DHCP assigning IP failed.

[ahmadqayyum7]

I am trying to give IP addresses to my PCs through the DHCP server. The problem is that the PCs attached with in the network of the DHCP server have been successfully  assigned IP but those that are in other network are unsuccessful. Please help me.

Password is ahmad and for configuring password is class.

[Seb Rupik]

Hi there,

You need to configure ip helper-address for the other layer interfaces:

!
int vlan 100
  ip helper-address <ip_of_dhcp_server>
!

cheers,

Seb.

[ahmadqayyum7]

in which mode do I have to enter the command int vlan 100 because I entered it in config mode and it said invalid marker

[Giuseppe Larosa]

Hello,

you need to customize the command for your scenario.

The command enabled the DHCP relay function and it is needed on each L3 interface in interface mode that is not the one where the DHCP server is connected to.

When multiple routers are involved you need it on each user facing L3 interface that is not the one where the DHCP server connects.

The DHCP relay function makes the DHCP request routable by changing the destination address from broadcast 255.255.255.255 to unicast IP address of the DHCP server specified in the ip helper command.

Hope to help

Giuseppe

[ahmadqayyum7]

I still don't understand what you are trying to say.

[Giuseppe Larosa]

Hello,

first of all, follow George's suggestions about creating multiple scopes on DHCP server.

When a PC client boots it tries to get an IP address by sending out a DHCP request packet with destination = broadcast = 255.255.255.255.

This packet is flooded to all devices in the same broadcast domain = IP subnet.

If the DHCP server is connected there it can answer directly.

IF the DHCP server is in a different IP subnet = different broadcast domain the client DHCP request cannot reach it.

If there is a router in the middle the router wil not route a packet with broadcast destination.

A router is a broadcast firewall is written in several books!

In order to avoid the need to setup a DHCP server for each Vlan/ IP subnet, the DHCP relay feature can be used.

When a router configured with an ip helper-address 10.1.1.1 (under an interface) command receive a DHCP request with broadcast destination it makes changes to make the packet routable to the DHCP server.

destination address 255.255.255.255 ---> 10.1.1.1 = DHCP server address

source IP address is changed to the router IP address of the interface in the path to the DHCP server

an internal field is set to the IP address of the router interface that received the original packet.

This allows the DHCP server to find the right scope ( right address pool) for picking up an available address that is in the correct IP subnet.

Hope to help

Giuseppe

[Georg Pauwen]

Hello,

what are you trying to accomplish ? Your DHCP server has only one scope, but your PCs are on different networks. If you want to assign IP addresses, you need to define the scopes first, and then, as indicated, configure IP helper addresses...

[ahmadqayyum7]

How do I accomplish that?

[Georg Pauwen · Accepted solution]

Hello,

I'll send the revised lab over...

[Georg Pauwen]

Hello,

here is the revised lab.

1. DHCP scopes added to DHCP server

2. IP helper addresses configured on all LAN router ports

3. Command 'spanning-tree portfast' added to access ports

4. All laptops set to DHCP