3750 and dhcp 1

[boymarty24]

Hi,

Have a 3750SI ( L3 switch ) acting as a vlan machine. I have 4 vlans configured. Routing between the vlans and to the internet works fine. Now i want to use dhcp for every vlan. Can i put a dhcp server on one vlan and let every client on the other vlans connect to it, and get correct subnets. I think there is a command on the ios forwarding dhcp request, right or wrong?

 

[ADB100]

Yes is the short answer. You could also use the 3750 as the DHCP Server if you wanted, but since you don't state this I will detail enabling IP Helpers:

To forward DHCP requests from one subnet to another the router on the VLAN where the DHCP Clients are needs an IP Helper-Address configured, this is the IP address of the DHCP Server:

interface Vlan10 !DHCP Server on this VLAN
ip address 10.1.1.1 255.255.255.0
!
interface Vlan20 !DHCP Clients on this VLAN
ip address 10.2.1.1 255.255.255.0
ip helper-address 10.1.1.10
!
interface Vlan30 !DHCP Clients on this VLAN
ip address 10.3.1.1 255.255.255.0
ip helper-address 10.1.1.10
!
interface Vlan40 !DHCP Clients on this VLAN
ip address 10.4.1.1 255.255.255.0
ip helper-address 10.1.1.10
!

Enabling IP Helpers by default forwards 9 types of UDP broadcasts; these are port 37 Time, port 42 Nameserver, port 49 TACACS, port 53 DNS, port 67 Bootstrap Protocol (Server), port 68 Bootstrap Protocol (Client) port 69 TFTP, port 137 NetBIOS Name Service and port 138 NetBIOS Datagram Service. Leaving all these protocols enabled can have an adverse effect on some network operations so I usually disable the unused ones and only allow BOOTP Client & Server (UDP ports 67 & 68). The following global config achieves this:

no ip forward-protocol udp tftp
no ip forward-protocol udp nameserver
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs

Obviously you need to create the scopes on the DHCP Server.

Good luck

Andy

 

[boymarty24]

Thx,

Great post Andy. I looked at the 3750 and couldnt find any
dhcp-server setup. Perhaps i need a different image then the one i use now.

 

[ADB100]

The DHCP Server functionality should be in all the images I think?

ip dhcp excluded-address 10.1.1.200 10.1.1.254
!
ip dhcp pool Network-10.1.1.0
network 10.1.1.0
default-router 10.1.1.254
domain-name somedomain.com
dns-server 10.1.1.60 10.1.1.61


Andy

 

[boymarty24]

Thx andy! Your posts have been very useful!

 

[boymarty24]

Hi again,

Anyone know how to configure a dhcpserver on a winserver2000/2003 to get this to work? I have tried but can´t make it work.

 

[ADB100]

It should be pretty simple with Windows 2000/2003. Point the IP Helper to the IP address of the Windows server, make sure your windows server has a static IP address and can reach each of the IP networks (default-gateway).
For a minimum you will need IP address, subnet mask, & default-gateway configured in each of the DHCP Scopes.

Andy