Loopback Interface on Router

[kuldeeprawat]

Hi,

I need to understand that why we create the loopback interface, can we do all the things with this as we do with physical interface(Serial/Ethernet). What are the diffrences between a physical and a virtual/loopback interface.

E.G this has been created in my router.........

I need to understand this.......

interface Loopback2

ip address 203.90.119.121 255.255.255.255

crypto map ABC

!

Urgent reply will be helpful for me...

[rsissons]

The advantage of using a loopback interface over a real physical interface is that it enables you to give the router an IP address which will always be reachable. If you use the IP address coded on a real physical interface and that interface goes down, then that address will no longer be reachable.

[kuldeeprawat]

Thanks for your response,

But again i have one question in mind,

In which circumstances we need the reachability of IP address always. Lets say I have one ISP if there is a problem in the ISP backbone then the interface doesn't count to me whether its up or down.

I hope you have got my confusion........

Please let me know

[scottmac]

If the ISPs connection to your router is down, you interface may show up/up or up/down, depending on the nature of the ISPs problem.

This is not related to Loopback addresses. Perhaps you are confusing when the carrier / ISP "loops" the connection for testing. In that case the "Loopback not set" becomes "looped" (if either you or the carrier sets up a loop).

CSU/DSUs and the "smartjack" (the carrier's demarc, a powered network termination, usually an RJ48) have the capability of connecting the incoming pair to the outgoing pair from either direction. It's used to diagnose cable and signaling proplems.

The process is like this: A signal can be sent to your smartjack or CSU to "loop up;" with the loop in-place, any signal sent from the looped side is sent back to the sender bit-for-bit. By observing the TX / RX stats, it can be determined if any information has been dropped or corrupted along (both sides) of the connection.

If you suspect a carrier media problem, you can loop up the connection and see the same thing for the media between the CSU/DSU and the smartjack.

There are instances where a (diagnostic) loop just pops up on the line. If your interface shows up/up, your TX and RX stats are identical (after resetting the counters) and you have no connectivity to the other side .... you probably have a loop up (and you may have an indication of "looped" instead of "Loopback not set".

The Loopback Address(es) have several functions, some are for internal administrative function, sometime (usually) it's used to establish a "Router ID" (as in OSPF), or, in the Lab, to generate some additional routes to filter, forward, or check out an ACL. Because a Loopback Address will not change state without administrative intervention, it becomes a good target when you need to telnet, SSH, or trace to the device.

Another useful tool is the NULL: interface. It uses less processor time to dispose of unwanted traffic then an access list. But that's a whole 'nuther story from your question ...

FWIW

Scott

[louie55]

Loopback addresses are important for the function of the network and it's processes. It is not an address that you would ever want to send traffic accept for testing purposes. For example, a loopback address is set on an OSPF router to force it to become the DR for the network segment and/or segments that it is on. It is used because the loopback address can never go down and therefore the OSPF process will never die. If you used a physical interface and it was the only active interface, if that inferface ever went down, even for a few seconds, the OSPF process would have to restart. The loopback provides stability for the OSPF process.

In other words, the loopback interface is used for routing processes and internal and external network testing, it is never used as an actual source or destination address in a production environment. (at least not to my knowledge)

Louie

[kuldeeprawat]

Louie what i understand from your viewpoint is that we uses the Loopback Interface incase in OSPF, Let's say if i am not using the OSPF then what are the benefits that can be provided to me in the static route environment.

In viewpoint specifically to Access-lists, VPN'S etc,.

[louie55]

Look here for a good explanation of the importance of Loopbacks. Hope it helps you.

http://www.ciscopress.com/articles/article.asp?p=27137

Louie55

[Craig Norborg]

To answer the question "in which circumstances do we need the reachability of the IP address always".

Ok, lets say in your scenario, one ISP, problem in the ISP backbone and your connection to them is down. From your own network you can telnet to your ethernet interface's IP address just fine to do diagnostics, no problem. If you had a loopback address, you could telnet to that IP address with no problem also. No real gain here.

Lets say you're out of town and your boss calls you in a panic that the internet is down. You get on via your hotels broadband connection and try telnet'ing to your ethernet interface's IP address. No luck. Your ISP tells you your connection is functioning great. Now you have to remember your serial interfaces IP address to get in, so you wait on hold for your ISP to remind you of what it is. Yet, if you tried your loopback interface you could get right in and figure out that there is a problem on the ethernet interface that caused it to shut down. Maybe someone bumped the cable or something.

When you get into having multiple connections it becomes even more useful. Lets say you have 5 T1's going to multiple locations. No matter what other interface might go down, the loopback will always be up. This is by far the best address to use for any type of management, whether it be telneting in or configuring Ciscoworks to monitor it or whatever.

Hope this helps!

[kuldeeprawat]

Thanks for this information..

Tell me do we actually implements these in the real scenerio forget about ospf in this case.

Also specific to VPN'S, if yes then can u send me a simple configuration file...

Thanks