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

Answered Question
May 9th, 2008
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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!

Correct Answer by Richard Burts about 9 years 2 months ago

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

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paolo bevilacqua Fri, 05/09/2008 - 17:37
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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 ?

izackvail Fri, 05/09/2008 - 18:21
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Are the interfaces on different subnets?

Difan Zhao Sat, 05/10/2008 - 19:53
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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 Sat, 05/10/2008 - 20:20
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A router by definition will not allow any of its interfaces to be on the same subnet.

paolo bevilacqua Sun, 05/11/2008 - 03:57
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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 Mon, 05/12/2008 - 11:55
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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 Mon, 05/12/2008 - 12:06
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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 Mon, 05/12/2008 - 12:16
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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.

izackvail Mon, 05/12/2008 - 14:02
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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 Tue, 05/20/2008 - 12:34
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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!

Correct Answer
Richard Burts Tue, 05/20/2008 - 12:46
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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 Wed, 05/21/2008 - 07:21
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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.

sirdudesly Tue, 05/20/2008 - 15:45
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Also, why are you running e0 and e1 in half duplex?

Difan Zhao Wed, 05/21/2008 - 07:16
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I just keep the default settings. I didn't do too much on the interface settings as you can see. Didn't go that far yet... Thank you for reminding me!

Richard Burts Wed, 05/21/2008 - 07:29
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Difan


My memory of the 1721 is that one restriction of the Ethernet interfaces (as compared to the FastEthernet) is that they only support half duplex.


HTH


Rick

Difan Zhao Wed, 05/21/2008 - 20:27
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You are right!! The 1721 supports either half duplex or full duplex but not auto-duplex. However most modems are not compatible if you set the router as full duplex. However if you set it as half duplex even it will sacrifice some performance however it won't bring you some very VERY wierd problems as in full duplex!! I was really pissed off by those modems but for testing purpose I don't have newer models available... However I don't think it will be the reason for our problem here. I think getting a second ISP would still be the solution.

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