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I was wondering what the router does to have an administration IP. This IP is joinable no matter which interface is up (if one is up at least). How does the router see this IP as ? Does it create a local L2 (juste like a VLAN) for this IP? I don't understand how an IP can be elswhere than in an "L2".

Edit: Let's take a Cisco 2000 router. I create a loopback interface, then assign an IP for this virtual interface : 172.16.0.1/32. I have 2 other interfaces :

•eth1, IP : 192.168.1.254/24

•eth2, IP : 192.168.2.254/24

So until both interface are down, my loopback is joinable. But i don't understand the mecanism behind the loopback address. The cisco2000 has to define some kind of "broadcast domain" somewhere to communique with the loopback address and then add a route to the rounting table (manualy or with a routing protocol). Does he make a local VLAN like a switch would do when we assigne an interface to a vlan ? I hope I was a bit more understandable.

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    Internet Protocol IPv4 and IPv6 is layer 3, MAC addresses, VLANs and LLC is layer 2. Commented Jul 24, 2017 at 9:03
  • What equipment are you referring to or could you provide an example to your question? I don't fully understand what you are asking. Commented Jul 24, 2017 at 9:07
  • @Cown I tried to be more specific Commented Jul 24, 2017 at 9:34
  • Loopback is primarily used as a Router ID in OSPF or BGP, because it never goes down. You should read this earlier answer from Ron Maupin, remember to read the comments also. https://networkengineering.stackexchange.com/questions/22894/whatss-the-function-of-a-loopback-interface-in-routing-protocols Commented Jul 24, 2017 at 11:05
  • @Cown, yes it's a good exlanation about what it's used for, i'm more intersted about how this works. Commented Jul 24, 2017 at 11:34

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Cisco Loop back address is not assigned to any physical ports and cannot connect any networks to it.

The most common being 127.0.0.1 that loops you back to your machine and checks pinging it checks if your TCP/IP stack is up and running.

With cisco devices, loopback addresses are virtual and are not assigned to physical interfaces that you can see. It can be a very powerful tool in any infrastructure as its strongest characteristic is that it never goes down unless the whole device goes down. This is very significant to processes that use IP addresses to point to a particular device.

One example is ospf that uses router id's to establish neighbor addresses. The router ID is determined as the highest active loopback address. If this is not available then the highest IP address is chosen. In a situation where a physical port goes down that happens to be the root-id then the router is deemed to be unavailable, resulting in a whole election process for all routes through that router, even though in reality the other routes are still available. Another example of the use of loopback addresses is in lab environments to simulate networks behind a router. Assigning a loopback address is a simple task as shown below:

Router(config)#interface loopback 1 Router(config-if)#ip address 192.168.0.1 255.255.255.0 Router(config-if)#exit

Where the the number "1" is the loopback interface number and is locally significant. The address can be verified on the running config as well as usinig the "show ip interface brief command"

Serial0/0 unassigned YES unset administratively down down Serial0/1 unassigned YES unset administratively down down Serial0/2 unassigned YES unset administratively down down Serial0/3 unassigned YES unset administratively down down FastEthernet1/0 unassigned YES unset administratively down down Loopback1 192.168.0.1 YES manual up up

As we can see, the interface immediately shows up as up and up even when all the other interfaces are down. Loopback addresses are great for management and if utilized properly can be a great tool to ping and check if your routers are up and running as well as for remote connection to a device. What other uses can you think of ?

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  • I understand what it's used for, but I don't get how it works. Router(config)#interface loopback 1 Router(config-if)#ip address 192.168.0.1 255.255.255.0 when you give those instruction, does the router do what a switch would do when you create a SVI ? Commented Jul 24, 2017 at 11:29
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    The loopback interface provides a stable address for protocols (eg BGP, Telnet) to use so that they can avoid any impact if a physical interface goes down. The loopback interface is not associated with any particular vlan, it's local to the device itself as an "internal" interface that needs to be routed to (within the device), to reach it. Commented Jul 24, 2017 at 12:05

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