Solved: Can one disable DHCP on one port of 3750?

murray-davis · ‎05-24-2007

3750 in routing mode acting as dhcp servr. Two vlans: client and staff. dhcp pool is set up relevant to client vlan. Only one port in staff vlan which is connected to a 2960, all of whose ports are in staff vlan. Off this 2960 is a w2k3 dhcp server. Obviously, I do not want any dhcp traffic from 3750 to staff switch. How do I disable/prevent dhcp on the 3750 staff port? The 2960 has a management interface in the staff vlan. The 3750 also has a management interface in the staff vlan. All other ports are in trunking mode since the neighbor switches have management interfaces in the staff vlan.

Richard Burts · ‎05-24-2007

Murray

You configure DHCP for a specific VLAN when you configure the network address in the DHCP pool. When the 3750 receives a DHCP request it looks at the address of the VLAN interface on which it received the request and looks for a network address in a pool that matches the interface address. So in your config network 172.17.0.0 255.255.128.0 matches VLAN 2 and will offer IP addresses to clients in that VLAN. Since there is no network address in any pool on the 3750 that matches 10.10.x.x then it will not offer DHCP on VLAN 1.

HTH

Rick

HTH

Rick

View solution in original post

Richard Burts · ‎05-24-2007

Murray

I am not sure that I have fully understood your explanation of the environment and your requirements. But it seems to boil down to the 3750 has 2 VLANs. You want the 3750 to provide DHCP on one VLAN and not provide DHCP on the other VLAN. It seems to me to have a simple solution: configure a DHCP pool for the VLAN you want it to serve and do not configure a DHCP pool for the other VLAN. If it does not have a pool configured for the other VLAN then it will not act as a DHCP server on that VLAN.

If I have misunderstood and the requirement is different then perhaps you can clarify for me.

HTH

Rick

HTH

Rick

murray-davis · ‎05-24-2007

Rick, thank you, that is correct. Here is a code snippet.

ip dhcp pool Clients

network 172.17.0.0 255.255.128.0

default-router 172.17.1.10

dns-server 202.194.28.230 198.161.156.1

However, how do I configure dhcp for a specific vlan, vlan 2?

More code:

interface Vlan1 (mgmt int & staff vlan)

ip address 10.10.1.10 255.255.0.0

!

interface Vlan2 (client vlan needing dhcp)

ip address 172.17.1.10 255.255.128.0

As previously mentioned all ports are in trunk mode except the staff port which is in the staff vlan (vlan 1).

Regards,

Richard Burts · ‎05-24-2007

Murray

You configure DHCP for a specific VLAN when you configure the network address in the DHCP pool. When the 3750 receives a DHCP request it looks at the address of the VLAN interface on which it received the request and looks for a network address in a pool that matches the interface address. So in your config network 172.17.0.0 255.255.128.0 matches VLAN 2 and will offer IP addresses to clients in that VLAN. Since there is no network address in any pool on the 3750 that matches 10.10.x.x then it will not offer DHCP on VLAN 1.

HTH

Rick

HTH

Rick

Richard Burts · ‎05-24-2007

Murray

Thank you for using the rating system to indicate that an answer did solve your issue (and thanks for the rating). It makes the forum much more useful when people can read a question and can know that an answer was provided which resolved the question. I encourage you to continue your participation in the forum.

HTH

Rick

HTH

Rick

murray-davis · ‎05-24-2007

This was the first time that I used a forum. I was aware of the forums' existence but never before considered a forum as an option for problem solving. I am happy to now have the the various forums as an option for solving future challenges.

Your help is greatly appreciated.

Regards,

Murray

Lee Mac · ‎12-31-2017

I suspect I should start a new thread since this is old, but I'll try this first. I have the same request text as the thread topic but with a twist.
The switch is the dhcp server for all the devices (voip phones) plugged into it, but I also have a connection to another lan (for maintenance purposes) that also has a dhcp server on it. As you might guess when I restart a phone, its a race to see which DHCP server gets there first.
I have been leaving the other lan unplugged until I need it, but can I set a single port to block any form of DHCP from passing through it?