![]() A loopback (or even many of them!) can serve as a great way to create distinction where none really exists - you could terminate distinct connections/services on multiple different loopbacks to provide uniqueness of source/destination without having to light-up additional physical ports or additional devices. Lastly, if you have many equal-cost paths (ECMP) between a router and another destination, it can be useful to substitute the address of a loopback interface in your communication with that destination - this enables your connection between the router and the other host to travel any and/or all of those ECMP options, and to automatically redirect among them should a failure to one path occur.īut maybe you're thinking more in terms 'how does it actually work', in which case I'd say 'as an imaginary point of contact or a virtual interface'. When it comes to routing protocols, it's also useful to have an identity independent of any of your physical (or logical!) interfaces from which to draw your identification to other routers. You can have the same thing built out on many operating systems, providing a type of 'host identity' as well as a unique point of origination/termination for services and connections such as GRE tunnels, management traffic, authentication requests, and other such functions. Imagine a loopback IP as an address that the router holds, and presents, but which isn't tied to any of the physical interfaces. This is why many IP based networks will have routers with one or more loopback interfaces with their own (usually RFC 1918) addresses, not 127.0.0.1/8. ![]() ![]() At the time, there was strong opposition to the idea of centralizing and codifying address policy in the routing components of the network. Cisco IOS-derived network operating systems generally don't configure a loopback interface by default, as the code base predates many of these conventions as well as the widespread adoption of TCP/IP. Linux, Windows, and other Unix-derived operating systems generally configure 127.0.0.1/8 by default, and pinging 127.0.0.1 does the expected thing. This is often useful for fingerprinting remote hosts. The IPv4 standards, specifically RFC1122, say that 127.0.0.0/8 is the loopback range, not 127.0.0.1/32, however various stacks have liberal interpretations of standards compliance and do not behave in a fully compliant manner. ![]() This concept of a loopback interface is different from the IPv4 addressing plan, and other protocols have their own loopback address conventions. Its more common modern use is to have a permanently "up" interface to use as a peer address for cases like multiple homed BGP or a logical management address which can be addressed independently of the physical addressing plan. Loopback interfaces were originally invented as a hack to allow early Ethernet systems to do testing of the rx and tx sides of their network stacks, as the original Ethernet hardware wasn't able to handle both sides at full line rate. ![]()
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