DHCP relay

[Mr.Vendelboe]

Hello.

I hope one of you can assist me with this issue.

I've connected an old Cisco 1812 series router to my SOHO wireless accespoint, which functions as a DHCP server as well.

The 1812 Router has two FE interfaces and 8 switchport interfaces.

The FE0 interface have I connected to my 'DHCP server' and used the IP ADDRESS DHCP to gain an IP address. This works.

Now I'd like to be able to connect my laptop to one of the switchports on the 1812 and receive and DHCP address from the DHCP server. This is what I'm having trouble with.

The switchports are part of a vlan 10, which I've created.

I'v tried the IP HELPER-ADDRESS on the FE0 interface, pointing to my default gateway on the DHCP server, as well as tried the IP HELPER-ADDRESS on the vlan 10 interface. Still my Laptop does not gain an IP address from the DHCP server.

What am I missing?

*** Config ***

Building configuration...

Current configuration : 1975 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
!
hostname JVA-ROUTER01
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$cKcv$ISBWlaif11Uu9qvqzPtPo.
!
aaa new-model
!
!
!
aaa session-id common
!
resource policy
!
!
!
ip cef
!
!
ip domain name JVA
ip ssh version 2
!
!
!
username adminjva privilege 15 password 7 0235145E0E021670181F59
!
!
!
!
!
!
interface FastEthernet0
 description WAN
 ip address dhcp
 ip helper-address 192.100.0.1
 duplex auto
 speed auto
!
interface FastEthernet0.10
 encapsulation dot1Q 10
 ip address dhcp
 ip helper-address 192.100.0.1
 no snmp trap link-status
!
interface FastEthernet1
 description NOT IN USE
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface BRI0
 no ip address
 encapsulation hdlc
 shutdown
!
interface FastEthernet2
 switchport access vlan 10
!
interface FastEthernet3
 switchport access vlan 10
 shutdown
!
interface FastEthernet4
 switchport access vlan 10
 shutdown
!
interface FastEthernet5
 switchport access vlan 10
 shutdown
!
interface FastEthernet6
 switchport access vlan 10
 shutdown
!
interface FastEthernet7
 switchport access vlan 10
 shutdown
!
interface FastEthernet8
 switchport access vlan 10
 shutdown
!
interface FastEthernet9
 switchport access vlan 10
 shutdown
!
interface Vlan1
 no ip address
!
interface Vlan100
 no ip address
!
interface Vlan10
 ip address dhcp
 ip helper-address 192.168.0.1
!
!
!
no ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
banner motd ^C \ \/\   /\/_\         /__\  /___\/\ /\/__   \/__\/__\/ _ \/ |
   \ \ \ / //_\\ _____ / \// //  // / \ \ / /\/_\ / \// | | | |
/\_/ /\ V /  _  \_____/ _  \/ \_//\ \_/ // / //__/ _  \ |_| | |
\___/  \_/\_/ \_/     \/ \_/\___/  \___/ \/  \__/\/ \_/\___/|_| ^C
!
line con 0
 logging synchronous
line aux 0
line vty 0 4
 transport input ssh
!
!
webvpn context Default_context
 ssl authenticate verify all
 !
 no inservice
!
end

*** config ***

*** Interface brief ***

JVA-ROUTER01#show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0              192.100.0.106   YES DHCP   up                    up
FastEthernet0.10           unassigned      YES DHCP   up                    up
FastEthernet1              unassigned      YES NVRAM  administratively down down
BRI0                       unassigned      YES NVRAM  administratively down down
BRI0:1                     unassigned      YES unset  administratively down down
BRI0:2                     unassigned      YES unset  administratively down down
FastEthernet2              unassigned      YES unset  up                    up
FastEthernet3              unassigned      YES unset  administratively down down
FastEthernet4              unassigned      YES unset  administratively down down
FastEthernet5              unassigned      YES unset  administratively down down
FastEthernet6              unassigned      YES unset  administratively down down
FastEthernet7              unassigned      YES unset  administratively down down
FastEthernet8              unassigned      YES unset  administratively down down
FastEthernet9              unassigned      YES unset  administratively down down
Vlan1                      unassigned      YES NVRAM  up                    down
Vlan100                    unassigned      YES NVRAM  up                    down
Vlan10                     unassigned      YES DHCP   up                    up
JVA-ROUTER01#

*** Interface brief ***

[Peter Paluch]

Hi,

This is not exactly how the DHCP Relay is meant to be used. The placement of the ip helper-address should be exclusively on interface Vlan10, and the following conditions must be met in order for this to work:

1. The interface Vlan10 must have a static IP address assigned. It cannot rely itself on DHCP to obtain its address.
2. The IP network on interface Vlan10 must be unique and different from the IP network on the WAN interface.
3. The DHCP server must be configured with a pool that corresponds to the IP network used on the interface Vlan10
4. The routing on DHCP server must be properly configured so that it knows how to reach the network on interface Vlan10. Notice that this essentially invalidates the use of DHCP-assigned address on your WAN interface: If this interface can change its address over time, you cannot configure your DHCP server with a static route for the network on interface Vlan10 because the next hop address (the interface on the WAN interface) is not stable and may change over time.

Is there any particular reason why you want both your WAN interface and the internal LAN network to be served by the same DHCP server?

Best regards,
Peter

[Edil Cajigas]

Thanks your post was very helpful to me.