The IPv4 loopback interface is a logical interface configure internally in the router. In this lesson, we should discuss the IPv4 loopback interface configuration but at this stage, it is necessary to know the Pv4 interface configuration before the IPv4 loopback interface configuration. I have already discussed the IPv4 interface configuration in one of my previous lessons. The address of the lesson is Cisco Router Interface Configuration
So let’s come to the topic as I said that the loopback interface is a logical interface and it is not physically found in the router. Therefore it is cannot connect to any other device physically. Its state is always up when a router is functioning. It is very useful in troubleshooting and managing a Cisco router because it ensures that at least one interface should always available for management. For example, we can use a loopback for testing purpose. We can use the loopback interface for routing purposes instead of a physical interface address.
Usage of Loopback Interface
We can use the loopback to recognize the device because the address of loopback never changes, therefore; it is the preferred method for identification of the router. It is always up and allows Border Gateway Protocol (BGP) neighbors between two routers to stay up even if one of the outbound physical interfaces connected between the routers is down. Loopback interfaces are also used as the termination points for Remote Source-Route Bridging and Data-Link Switching Plus.
The OSPF also uses the loopback address to determine protocol-specific properties for the device or network. Additionally, ping and mpls also need a loopback address to function correctly. We can also apply stateless firewall filters to the loopback address to filter packets originating from, or destined for, the Routing Engine.
The configuration of the Loopback interface
We can enable multiple loopbacks on the Cisco router. Each loopback required a unique IPv4 address. The number is different for routers but it starts at 0 and it is going up to billions.