DHCP on different VLANs

[ilyaskhan]

Hi,

Here is my scenario. I have a DHCP server(10.25.16.6/20) on a VLAN with the following configuration:

interface Vlan256

ip address 10.25.17.100 255.255.240.0

no ip proxy-arp

And i have a client on another VLAN with the following configuration:

interface Vlan140

description " Gateway Address for the vlan 140 users"

ip address 10.24.140.1 255.255.255.0

ip helper-address 10.25.16.6

no ip proxy-arp

Here's the problem, when i create a scope on the DHCP server as 10.24.140.0/24 , it takes an hour before the client gets an IP and i see the following in the debug logs of the DHCP:

11,08/22/05,08:53:58,Renew,10.24.140.15,mahmood.XXX.com,000D56DF510A,

15,08/22/05,08:53:59,NACK,10.24.140.15,,000D56DF510A,

31,08/22/05,08:53:59,DNS Update Failed,10.24.140.15,mahmood.XXX.com,2,

30,08/22/05,08:53:59,DNS Update Request,15.140.24.10,mahmood.XXX.com,,

11,08/22/05,08:53:59,Renew,10.24.140.15,mahmood.XXX.COM,000D56DF510A,

15,08/22/05,08:54:00,NACK,10.24.140.15,,000D56DF510A,

31,08/22/05,08:54:00,DNS Update Failed,10.24.140.15,mahmood.XXX.com,2,

30,08/22/05,08:54:00,DNS Update Request,15.140.24.10,mahmood.XXX.com,

The thing is it shows on the server under the address leases section that it has given an IP to the client, but on the client it gets stuck at the 'acquiring network address' stage on the LAN connection for an hour before it gets the IP.

The funny thing is, this does not happen at all if the scope i created on the DHCP server was with /8 subnet and not /24.If i make a scope of 10.0.0.0/8, things work fine.But my requirement ofcourse is that the client should have a subnet of /24.

Any ideas?

PS- i tried adding 'no ip directed-broadcast' command also on both vlan interfaces.

[amit-singh]

Do you have spanning-tree portfast enable on the host ports. If not enable that and then observe the pattern.

Where is your DHCP server localted, local or across Wan.Please attach the switch config.

regards,

-amit singh

[Georg Pauwen]

Hello,

from your debug output, it appears that the problem might be related to (dynamic) DNS updates.

Are you using a Windows (2000/2003) based DHCP server ?

Check how your DHCP server is configured and try the following (this is from the Microsoft support site):

To configure the DHCP server to register client information according to the client's request, open the DHCP properties for the server, click the DNS tab, click Properties, and then click Update DNS only if DHCP client requests.

Regards,

GP

[ilyaskhan]

Yes, the DHCP server is windows 2003 based.

Yes, portfast is enabled on the ports

The server and client are located on the same switching arena. Server connected to 4500 series having intervlan routing and client connected to 2950 connected to port on 4500 switch.

Attached is the 4500 switch config.

Yes, the DNS tab in the properties of the DHCP server is already set to 'dynamically update DNS only if requested by DHCP clients'

????

[Richard Burts]

I have looked at the configuration that you posted. I do not see anything in it that looks like it would relate to the problem being caused by anything on the 4500. I assume that the issue is something on the DHCP server, or in the client.

Do I understand correctly that the client PC attempts to get an address (or according to the log messages attempts to renew an address previously assigned), has a problem for an hour, and after an hour does correctly get the assigned address?

HTH

Rick

[vedran.djakovic]

Try this

interface Vlan256

ip address 10.25.16.1 255.255.255.0

instead

interface Vlan256

ip address 10.25.17.100 255.255.240.0

Regards

Vedran

[ilyaskhan]

Well, i double checked with the DHCP server and many other clients also. These both are working fine. As i said before the DHCP gives the IP if i put a different scope of /8. And there is nothing wrong on the clients side as it gets another IP if i put a test DHCP on the same vlan. It is only when the DHCP is on VLAN /20 and client on VLAN /24 and scope of /24, things dont work.It does not get an IP to begin with let alone renew. I dont know how come its showing the 'renew' in the log.

I feel there is a problem with the way broadcasting is happening betweem the VLANs. When i used network monitor to view the packets on the DHCP server, i got the attached output.

File 1(slash8) is when the scope on the DHCP is with subnet 255.0.0.0 and in this case, the client gets the IP.

File 2(slash24) is when the scope on the DHCP is with subnet 255.255.255.0 and in this case, the client doesnt get the IP.

Also check out interesting File 3(helper). In this case, the client is on the same VLAN as the server and gets the IP, yet the helper defined on the other VLAN is still broadcasting on behalf of this client.

???

[vedran.djakovic]

I had a similar problem when I use different net masks for 4 VLAN-s, I recreate all VLAN and scopes with same /25 mask and it works.

Only other solution I could think of is using 802.1Q NIC on DHCP server but I did not have chance to test it.

I think that problem is that net mask information from interface with ip helper is not correctly recognized by DHCP server.