The dark secrets of loopback addresses

Contributor Icon Contributed by aaronm Date Icon January 22, 2007  
Tag Icon Tagged: Cisco networking

Just some info on loopback virtual interfaces and their uses.


Loopback interfaces are primarily used for troubleshooting, router management and protocol enhancement. Over the next three paragraphs, I’ll explain.

First, troubleshooting can be made easier with loopback interfaces. For example, If I am configuring MY network stack with a new network, I can create a loopback address on a router and test (ping) through the network to ensure I configured the routes and access-lists correctly. There are more uses, especially when it comes to ATM interfaces and such.

Next, some administrators use loopback interfaces for SNMP management. It saves on real interfaces and can provide the same info. I’ve seen networks that utilize loopback interface exclusively for SNMP, they do this to avoid sending their real IP’s over the Internet, or at least that is what he told me. I don’t much experience with this one.

Finally, OSPF can use the loopback interfaces in determining the next feasible successor in case a router goes down. OSPF uses the lowest numerical address as the best and you can configure loopback interfaces to accomplish this goal.

Loopback interfaces are commonly configured as such:

router#(config) interface loopback0 * you can use any number you want
router#(config-int) * from here you can configure it pretty much like any other interface (ip address, shut, ect.)

There is a lot of info on the net about this, as its a pretty in depth topic. I hope this article just provides some basic info. Good luck!

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    I don't know quite where to start with this but if you're looking for info I'd suggest looking elsewhere.

    First, loopbacks don't have much to do with troubleshooting. You can ping one or use it as the source for pings but you can do that with any interface with an IP.

    Loopbacks are often used for router management but can also be used to provide virtual interfaces for things like GRE tunnels. The reason you use a loopback is that a physical interface can go down but the loopback, being virtual, is up as long as the router is up.

    Regarding OSPF, the term "feasible successor" is from EIGRP, it's not used in OSPF. I think the author was looking for is Router ID. This the IP the router uses to identify itself to other routers running OSPF. The router is determined thusly:

    1 Use the address configured by the ospf router-id command
    2 Use the address of the loopback 0 interface
    3 Use the highest IP address of any interface
    4 If no interface exists, set the router-ID to 0.0.0.0
 
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