Specifications

Basic Router Configuration
Configuring a Loopback Interface
8
Cisco 3900 Series, 2900 Series, and 1900 Series Integrated Services Routers Software
DETAILED STEPS
Example
The loopback interface in this sample configuration is used to support Network Address Translation
(NAT) on the virtual-template interface. This configuration example shows the loopback interface
configured on the gigabit ethernet interface with an IP address of 200.200.100.1/24, which acts as a static
IP address. The loopback interface points back to virtual-template1, which has a negotiated IP address.
!
interface loopback 0
ip address 200.200.100.1 255.255.255.0 (static IP address)
ip nat outside
!
interface Virtual-Template1
ip unnumbered loopback0
no ip directed-broadcast
ip nat outside
!
Verifying Configuration
To verify that you have properly configured the loopback interface, enter the show interface loopback
command. You should see verification output similar to the following example.
Router# show interface loopback 0
Loopback0 is up, line protocol is up
Hardware is Loopback
Internet address is 200.200.100.1/24
MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation LOOPBACK, loopback not set
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Queueing strategy: fifo
Output queue 0/0, 0 drops; input queue 0/75, 0 drops
Command Purpose
Step 1
interface type number
Example:
Router(config)# interface Loopback 0
Router(config-if)#
Enters configuration mode for the loopback
interface.
Step 2
ip address ip-address mask
Example:
Router(config-if)# ip address 10.108.1.1
255.255.255.0
Router(config-if)#
Sets the IP address and subnet mask for the
loopback interface.
Step 3
exit
Example:
Router(config-if)# exit
Router(config)#
Exits configuration mode for the loopback
interface and returns to global configuration
mode.