Can a router use one IP address for more than one Interface?

Answered Question
May 20th, 2010

Hi People,

Basically I need a router to 'pass through' traffic without it changing the ip subnet. I have live IP addresses behind the router, attached to fa0, and they have the same ip range/subnet as the routers atm0 interface (I purchased a Non-NAT setup from my ISP).

My question is how to setup the router interfaces so they use the same IP address for the ATM interface and the Ethernet interface?

On standard consumer products you can simply turn off the NAT and it disables the LAN setup allowing live IPs behind the router.

I am using a /29 setup so have 6 ips. Everything I have looked at shows the usage of NAT but I guess I don't need that.

If I try to ping the IP behind the router from the command prompt it gets stuck in a loop between itself and my ISPs gateway.

Is that because of this - ip route 0.0.0.0 0.0.0.0 dialer 0 ??? I guess so but what is the alternative?

Oh, I am using an SR520 series router with an ADSL over POTS Wan link.

Regards,

Chris Snape (UK)

I have this problem too.
0 votes
Correct Answer by Mlman1965 about 6 years 8 months ago

I thought I had a solution for you, but didn't look at the capabilities of the router first.

It is possible in the entreprise series routers (72XX, 76XX, 65XX), but I don't think the SR520 will support it.

I recently did a BGP exercise where we assigned a router IP adress to a loopback & then configured the ports to use the loopback.

Ex:

interface Loopback0

ip address XX.XX.XX.XX 255.255.255.248

interface ATMY/Y

ip unnumbered loopback0

interface FastEthernetZZ

ip unnumbered loopback0

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Mohamed Sobair Thu, 05/20/2010 - 10:44

Chris,

Its impossible to have 2 or more layer-3 interfaces (Router Interfaces) with the same IP Addresses.

Any reason for you to do that?

HTH

Mohamed

Correct Answer
Mlman1965 Thu, 05/20/2010 - 10:20

I thought I had a solution for you, but didn't look at the capabilities of the router first.

It is possible in the entreprise series routers (72XX, 76XX, 65XX), but I don't think the SR520 will support it.

I recently did a BGP exercise where we assigned a router IP adress to a loopback & then configured the ports to use the loopback.

Ex:

interface Loopback0

ip address XX.XX.XX.XX 255.255.255.248

interface ATMY/Y

ip unnumbered loopback0

interface FastEthernetZZ

ip unnumbered loopback0

Mohamed Sobair Thu, 05/20/2010 - 11:09

Chris,

I dont need to fire up GNS3,  and try it..

I am telling you its impossible to have the same IPs on different layer-3 interfaces at the same router, this is according to my knowledge.

If you dont want to take my answer then its your call, you can consult some one else about it.

I hope I made my self clear,

Mohamed

Mlman1965 Thu, 05/20/2010 - 11:14

Double check the post.  Chris didn't post that, I did.  If you don't want to believe what I said that's fine also, however I know that in the routers I specified you can do it.

Mohamed Sobair Thu, 05/20/2010 - 11:25

Apologize Chris for addressing my last post to you,

However, for the second poster, The original poster wanted to have this setup on a realy hardware and in a production Network, so I dont think he is looking to test this in A GNS, this is as per his post and what I understood.

Another point is that, if the GNS accepts it, it doesnt mean it should be taken into consideration also, as this is incorrect.

HTH

Mohamed

merryllem Thu, 05/20/2010 - 12:52

Hello Chris,

What you are trying to achieve is possible using "ip unnumbered"

Here is a good doc you can read through.

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml

As far using the same IP for more than one interface, it is technically possible using VRF's. However, that might not be the way to go with what you are trying to do.

Hope that helps

Mhel

Chris Snape Fri, 05/21/2010 - 06:24

Thanks for all the info guys.

IP UNNUMBERED worked like a charm. At first I tried doing it off the vlan but that wouldn't let me so I took your advice and created the loopback0 interface to hold the permanent IP address. Then I linked the dial0 (ADSL over POTS) and vlan to the loop via unnumbered.

Worked great, thanks.

Regards,

Chris Snape

Mohamed Sobair Fri, 05/21/2010 - 08:27

Chris,

What I understand from your post that the ISP provided you with one subnet , and you want to use two of the free IPs and configured it on different interfaces.

If so, then I still insist its impossible, even if you want to use loopback and IP unnumbered.

However, if you managed to solve your problem, then I might mis understood your question.

Regards,

Mohamed

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