Silly question: Can't get the second IP on a router through DHCP

[Difan Zhao]

I can't believe I can't make it work!

I am using a Cisco 1721 router with two Ethernet ports and one FastEthernet ports. Here is my config:

interface Ethernet0

ip address dhcp client-id Ethernet0 hostname E0

half-duplex

!

interface Ethernet1

ip address dhcp client-id Ethernet1 hostname E1

half-duplex

!

interface FastEthernet0

no ip address

shutdown

speed auto

E0 and E1 are connecting with ISP's ADSL modem. However only one can get an IP address! ISP is fine because if I plug one cable to my laptop my laptop gets the second IP address without problems. Somebody please HELP!! Thanks a lot!

[paolo bevilacqua]

Some BB modem/routers don't respond to DHCP requests coming from by cisco IOS. I understand it can be difficult to have an ISP to look into that when a regular PC works. Is it a public or private address you are supposed to receive ?

[Difan Zhao]

Thank you for reply. It's public IP...

[izackvail]

Are the interfaces on different subnets?

[Difan Zhao]

Two ports are on two network modules. They are not switchports. I think they should be under different subnets. Do you mean that if the ISP assign two IP addresses which are of same subnet, my router won't accept them?? Thanks!

[izackvail]

A router by definition will not allow any of its interfaces to be on the same subnet.

[paolo bevilacqua]

I didn't thought of this, but it's easy to verify, just use one interface at time, and see if the problem stays always with a specific one.

[Difan Zhao]

Good point. I just gave it a try. If I connect only one port to the ISP they can get IP addresses just fine. The IP addresses they got are different but under same subnet. It seems that it's the problem.

[Difan Zhao]

Is there anyway to remove this restriction? Nobody would ever need to plug two ports into the same subnet? What if people want to make one port as redundency of another port? Is my question getting more and more ridiculous??

[paolo bevilacqua]

No, is not ridiculous. Router doesn't allow that for reasons buried in history. I don't really see an easy way t work around that.

[Difan Zhao]

It makes me feel a lot better... Thanks!

[izackvail]

What is your goal? Why do you want both interfaces plugged into the modem? If they plug into the same DSL modem I don't know what you gain by having them both connected?

You may be able to talk to your ISP and get them to give you 2 connections from different subnets but beyond that I think you are stuck. Sorry.

[Difan Zhao]

I am sorry for the late reply. Actually I want to load balance between 2 ISP. The reason why I am connecting both ports to one ISP is because I am at testing stage so I just want to try before I get my second one. I want to use 2 ISP to double the bandwidth for VPN connection. Thanks a lot for your help!

[Richard Burts · Accepted solution]

Difan

With this explanation the situation makes a lot more sense. And I believe that when you are actually connecting to 2 different ISPs that it will work ok. But, as you found out, trying to connect both router interfaces to the same provider does not work.

I am not sure that there is any good way to test when you are connected to only a single provider. I had thought some about configuring one interface with an address (not DHCP) and doing something like splitting the address range (create 2 small subnet rather than the original single subnet) which would allow both interfaces to have an IP address. But the problem is that there is only one next hop address for any routing that you would be doing. So you would not be able to really test redundancy or failover logic.

I believe that if you want to do any meaningful testing that you will need 2 separate providers active during the test.

HTH

Rick

[Difan Zhao]

I think you are right. I probably will need a second ISP link. I will continue my testing then. I will open a new post for my future questions! Thanks a lot for your and all other's help.