A loopback interface is generally used by routers or Layer 3 capable switches so that they have a reliable IP interface for routing updates. A loopback interface is a virtual interface on Cisco Router, It appears like any physical interface and unlike a physical interface, it never goes down unless the entire Router goes down and hence has many uses. It is primarily used for troubleshooting, Router management and protocol enhancement. It is also used to establish peering between neighbours in BGP, so that if even if the physical interface went down for a short time the session does not go down.
Key Benefits of Loopback interface
1. Loopback interface is a virtual interface created in software.
2. Loopback Interface has no associated hardware interface and is therefore always up
3. Loopback Interface allows the assigned IP address to be used to access the Router, for example, Telnet, regardless of which real interfaces are up or down.
4. Loopback interfaces do not have internal VLAN IDs or MAC addresses
5. The Loopback IP address can be used to assign the OSPF Router ID , establish a External BGP neighborship and load balancing on multiple links with BGP also.
Create a Loopback Interface on Cisco Router
To create a loopback interface, all you need to do is enter configuration mode for the interface:
Cisco-Router(config)#int loopback ?
<0-2147483647> Loopback interface number
Cisco-Router(config)#int loopback <Number>
In this command we have an option to create loopback interfaces range from 0 to 2147483647. Cisco IOS Software gives you plenty of loopback interfaces; if you want, you can use all of them also. When entering this command, Cisco IOS Software automatically creates the loopback interface, places you into interface configuration mode, and removes the interface from shutdown mode. Once interface created, you only need to assign an IP address to the interface.
Configure IP addresss on Router Loopback Interface
You can assign any network address on the loopback interface. If the loopback address is included in the Routing process, the route gets advertised. Choose the address carefully. Use an address that is not used elsewhere on your network. Some engineers used the loopback for managing Routers and reserve a network segment for the loopbacks. For example, Router 1 has the loopback 192.168.0.1/32, Router 2 has the loopback 192.168.0.2/32 and so on.
Configure IP addresses for the loopback interfaces as follows:
Cisco-Router-1(config)#int loopback 0
Cisco-Router-1(config-int)#ip address 192.168.0.1 mask 255.255.255.255
Cisco-Router-1#copy running-config startup-config
Cisco-Router-2(config)#int loopback 1
Cisco-Router-2(config-int)#ip address 192.168.0.2 mask 255.255.255.255
Cisco-Router-2#copy running-config startup-config
Filed Under: Cisco General
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