Loopback Interface - Cisco 300 Series Cli Manual

Introduction
System Modes

Loopback Interface

OL-32830-01 Command Line Interface Reference Guide
Sx300/ESW2-350 and Sx500 devices function in either Router (Layer 3) or Switch
(Layer 2) system mode, while SG500X devices always function in Switch plus
Router system mode. Therefore, the parts of this section that refer to switching
between the two system modes are not relevant for SG500X devices.
In Switch system mode, the switch forwards packets as a VLAN-aware bridge. In
Router system mode, the switch performs both IPv4 routing and VLAN-aware
bridging.
If Router system mode is selected, a single IP address is supported on the default
VLAN. The user also must configure a default gateway.
If Switch system mode is selected, the user can manage the device on any IP
interface configured on the device, as long as a default route is configured. In
Router system mode, the switch routes traffic between IP VLANs, and bridges
traffic within VLANs.
When the switch operates in Router system mode, the following features are not
supported:
• Protocol-based VLANs
• MAC-based VLANs
• DVA, Multicast TV VLAN
• Per flow policing
When an IP application on a router wants to communicate with a remote IP
application, it must select the local IP address to be used as its IP address. It can
use any IP address defined on the router, but if this link goes down, the
communication is aborted, even though there might well be another IP route
between these IP applications.
The loopback interface is a virtual interface whose operational state is always up.
If the IP address that is configured on this virtual interface is used as the local
address when communicating with remote IP applications, the communication will
not be aborted even if the actual route to the remote application was changed.
The name of the loopback interface is loopback1.
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