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A loopback interface is a virtual interface in our network device that is always up and active after it has been configured. Like our physical interface, we assign a special IP address which is called a loopback address or loopback IP address.
- Router# ethernet loopback start local interface gigabitEthernet 0/4/1 service instance 10 external dot1q 10 cos 1 destination mac-address 0000.0000.0001 timeout none This is an intrusive loopback and the packets matched with the service will not be able to pass through.
- Layer 2 Configuration Guide (Cisco ASR 920 Series) 1. This is an intrusive loopback and the packets matched with the service will not be able to pass through.
- This article examines the concept of NAT Reflection, also known as NAT Loopback or Hairpinning, and shows how to configure a Cisco ASA Firewall running ASA version 8.2 and earlier plus ASA version 8.3 and later, to support NAT Reflection.
Loopback interfaces should be supported on all Cisco platforms, and unlike subinterfaces, loopback interfaces are independent of the state of any physical interface. Most IP implementations support a loopback interface (lo0) to represent the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer.
The loopback interface can be considered stable because once you enable it, it will remain up until you issue the shutdown command under its interface configuration mode. It's very useful when you want a single IP address as a reference that is independent of the status of any physical interfaces in the networking device.
Loopback addresses are not limited to the 127.0.0.0/8 block.
Benefits of Having a Loopback Address
We would reap the following advantages and benefits of having loopback interfaces and loopback addresses in our network:
- A good example is mapping a router's loopback IP address to its DNS server address.
- You can also configure the loopback address as the Router ID for routing protocols like OSPF and BGP.
- A loopback interface can also be used to establish a Telnet session from the console port of the device to its auxiliary port when all other interfaces are down.
- TACACS+ or RADIUS can use the loopback address as a source address for AAA functions and thereby reducing the administration overhead of having to add every IP address of the router to the AAA server.
- It is also recommended that the unnumbered interface points to a loopback interface since loopbacks do not fail. The ‘ip unnumbered' configuration command allows you to enable IP processing on a serial interface without assigning it an explicit IP address.
- Loopback address becomes even more important in large fully meshed or route-reflector BGP environments where many routers will have multiple peering with many other BGP speakers. Adding and troubleshooting other services like LDP for MPLS or building MPLS L2 cross-connects, loopback address becomes extremely important to easily identify the peer routers to deliver the services in large Service Provider environments.
- It can also be used as a source and destination address for testing network connectivity and lab environments.
- It also allows us to test IP software without worrying about broken or corrupted drivers or hardware.
Loopback Configuration
I am going to show you how to configure the loopback interface and loopback address and use it as the Router ID for OSPF. We'll use the network topology below for the sample configuration. Let's start by configuring the physical and loopback interfaces.
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R2
Then, we configure OSPF.
R1
R2
Next, it's time to verify. We can now see that the OSPF neighborship is established.
Let's ping the loopback IP address from both R1 and R2.
If there is no OSPF Router ID configured, the highest loopback IP address is selected as the OSPF Router ID.
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In many Ethernet testing use cases its always convenient to have an option of doing a real Ethernet loopback. The key to Ethernet loopback testing is for the device doing the loopback function to swap the source and destination MAC addresses within the Ethernet frame. The reason for this swap is to make the frame appear to be sourced from the destination of the original frame and destined to the original source.
With that functionality available we can connect an Ethernet test set, such as those from EXFO or JDSU, and run an Ethernet test with one test kit and one technician. Having an Ethernet test set on each side of a link is not needed in this case.
I verified this configuration in a lab environment using a Cisco ASR901 router and a laptop. I applied the following configuration of the ASR901 router:
This configuration creates a service instance on an GigE interface that matches untagged traffic and forwards it to bridge-domain 3. I then connected my laptop to port Gig0/2 and once the interface is up I issued ‘ethernet loopback' command in EXEC mode.
This command tells the A901 router to perform an Ethernet loopback on interface Gig0/2, service instance 1. This will be a facility loopback, which means it will be a loopback on an interface back out that same interface. It will match any frames sourced from MAC address d4be.d930.e0d0 and the perform a mac swap of the source to destination and destination to source.
In order to test this I placed a static ARP entry on my PC so I could avoid performing ARPs for this test. To do this I issued the following command:
I previously had set the IP address on my PC to 10.10.10.10 255.255.255.0.
Loopback Interface Cisco
Once this once complete I issued a ping command.
This resulted in the following output in Wireshark on my PC:
This shows the ICMP echo (ping) message sent with src MAC of the PC and destination of the fictitious destination. As expected.
We then receive a packet that has been loopbacked by the ASR901 router.
You notice it was swapped and now the src is the fictitious MAC address and the destination is now the PC. Notice that the IP header is unchanged, the SRC and DST IP addresses remain the same throughout.
Loopback Address Cisco
Once testing is complete you can remove the loopback by issuing the following command:
