Loopback port configuration

Configure logical Port
A logical port is relative to a physical port. It refers to a port that can implement data exchange but does not exist physically and needs to be configured. Vrouters generally provide five types of logical ports: Loopback port, NULL Port, Tunnel port, Dialer port, and sub-port.
12.3.1 Loopback port configuration
The Loopback (Loopback) port is a local port of the router simulated by software. It is always in the UP state. Data packets sent to the Loopback port are processed locally on the vro, including route information. The IP address of the Loopback port can be used as the router ID of the OSPF routing protocol, as the network port for Telnet or remote Telnet access. Configuring a Loopback port is similar to configuring an Ethernet port. You can regard it as a virtual Ethernet port.
Set the Loopback port. After creating the Loopback of the specified port number, you can configure the communication parameters (such as IP addresses) of the Loopback port just like configuring the Ethernet port.
Router (config) # interface loopback-interface-number
Delete the Loopback port. Because Loopback is a virtual port, it only exists logically. You can use the no command to delete the specified Loopback port as needed.
Router (config) # no interface loopback-interface-number
Displays the Loopback port status.
Router # show interfaces loopback-interface-number
 
NULL Port Configuration
The router also provides NULL (NULL) virtual ports. This virtual port is equivalent to only one available system device. NULL (NULL) is always in the UP state and never sends or receives network data actively. Any data packet sent to the NULL Port will be discarded, attempts to encapsulate any link layer protocol on a NULL Port will not succeed.
Enter the NULL Port configuration.
 
Router (config) # interface null 0
Allow NULL ports to send ICMP unreachable messages.
Router (config-if) # ip unreachables
Disable the NULL Port from sending ICMP unreachable messages.
Router (config-if) # no ip unreachables
The NULL Port is more used for filtering network data streams. If you use a NULL Port, You can route the network data stream that you do not want to process to the NULL Port without using the access list, for example:
Router (config) # ip route 127.0.0.0 255.0.0.0 null 0
Because the NULL Port is a "NULL" system device, it is not displayed like Ethernet or other ports (for example, you cannot see it by running the show running-config command ). At the same time, as a system device, you cannot use the no interface null command to delete the NULL Port.

 

This article is from cisco"