bandwidth is inheritedRouter(config-if)#bandwidth 64
Router(config-if)#
Notice that, unlike the clock rate command, the bandwidth command is configured in kilobits!
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