Loopback Interface question

Answered Question
Mar 18th, 2008

I'm configuring a PVDM2-24DM module for dial-up access and I'm not sure what to do with the loopback interface setting. The documentation ( ) states:

"Configuring the Loopback Interface

The loopback 0 interface is the interface that dial-in users access when dialing in to the network. Usually, all dial-in users are assigned to a single IP subnet. This subnet can be identified with the loopback 0 interface, a logical interface whose network number can be borrowed by each asynchronous dial-in interface."

Being that you also need to identify an IP pool for dial-in users to use I'm not sure what this does. Do I just identify the subnet that I'm using for the dial-in access. Would this work? - loopback - - IP Pool (there are only 24 channels so I used a 24 IP pool)

I have this problem too.
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Correct Answer by Makarand Chitale about 8 years 6 months ago

The loopback does not have to be from the same ip pool range, however it can be if that is what you want. Either case will work.

Thanks, Mak

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Overall Rating: 5 (2 ratings)
Paolo Bevilacqua Tue, 03/18/2008 - 13:55

In fact, if you use only one asynchronous interface, you would not need the loopback neither. But, a loopback is always useful. The mask on the loopback is usually /32. Since PPP links do not really have the concept of netmask, that is fine. Then again, mask doesn't really matter. Sometime for simplicity you make the async unnumbered off the lan interface.

For the pool, really doesn't make a difference. Sometime, it is wanted to give a static IP to each user, to help in doing ACLs and stuff. Pool and static assignement can coexist.

qbakies11 Tue, 03/18/2008 - 14:04

Thanks for the response. My end goal is to have PDAs in the field dial-in, connect to this router, and synch info with a server on my internal network. I use a /21 network so don't I have to declare that in the loop back? Then provide a pool of IPs that reside in that subnet so they can communicate with my internal network?

I'm not sure what you mean by "if you use only one asynchronous inerface, you would need the loopback neither."

Paolo Bevilacqua Tue, 03/18/2008 - 15:28

Yes, you can declare the ip pool to be in the same internal lan, however to make things nicer I would use a different subnet.

Sometime people especially ISP have multiple group-async interfaces, so to save on addresses, they make them all unnumbered off the same loopback. But if you have a single group-async, is not necessary to create a loopback only for the purpose, as it works anyway no matter which address you give it it. However, the cleaner way is a subnet that comprises the ip pool.

Makarand Chitale Tue, 03/18/2008 - 22:23

In a dial senerio specifically when using group-async interface we always recommend using an unnumbered interface, this because there is a limitation in IOS where if the group-async interface is with an ip address (statically assigned) then we can successfully negotiate 5 (or 6) ppp connections - after that the calls will either not negotiate ipcp or have ip connectivity issues.

interface group-async x

ip unnumbered

We can use ip unnumbered to any interface (Ethernet or loopback or any interface) - loopback is preferred coz it's always UP & UP (where as an Ethernet could go down causing ip connectivity issues after ipcp is negotiated).

Again the loopback can have any mask since mask info is not exchanged in ppp nego. The loopback need not be from the same subnet as the ip address pool (doesn't hurt to have thought but then make sure to exclude that ip from the pool). From the routers perspective all the ppp dialin connections are host routes of /32.

Thanks, Mak

qbakies11 Wed, 03/19/2008 - 20:10

OK, I appreciate the help but that's a lot of information there I'm not familiar with and I'm a bit lost. So do I need to declare the loopback as 'ip unnumbered' or declare it with a static ip in the sunbnet that I'm going to assign the ip pool in?

I need the dial-in users to be able to talk to my internal server for synch processing so I'm going to make the ip pool within my internal subnet.

Internal -

Proposed Dial-in IP Pool - -

Loopback - IP Unnumbered or Does it matter?

Also, I'm confused about the group-async interface and why it has it's own loopback interface. I have never set something like this up before and am trying to follow step by step with the document http://www.cisco.com/en/US/docs/ios/12_4t/12_4t11/htpvdm2.html

in the 'Tasks Common to all configurations' section. It's a bit overwhelming as I don't have experience in this and am be pushed hard to get it working. All help is greatly appreciated.

qbakies11 Thu, 03/20/2008 - 08:41

Another problem...I tried to assign the loopback 0 interface with an IP on my local subnet but I got an error back stating "% overlaps with FastEthernet0/0" so I guess I have to give it an address on a different subnet.

If I do that and all dial-in users are assigned IPs with a /32 how does it know to route that traffic into my interal LAN?

Makarand Chitale Thu, 03/20/2008 - 15:36

I am responding to both your question here:

Loopback 0 needs to have a static address which we will use for the group-async interface (using "ip unnumbered loopback 0")

This static address can be from the same pool subnet or a totally different subnet however like any router requirement - 2 interfaces cannot be from the same subnet (so your Ethernet and loopback are overlapping).

To make it simple, give the loopback any ip address, say and have the group-asynch unnumbered to loop 0.

As said earlier all dial in users will be assigned a host route (/32) from the address pool, any packet from the dialin client will ve forwarded to the router (def route) which will further forward it to the internal LAN (for the router will appear as directly connected).

Let me know if you have any further questions.

Thanks, Mak

qbakies11 Fri, 03/21/2008 - 05:21

That's much clearer, thank you.

Do I need to have the default ip pool that dial-in users are going to pull from in the same subnet as the loopback?

Correct Answer
Makarand Chitale Fri, 03/21/2008 - 15:46

The loopback does not have to be from the same ip pool range, however it can be if that is what you want. Either case will work.

Thanks, Mak


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