Femi

What is happening is not an issue about loopback interface or secondary address. You would have exactly the same problem on both types of interfaces.

The issue here is that the address and mask that you are using create a subnet that overlaps with an existing subnet on the router. And IOS will not allow two overlapping subnets.

To make it work you need to use and address and a mask that do not overlap with an existing subnet. Since we have no knowledge of what addresses you are using there is not much other advice that we can give you.

HTH

Rick

Hello Richard,

Sorry that i missed out some info. Please see details of the IP addresses below:

IP configured on Loopback 11 = a.b.c.95 255.255.255.255

Default IP route = ip route 0.0.0.0 0.0.0.0 a.b.c.65

IP to be configured on the FE port that throws overlap error =a.b.c.94 255.255.255.192

Additional IPs issued that have been attempted to add on additional loopback interfaces: a.b.c.96 to a.b.c.100

Please note that a.b.c is same for all the IPs.

Regards,

Femi

Femi

Thanks for the additional information. If a.b.c is the same for all addresses then the problem is in the mask that you are using of 255.255.255.192. This defines a subnet containing 64 addresses. If you have multiple addresses (and especially if the addresses are like 95, 96, etc which are sequential) then you probably should use a mask of 255.255.255.255.

HTH

Rick

Hello Richard,

So I try to set the FE to IP address a.d.c.94 255.255.255.255 but got a bad mask error. Then I used subnet mask 255.255.255.252, after which i tried adding the IP a.b.c.95 255.255.255.255 to the loopback interface and got the overlap error. However, when i tried using any other IP between .96 and .100, it worked. Apparently, using the .252 mask makes .95 a broadcast IP which is still in the same subnet with .94

Does this imply that i cannot use .95 along with .94 in this configuration? That would be painful as I had to pay extra to get these additional IPs from the ISP.

Regards,

Femi

Femi

Yes to configure an address that will operate on a physical interface such as FE then you need a subnet mask of at least 255.255.255.252. And that will create a subnet that occupys 4 addresses (94, 95, 96, 97). Sometimes when you are getting IP addresses from the ISP you can ask for a /30 for the connection and another small block that falls outside the range of the /30. Perhaps that might work for you?

We do not know how you are using these addresses (other than trying to put some on loopback interfaces) and so it is difficult to know if there are alternatives. I am guessing that you are using at least some of these addresses for address translation, so that devices inside your network on private addresses can access the Internet and have fixed public addresses. If that is the case then perhaps it is possible to set up some of these new addresses for address translation without requiring that they be configured on an interface?

HTH

Rick

Hello Richard,

Thanks for the feedback.

The IPs configured on the physical FE interfaces are primarily used to setup site-to-site VPN tunnels to 3rd parties.

The IPs configured on the loopback interfaces are used for the following:

• One-on-one Server to Public IP mapping
• VPN server to remote clients

So the main use of the loopback interfaces is for one-on-one server to public IP mapping.

Hope this helps as I would rather explore an aternative configuration than to ask the ISP for new sets of IPs.

Regards,

Femi

Hi again,

Sorry i apparently didnt finish reading your post before i hit the "add reply" button.

You typed:

I am guessing that you are using at least some of these addresses for address translation, so that devices inside your network on private addresses can access the Internet and have fixed public addresses.

Yes, that is the main aim for getting the additonal IPs up actually. Also to ensure that services coming from the internet can reach those devices directly

If that is the case then perhaps it is possible to set up some of these new addresses for address translation without requiring that they be configured on an interface?

Would love to explore any other possible option to configure.

Regards,

Femi

Hello Rick,

Thanks for the comments. I actually never knew that i could configure a static address translation of an inside address to an outside address without configuring that outside address on an interface.

So why then do you think the loopback interface was used in my case?

Regards,

Femi

Femi

We do not know how the loopback interfaces are being used and so is difficult to know for sure why they were configured. But my guess is that they were configured because someone else, who was configuring your routers first, did not know that you could configure translation without having interfaces applied to an interface. Or perhaps they needed to be able to advertise the subnet of the extra IP addresses, and having it on a loopback is the easy way to get a subnet into the routing protocol.

HTH

Rick

Hello Rick,

The ISP gave us the additional IP addresses with a mask of /32, so I do not think advertising the subnet mask of the extra IP addresses may have been the reason as /32 would not be anything to broadcast I presume.

It could be that the person didnt also know that translation can be done without applying the IP addresses to a specific interface on the router.

Thanks a lont for your feedbacks.

Regards,

Femi