Configuring dhcp on cisco 3750

Why do you have an IP-helper on VLAN 2? While I would think this wouldn't hinder anything, I would definitely remove that first, especially since the switch sees those frames before anything else.....

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At first I did a shutdown cmd on the default vlan 1 and simply connected the dhcp server and the clients. I did not configure the helper- address though. Then I decided that I want an isolated subnet, in which I want to perform dhcp operations. All this is for testing network boot by-the-way. So now I have the vlan #2 (192.168.2.x). By-the-way, when I looked at the cisco docs and used the ip helper-address cmd, the directions were to configure the helper-address per vlan. I tried do the helper-address per interfaces connected to the clients and this is unsupported by the cisco f/w I have 12.2(25)SEB4.

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Ip-helper is only needed for subnets that do not have a directly connected dhcp server. Additionally, you are saying your dhcp server is .100, and your ip-helper says .150. I would either make that match, or get rid of it, especially since these clients are on the same vlan. Let me know how you fare and we can move to the next phase of looking at your issue.

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As I already mentioned, in my previous simple configuration, I simply connected the dhcp server to port 1 and clients to ports 3 through 5. I did not do anything else. This did not work. So then I found out about helper- address and proceeded to perform the current configuration. If you look once more, the helper-address is set to 192.168.2.100 and the vlan 2 ip address is set to

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I'm sorry, the helper address does match. Regardless, you shouldn't need it on the same vlan as the dhcp server, so I'd still try to remove and test. Additionally, your show ip int brief show all ports as up/active as needed?

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Ok, I saw your latest post. Can you ping the dhcp server from the switch? What happens when you do an extended ping (and choose a source interface of the VLAN 2 IP address)? If ping is successful, can you try to set one of the clients to a hard coded IP and do the same test? Can you ping between the static IPed client and the dhcp server? Does show mac-address-table show macs for the clients when they first connect as they should?

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Did several of these from the switch and it works:

Switch#ping 192.168.2.100 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.100, timeout is 2 seconds: !!!!!

On the dhcp client system:

# ping 192.168.2.100

As I understand, broadcast pkts to 255.255.255.255 are not allowed to be propagate across the switch ports by default and my guess is that this is the problem I am facing with the dhcp operation. But then I though that the ip helper-address was meant to address this issue. But you are saying that the helper-address is used for subnet to subnet traffic flow in particular for dhcp. Anyway, I do appreciate your help so far. All the checks you suggested work so far. I tried the dhcp boot and it still fails. I have not changed anything yet.

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dhcp server: Internet Systems Consortium DHCP Server V3.0.4b2

Cisco 3750 VERSION (reformatted): Switch =3D 1 Ports =3D 28 Model =3D WS-C3750G-24TS SW Version =3D 12.2(25)SEB4 SW Image =3D C3750-IPSERVICES-M

Back2Back: If you look in my earlier posts, you'll see that I've conformed that it works. Anyway, I reconfirmed again, and back2back works.

Got rid of the ip dhcp snooping, and it still does not work! I am running out of ideas here.

VLAN 2 contains a DHCP server and several client computers. The layer 3 switch (Cisco 3750) does not have to do anything for DHCP to work. Remove the IP helper configuration completely. There is no doubt that IP helper forwards DHCP requests from a VLAN to another VLAN containing a DHCP server when the DHCP server and DHCP clients are on different broadcast domains, such as the case when they are seperated by a router.

Quote from Cisco documentation: DHCP snooping is a DHCP security feature that provides security by filtering untrusted DHCP messages and by building and maintaining a DHCP snooping binding table. An untrusted message is a message that is received from outside the network or firewall and that can cause traffic attacks within your network.

Based on your below posted configuration, enter the following:

no ip dhcp snooping vlan 2 no ip helper-address 192.168.2.100

Test that without the IP helper-address. If it works, add DHCP snooping back in but do not use IP helper-address if the DHCP server is within the same VLAN on that switch as the DHCP clients.

in message

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Ok got rid of ip helper-address and dhcp snooping. It works. But interestingly it takes a long time. Here is an example dhcp session on a Sun SPARC at the OBP ok prompt:

ok load net:dhcp,192.168.2.100,hello Boot device: /pci@1f,700000/network@2:dhcp,192.168.2.100,hello File and args:

-v

Server IP address: 192.168.2.100 Client IP address: 192.168.2.130 Router IP address: 192.168.2.1 Subnet Mask : 255.255.255.0 ok