Ip Helper on 2950

Unanswered Question
Jan 29th, 2008

Hi,

Usually the Ip-Helper command is placed on a router. I've been wondering if the same command can be placed on a 2950 switch.

thx

I have this problem too.
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Richard Burts Tue, 01/29/2008 - 08:54

Tony

Since the 2950 is a layer 2 switch it forwards based on MAC address and not on IP address. This would prevent the 2950 from being able to implement the helper-address function which requires forwarding based on IP address.

HTH

Rick

tonyp8581 Tue, 01/29/2008 - 09:03

Hi Rick,

My understanding of ip-helper, it transforms the broadcast packet to a unicast packet in order for the router to send the packet on another network.

Like any other unicast packet that starts at a layer 2 switch, it will end up at the router interface. (could be mistaken...)

Final thought, using the IOS help, the 2950 does give you the opportunity to enter a ip-helper address. why is that?

Tony

Richard Burts Tue, 01/29/2008 - 09:25

Tony

To my great surprise when I tested this on a 2950 it did allow me to configure ip helper-address. I did not have a client sending DHCP requests so I can not say for sure whether it works or not. But the configuration is allowed and my guess is that it would work.

I am puzzled how a layer 2 device can do this. But it looks like my original answer was incorrect. Sorry about that.

HTH

Rick

tonyp8581 Tue, 01/29/2008 - 10:07

Rick,

This week i'll try to tested on a 2950. If i find anything valuable, i will post it.

thanks

jpodolanko Thu, 02/07/2008 - 11:57

A 2950 is Layer 2 but it can have a Layer 3 IP address for one vlan at a time for management. If it could have more than one working IP address at a time, it would then be a router or L3 switch. The fact that it can only hold one IP address makes it Layer 2. The ip helper-address is used for DHCP on a different VLAN (which requires a L3 routing/switching device).

tonyp8581 Thu, 02/07/2008 - 12:12

Hi,

to my surprise, when you place the ip helper with the management ip address it does actually work.

thx for all your help.

glen.grant Thu, 02/07/2008 - 17:13

It can't work on the management ip as it is to manage the switch only , all ip forwarding for the clients is done at the layer 3 router or switch level so I am not sure what you saw. The clients do not talk to that address on the switch .

tonyp8581 Fri, 02/08/2008 - 06:09

Hi glen.grant,

FYI,

your saying that it won't work

but this is what i've done:

i placed the ip helper command under the managemenent ip port of the 2950. On my DHCP server that's found on another network segment, i can see the management ip address showing up in the logs. That ip address is enter in my Bind to Interface of my DHCP server which identifies the scope that was created.

so, according to this test. it does work. Why would the IOS of the 2950 permit you to enter an ip helper ??

One more thing i need to add, obviously, my 2950 is connected on a router. If you recall, my initial question was can you add an ip helper command on 2950.

i hope this helps.

thx

Richard Burts Fri, 02/08/2008 - 06:56

Tony

There are a couple of things that you can tell us which may help to clarify this issue. You have told us that there is an ip helper-address configured on the 2950, that the 2950 is connected to a router, and that you are seeing the management IP address in the logs of the DHCP server which is on a different network segment.

Can you tell us these things:

- can you verify that clients connected to the 2950 are successfully getting addresses from the DHCP server? (I believe this is what you mean when you say that you see the management IP in the logs of the server, but it would be nice to confirm this)

- can you tell us whether there is an ip helper-address configured on the router interface for this VLAN?

HTH

Rick

tonyp8581 Fri, 02/08/2008 - 07:02

Rick,

Clients that are connected on the 2950 do receive the right ip address coming from the DHCP server.

This week i spoke with the provider that services our router and he confirmed that they don't have an ip helper-address on the interface.

Tony

Richard Burts Fri, 02/08/2008 - 07:17

Tony

Thanks for the additional information. It seems to me to confirm that the helper-address on the 2950 is really working.

I had originally thought that it would not work. And I am still a bit surprised by the logic that a layer 2 device can do the DHCP request forwarding which I had assumed would be done only by layer 3 devices. But I do see that the 2950 allows the helper-address to be configured. And your experiment seems to demonstrate that it does work. I have learned something new. Thank you.

HTH

Rick

tonyp8581 Fri, 02/08/2008 - 07:22

Rick,

no problem, i also thought it wouldn't work.

if you have some time i posted another question called: Adding a third 2948g-l3 switch.

can you take a look and let me know what you think.

thx again for your help.

Tony

Anonymous (not verified) Thu, 08/14/2008 - 12:40

Hi Tony,

can you tell me what version of IOS your running. I just don't get it!

so you just put the ip helper-address under the interface vlan and it work?

Thanks,

yanic

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