Can someone give me an explanation of a loopback interface?
Loopback is always 'Up' so always available. If you have G0/0/0 with IP 192.168.1.1 (for example) and that link is down, then it won't do anything with that 192.168.1.1 IP.
If you have 192.168.1.2 set as a Loopback, it will always be accessible as long as the router is up and there's a way to get to it.
In practice I see the primary use of loopbacks as the 'canonical' management IP for devices. They're also used for several VoIP tasks which require an IP to bind to.
This is what I do for everything. Loopback with /32 for all management. It's always up, and always in my OSPF if it has a way to talk to a neighbor. Unlike a Vlan, where is there is nothing on the vlan, the vlan isn't up and it isn't advertised.
Now VXLan, DHCP and anycast-gateways, loopbacks get wildly interesting.
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. 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. This concept of a loopback interface is different from the IPv4 addressing plan, and other protocols have their own loopback address conventions.
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 is often useful for fingerprinting remote hosts.
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. 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. At the time, there was strong opposition to the idea of centralizing and codifying address policy in the routing components of the network. 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.
Not always configured, and you can have many logical loopbacks. The address can be anything, and is not assigned to a physical interface, so it cannot go down unless you command it. Can be used to source routing, tunnel endpoint etc. Using it as a management interface means you can still access in band but isolate and control access to the device.
127.0.0.0 is an internal address range for a configured interface. If you ping an address in that range, it sort of goes down the stack to the nic then back up. Internal to the device.
But a loopback address on a router/switch can be advertised
Imagine a loopback IP as an address that the router holds, and presents, but which isn't tied to any of the physical interfaces.
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.
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.
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.
But 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'. 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.
Hope this helps!
So you have real physical interfaces. A loopback is a virtual interface. It not ethernet. Its not fiber. I just exists, because you told the router about it.
Like any other interface, it has to be routed to. So if a packet came in on a physical interface, to get to the loopback, it would need to be routed to it. If you just had one router, and it is the default gateway, then the router would already know how to route to it. But if you are multiple hops away, it needs to be advertised like any other network through a routing protocol or static routes.
What is its purpose. Usually its used for management. As someone else stated below, its always up. So in the case of a router with multiple paths to get to it, the loopback will always be reachable regardless of the one down network. There are more uses for loopbacks besides management, but its too much to fit in to a summary like this.
It probably doesnt make allot of sense right now. But it will. Especially when you start dealing with Layer 3 switching; which use SVI's.
A loopback is an address for "me" that everyone else can use.
The idea being that if you don't set it, "you" don't exist, but are only a black box behind interfaces. The loopback allows you to identify yourself consistently across all interfaces.
It also allows you to set clearly defined rules for management distinct from rules for traffic.
In the networking world, we use IP Addresses to identify hosts, links, services, devices, and many other things.
Loopbacks are addresses separated from an interface or a link.
[Considering a management address,] let's say we apply the management address directly to the physical interface. What happens when that link goes down? The management address is no longer reachable and all traffic to or from it stops.
If instead, we place that management IP on a loopback, and use the rest of the network and routing protocols to "spread it around", then when that same link goes down, only the interface IP disappears. The management IP will still be attached to an interface that is up and may be reachable via any number of other routes.
Edit: So I'm a little surprised by how this blew up, and therefore a lot embarrassed by my use of the phrase "spread it around". I, of course, meant to write "advertise the location of the management address across those private links" but was clearly too befuddled by stupid user requests due to being at work at the time.
Now that this is such a well-viewed topic, I'll elaborate on what we might use a loopback for, now that we know it's superpower.
Management addresses that work regardless of how bad your network has failed--assuming there is still at least one link between you and the device
iBGP endpoint addresses so we keep our connection with the outside world as robust as possible
Service locators: you can move a loopback address between devices easily because it's not part of the infrastructure, so if a service moves, the address can move with it without breaking anything, or invoking the darkest curse of the network engineer: renumbering
Anycast addresses: let's say you have a pool of DNS servers scattered across your infrastructure, you only need to talk to the nearest one, so you give each server the same loopback and advertise all of them to the network. Each device will route to the nearest DNS server, or the next closest if that one goes down, or the next, etc. etc.
The Anycast item above is the core of how overlays like VXLAN work: each host thinks the one gateway is local, but in fact, all the gatways are anycast addresses attached to virtual interfaces. The underlying network can get the frame wherever it needs to go without the host realizing it's not actually in the same broadcast domain as it's destination.
Loopback is always 'Up' so always available. If you have G0/0/0 with IP 192.168.1.1 (for example) and that link is down, then it won't do anything with that 192.168.1.1 IP.
If you have 192.168.1.2 set as a Loopback, it will always be accessible as long as the router is up and there's a way to get to it.
In practice I see the primary use of loopbacks as the 'canonical' management IP for devices. They're also used for several VoIP tasks which require an IP to bind to.
This is what I do for everything. Loopback with /32 for all management. It's always up, and always in my OSPF if it has a way to talk to a neighbor.
Unlike a Vlan, where is there is nothing on the vlan, the vlan isn't up and it isn't advertised.
Now VXLan, DHCP and anycast-gateways, loopbacks get wildly interesting.
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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. 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. This concept of a loopback interface is different from the IPv4 addressing plan, and other protocols have their own loopback address conventions.
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 is often useful for fingerprinting remote hosts.
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. 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. At the time, there was strong opposition to the idea of centralizing and codifying address policy in the routing components of the network. 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.
Not always configured, and you can have many logical loopbacks. The address can be anything, and is not assigned to a physical interface, so it cannot go down unless you command it. Can be used to source routing, tunnel endpoint etc. Using it as a management interface means you can still access in band but isolate and control access to the device.
127.0.0.0 is an internal address range for a configured interface. If you ping an address in that range, it sort of goes down the stack to the nic then back up. Internal to the device.
But a loopback address on a router/switch can be advertised
Imagine a loopback IP as an address that the router holds, and presents, but which isn't tied to any of the physical interfaces.
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.
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.
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.
But 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'. 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.
Hope this helps!
So you have real physical interfaces. A loopback is a virtual interface. It not ethernet. Its not fiber. I just exists, because you told the router about it.
Like any other interface, it has to be routed to. So if a packet came in on a physical interface, to get to the loopback, it would need to be routed to it. If you just had one router, and it is the default gateway, then the router would already know how to route to it. But if you are multiple hops away, it needs to be advertised like any other network through a routing protocol or static routes.
What is its purpose. Usually its used for management. As someone else stated below, its always up. So in the case of a router with multiple paths to get to it, the loopback will always be reachable regardless of the one down network. There are more uses for loopbacks besides management, but its too much to fit in to a summary like this.
It probably doesnt make allot of sense right now. But it will. Especially when you start dealing with Layer 3 switching; which use SVI's.
A loopback is an address for "me" that everyone else can use.
The idea being that if you don't set it, "you" don't exist, but are only a black box behind interfaces. The loopback allows you to identify yourself consistently across all interfaces.
It also allows you to set clearly defined rules for management distinct from rules for traffic.
https://www.reddit.com/r/networking/comments/bp4uzv/i_feel_like_i_run_into_a_new_use_for_loopbacks/
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