Loopback Interface - Cisco Small Business 200 Series Administration Manual

IP Configuration
Overview
IP address collisions occur when the same IP address is used in the same IP subnet by more than one
device. Address collisions require administrative actions on the DHCP server and/or the devices that collide
with the device.
When a VLAN is configured to use dynamic IP addresses, the device issues DHCPv4 requests until it is
assigned an IPv4 address from a DHCPv4 server. The management VLAN can be configured with a static or
dynamic IP address.
The IP address assignment rules for the device are as follows:
• Unless the device is configured with a static IPv4 address, it issues DHCPv4 queries until a response
is received from a DHCPv4 server.
• If the IP address on the device is changed, the device issues gratuitous ARP packets to the
corresponding VLAN to check IP address collisions. This rule also applies when the device reverts to
the default IP address.
• The system status LED changes to solid green when a new unique IP address is received from the
DHCP server. If a static IP address has been set, the system status LED also changes to solid green.
The LED flashes when the device is acquiring an IP address and is currently using the factory default
IP address 192.168.1.254.
• The same rules apply when a client must renew the lease, prior to its expiration date through a
DHCPREQUEST message.
• With factory default settings, when no statically-defined or DHCP-acquired IP address is available,
the default IP address is used. When the other IP addresses become available, the addresses are
automatically used. The default IP address is always on the management VLAN.

Loopback Interface

Overview
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 operational state of a loopback interface is always up. You define an IP address on it and use this IP
address as the local IP address for IP communication with remote IP applications. Communication remains
intact as long as the remote applications can be reached from any one of the switch's active (non-loopback)
IP interfaces. On the other hand, if the IP address of an IP interface is used in communicating with remote
applications, the communication will be terminated when the IP interface is down.
A loopback interface does not support bridging; it cannot be a member of any VLAN, and no layer 2
protocol can be enabled on it.
Cisco Small Business 200 Series Smart Switch Administration Guide
18
225