DHCP Scope Question

Smallguy

Hi

I've been looking around but not able to find what I'm looking for right now.

I know there must be away but it is slipping my mind right now.

I have one DHCP server configured with a single NIC and I want to have multiple scopes

say 10.10.10.0/24 and 10.10.11.0/24

The issue is I forget how to force the users with a connection on the 9th and 10th floor to get an address in 10.10.10./24 and the users on the 8th to get a 10.10.11.0/24 address


if anyone can point me in the right direction it would be appreciated

tiersten

Use a VLAN for each floor or have a big list of MAC addresses for each floor.

Smallguy

tiersten wrote: »

Use a VLAN for each floor or have a big list of MAC addresses for each floor.

cool

I've done sum some digging and what your saying seems to hold true

I guess where I'm confused is say we put the 9th and 10 on Vlan3 and users on the 8th on vlan4

are there any special configuration options I need to configure or do I just set them up like I normally would and the information passed on by IPhelper will let my DHCP box know 9th & 10th get 10.10.10.0/24 and 8th gets 10.10.11.0/24.

I'm confused on how the server is intelligent enough to differentiate I guess

BroadcastStorm

Smallguy wrote: »

cool

I've done sum some digging and what your saying seems to hold true

I guess where I'm confused is say we put the 9th and 10 on Vlan3 and users on the 8th on vlan4

are there any special configuration options I need to configure or do I just set them up like I normally would and the information passed on by IPhelper will let my DHCP box know 9th & 10th get 10.10.10.0/24 and 8th gets 10.10.11.0/24.

I'm confused on how the server is intelligent enough to differentiate I guess

If you can create a VLAN you can also set an IP Helper-Address.

ip helper-address

To have the Cisco IOS software forward User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address interface configuration command. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.
ip helper-address address
noip helper-address address
Syntax Description

address
Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.

Defaults

Disabled
Command Modes

Interface configuration
Command History

Release
Modification
10.0
This command was introduced.

Usage Guidelines

Combined with the ip forward-protocol global configuration command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.

Note In order for the ip helper-address command to function correctly, the ip bootp server command must be enabled.
One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP), which is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Because BOOTP packets are forwarded by default, DHCP information can now be forwarded by the router. The DHCP server now receives broadcasts from the DHCP clients.
All of the following conditions must be met in order for a UDP or IP packet to be helpered by the ip helper-address command:
•The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).
•The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface; or major-net broadcast for the receiving interface if the no ip classless command is also configured.
•The IP time-to-live (TTL) value must be at least 2.
•The IP protocol must be UDP (17).
•The UDP destination port must be for TFTP, Domain Name System (DNS), Time, NetBIOS, ND, BOOTP or DHCP packet, or a UDP port specified by the ip forward-protocol udp global configuration command.

Note The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot determine if the packet was intended as a physical broadcast.
Examples

The following example defines an address that acts as a helper address:
interface ethernet 1
ip helper-address 121.24.43.2
Related Commands

Command
Description
ip bootp server
Enables the BOOTP service available from hosts on the network.
ip forward-protocol
Specifies which protocols and ports the router forwards when forwarding broadcast packets.

lex0429

sorry this command is only for Cisco but the command you put on the vlan interface is
ip helper-address <ip of DHCP server>
without the brackets. I could be wrong about this but basically my understanding is that since by default a broadcast packet, which a dhcp packet is, wouldnt pass through that interface. that command pushes the packet through and since its coming from the subnet 192.168.2.0/24 instead of say 192.168.1.0/24 an IP is assigned from the correct DHCP scope..i know the command is right, not 100% sure about my logic

BroadcastStorm

If your trying to accomplish this on Windows Server, you might need to configure a DHCP relay agent, I am not sure how you can force different VLAN to get an IP address to a specific DHCP server, but you might have the option here, up to you to research the rest.

sprkymrk

Whether it's an "ip helper-address" or DHCP relay agent on a RAS server or any other vendor's name for the same thing doesn't matter. The DHCP server will assign an address from it's range based on the network address of the request. So as long as your 9th and 10th floor are in a different subnet, and you have some way to pass the DHCP requests, the server will assign them an address from the appropriate scope.