IP-Helper configuration tasks

[uupsala]

I have a few questions on working with ip helper.

NETWORK: Client--Router--Router--DHCPServer.

Can anyone tell me how ip helper works. Is it using some kind of encapsultion? How is the answer from the DHCP server going back to the client and finally what configuration tasks I have to do when multiple hops are involved to reach the Server? I plan to use this feature in my network but did not find helpful information regarding to the questions above. Any link or description would be very helpful to me.

[pflunkert]

Hi,

if the DHCP server and the DHCP clients are on different networks or subnets, you must configure the switch with the ip helper-address address interface configuration command. The general rule is to configure the command on the Layer 3 interface closest to the client. The address used in the ip helper-address command can be a specific DHCP server IP address, or it can be the network address if other DHCP servers are on the destination network segment. Using the network address enables any DHCP server to respond to requests.

Here is a configuration example:

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch(config)# service dhcp

Switch(config)# ip dhcp relay information option

Switch(config)# interface vlan 10

Switch(config-if)# ip address 10.0.0.1 255.0.0.0

Switch(config-if)# ip helper-address 30.0.0.2

Switch(config-if)# exit

Switch(config)# interface range gigabitethernet0/1 - 2

Switch(config-if)# switchport mode access

Switch(config-if)# switchport access vlan 10

Switch(config-if)# exit

Normal a DHCP Packet is a udp broadcast packet. The router/switch sent this packet as a unicast packet to the dhcp server. The dhcp server anseralso a unicast packet.

Regards

Peter

[aashish.c]

Hi

"ip helper works" specifies the DHCP packet forwarding address. If you have multiple servers, you can configure one helper address for each server.

You can refer to the following document for DHCP on a switch :

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12120ea2/3550scg/swdhcp82.htm#wp1112175

Also refer to this document for sample config of DHCP :

client-switch-router-DHCP

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080114aee.shtml

Regards

aashish C

[bpotschien]

For your constellation:

client - Router A - Router B - DHCP server

1. The client sends out a broadcast dhcp request

2. Router A needs an config entry for this subnet, the client is in (ip dhcp helper-address SERVER_IPADRESS)

3. Router A takes the broadcast from the client and transforms it to a unicast with destination SERVER_IPADRESS and source ROUTER_A_IPADRESS.

4. Router B is transparent for this process. It only forwards unicast.

5. The DHCP server recieves the unicast dhcp request and replies to Router_A_IPADRESS (through Router B)

6. Router A recieves the Packet and sends it to the client (sends out broadcast with the specific client MAC Adress*)

*don't really know what typ of packet this is :-(

AFAIK this is the correct process.