Chapter 9 Configuring Virtual Loopback And Null Interfaces; Information About Configuring Virtual Interfaces; Virtual Loopback Interface Overview - Cisco NCS 5500 Series Configuration Manual

Configuring Virtual Loopback and Null Interfaces
This module describes the configuration of loopback and null interfaces. Loopback and null interfaces are
considered virtual interfaces.
A virtual interface represents a logical packet switching entity within the router. Virtual interfaces have a
global scope and do not have an associated location. Virtual interfaces have instead a globally unique
numerical ID after their names. Examples are Loopback 0, Loopback 1, and Loopback 99999. The ID is
unique per virtual interface type to make the entire name string unique such that you can have both Loopback
0 and Null 0.
Loopback and null interfaces have their control plane presence on the active route switch processor (RSP).
The configuration and control plane are mirrored onto the standby RSP and, in the event of a failover, the
virtual interfaces move to the ex-standby, which then becomes the newly active RSP.
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Information About Configuring Virtual Interfaces

To configure virtual interfaces, you must understand the following concepts:

Virtual Loopback Interface Overview

A virtual loopback interface is a virtual interface with a single endpoint that is always up. Any packet transmitted
over a virtual loopback interface is immediately received by the same interface. Loopback interfaces emulate
a physical interface.
In Cisco IOS XR Software, virtual loopback interfaces perform these functions:
• Loopback interfaces can act as a termination address for routing protocol sessions. This allows routing
• You can ping the loopback interface to verify that the router IP stack is working properly.
In applications where other routers or access servers attempt to reach a virtual loopback interface, you must
configure a routing protocol to distribute the subnet assigned to the loopback address.
Information About Configuring Virtual Interfaces, page 139
protocol sessions to stay up even if the outbound interface is down.
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