836 Configuring Routing InterfacesFor each VLAN routing interface you can assign a static IP address, or you canallow a network DHCP server to assign a dynamic IP address.When a port is enabled for bridging (L2 switching) rather than routing,which is the default, all normal bridge processing is performed for an inboundpacket, which is then associated with a VLAN. Its MAC Destination Address(MAC DA) and VLAN ID are used to search the MAC address table. Ifrouting is enabled for the VLAN, and the MAC DA of an inbound unicastpacket is that of the internal router interface, the packet is routed. Aninbound multicast packet is forwarded to all ports in the VLAN, plus theinternal bridge-router interface, if it was received on a routed VLAN.Since a port can be configured to belong to more than one VLAN, VLANrouting might be enabled for all of the VLANs on the port or for only some ofthe VLANs on the port. VLAN Routing can be used to allow more than onephysical port to reside on the same subnet. It could also be used when aVLAN spans multiple physical networks, or when additional segmentation orsecurity is required.What Are Loopback Interfaces?A loopback interface is a logical interface that is always up and, because itcannot go down, allows the switch to have a stable IP address that othernetwork devices and protocols can use to reach the switch. The loopback canprovide the source address for sent packets.The loopback interface does not behave like a network switching port.Specifically, there are no neighbors on a loopback interface; it is a pseudo-device for assigning local addresses so that the other layer 3 devices cancommunicate with the switch by using the loopback IP address. The loopbackinterface is always up and can receive traffic from any of the existing activeinterfaces. Thus, given reachability from a remote client, the address of theloopback can be used to communicate with the switch through variousservices such as Telnet and SSH. In this way, the IP address on a loopbackbehaves identically to any of the local addresses of the VLAN routinginterfaces in terms of the processing of incoming packets.NOTE: In this context, loopback interfaces should not be confused with theloopback IP address, usually 127.0.0.1, assigned to a host for handling self-routedpackets.
