ip helper address dosent work

[Roger Base]

Guys I have some ip helper address problems in my production environment. It seems that if I use my SW1 as dhcp pool and my clients are on on same switch then my ip helper-address doesn't work (no data received on MYSERVER). If I move the dhcp pool to R1 everything works again. 

Is there any limitation regarding using ip dhcp helper address on same switch where you have dhcp pool too?

PC1---SW1---R1--MYSERVER

My DHCP pool is on SW1.

SW1 Config:

ip dhcp pool VLAN10
   network 10.10.10.0 255.255.255.0
   default-router 10.10.10.1 
   dns-server 172.16.10.100 
   domain-name mydomain.local
!
interface Vlan10
 ip address 10.10.10.10 255.255.255.0 
 ip helper-address 172.16.3.100 --- MYSERVER
!
interface FastEthernet1/0/5

/CLIENT
 switchport access vlan 10
 switchport mode access
 switchport voice vlan 47
 logging event spanning-tree
 authentication host-mode multi-auth
 authentication port-control auto
 mab
 spanning-tree portfast
end

[ammahend]

remove the local DHCP pool for VLAN 10 from your switch and then try it should work.

rate this post if it helps you !

[Roger Base]

I want to keep the dhcp pool locally on Sw1. 

[Richard Burts]

If you want to keep the dhcp pool on the switch it should be quite possible. In that case I do not see the need for the helper address configuration. And I am puzzled why you are concerned that they are not requesting addresses from the server. Can you give us some clarification?

HTH

Rick

[Charles Hill]

Hey Roger,

Is DHCP snooping enabled?

If so, you can try disabliing it and test.

What is the switch model #?

What is the ios?

[Richard Burts]

I am not clear what the problem is. Given the partial config that was posted a client should receive an IP address from the switch and not need to communicate with the server. Is that not what was intended? Perhaps we need a better explanation of what is going on and what the problem is.

HTH

Rick

[Roger Base]

Hi Richard, 

I want to use DHCP request information from my CLIENTS for my MYSERVER. Which is a analyser engine. But I dont get any dhcp request when I move the DHCP pool from R1 to SW1 (where my clients are directly conneted) even if I I use DHCP HELPER statement. It works fine when the dhcp pool are on R1. 

Thats why I am asking if there is any limitation using the ip dhcp helper statement when the dhcp pool are on the same same device too. 

[Richard Burts]

Thanks for additional information. But I still do not understand what the issue is. So let me make a couple of comments and then perhaps you can tell me some more that would help me understand the issue.

First let us be sure that we understand ip helper-address in the same way. Helper address takes certain types of UDP broadcasts (which include DHCP requests) and forwards them to an address that is outside of the local subnet on which the broadcast was received. So if the switch receives a DHCP request and the DHCP server is remote (as your server is) then the switch forwards the request. And as noted in another response in this thread you can use forward-protocol to have helper forward other types of broadcast messages.

So helper address makes sense when the server is remote. But when you create the DHCP pool on the switch then you effectively are making the switch into a DHCP server. So what is the point of keeping the helper address for a remote device when the client will receive its IP address from the local switch?

HTH

Rick

[Roger Base]

Yes. We both understand ip helper-address functionality. My SW1 are the DHCP server. So I don't need to specify ip dhcp helper address on my layer3 interface for it self.. 

But in this senerio I want send dhcp request information from my clients to my analyser server. That is NOT an dhcp server. So it will never reply back with anything. It will only keep incoming information from my clients to analyse certain things. Thats why I am using ip dhcp helper address on my layer3 interface. 

But this dosent work when I move my dhcp pool R1 to SW1.

[Richard Burts]

Thanks for the explanation. I had thought that the server was a DHCP server. In this case the helper address doe make sense. And as I think about it the helper address should still be forwarding the broadcast even if the IP address will be assigned locally. What kind of switch is this? If the switch is covered by a maintenance plan I would suggest opening a case with TAC because this behavior does not sound correct to me.

HTH

Rick

[Roger Base]

I think there is limitation with IP DHCP helper functionality in this scenario but I cannot prove it. And that's why asking about it..

[Richard Burts]

If it does not work when the DHCP pool is configured on the local switch and the only change that you make is to remove the DHCP pool from the switch, and then it does work then certainly it demonstrates that a limitation does exist.

But I do not believe that would be the proper behavior and believe that it is buggy behavior. In that case it would be appropriate to open a case with Cisco TAC. Or perhaps to try a different version of code and see if the behavior changes.

HTH

Rick

[ammahend]

Also if you want you can define which class of broadcast message get changed to unicast and forwarded to the ip helper address by configuring

ip forward-protocol udp <port>  for e.g bootpc for dhcp discovery, bootps for reply etc.
 you can add multiple broadcast message types.

[Roger Base]

I never used the ip forward-protocol. Can you give example of that statement in my scenario ?

[Richard Burts]

In your scenario I do not see a need for forward-protocol. The scenario where I usually see forward protocol used is someone needing to do Wake On LAN from a server in one subnet to clients in a different subnet. But I do not see any need for forward-protocol in your situation.

HTH

Rick