DHCP ip helper-address

[helplesskheng]

Hi, I am currently learning about DHCP and would like to know more.

I have the set up shown in the image with Router0 being the DHCP in this case. I have got PC4 and 5 in the same VLAN and there is a trunk from Switch1 to Switch2, likewise from Switch1 to Switch3.

I am unable to get an IP from both PC4 and 5, thus I have tried the DHCP ip helper-address at Switch1 and now PC3,6 and 7 cannot get an IP form the DHCP.

I would like to know where I went wrong. Thanks in advance!

[Richard Burts]

You have given us a description of a problem but no detail about it to help us identify the issue and suggest solution. A good starting point would be to post the config of the router.

[helplesskheng]

Sorry bout that!

========

Building configuration...

Current configuration : 1145 bytes

!

version 15.1

no service timestamps log datetime msec

no service timestamps debug datetime msec

no service password-encryption

!

hostname Router

!

!

!

!

!

ip dhcp pool round1

network 192.168.1.0 255.255.255.0

default-router 192.168.1.254

ip dhcp pool round2

network 192.168.2.0 255.255.255.0

default-router 192.168.2.254

ip dhcp pool vlan10

network 192.168.10.0 255.255.255.0

default-router 192.168.10.254

!

!

!

ip cef

no ipv6 cef

!

!

!

!

license udi pid CISCO1941/K9 sn FTX1524XTVU-

!

!

!

!

!

!

!

!

!

!

!

spanning-tree mode pvst

!

!

!

!

!

!

interface GigabitEthernet0/0

no ip address

duplex auto

speed auto

!

interface GigabitEthernet0/0.1

encapsulation dot1Q 1 native

ip address 192.168.1.254 255.255.255.0

!

interface GigabitEthernet0/1

no ip address

duplex auto

speed auto

!

interface GigabitEthernet0/1.2

encapsulation dot1Q 2

ip address 192.168.2.254 255.255.255.0

!

interface GigabitEthernet0/1.10

encapsulation dot1Q 10

ip address 192.168.10.254 255.255.255.0

!

interface Vlan1

no ip address

shutdown

!

ip classless

!

ip flow-export version 9

!

!

!

!

!

!

!

!

line con 0

!

line aux 0

!

line vty 0 4

login

!

!

!

end

[Georg Pauwen]

Hello,

if this is a Packet Tracer project, post the (zipped) .pkt file here...

If all links between the switches and the router are trunks, you don't need any ip helper-addresses. Just make sure that you exclude the default router IP addresses:

Current configuration : 1145 bytes
!
version 15.1
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
--> ip dhcp excluded-address 192.168.1.254
--> ip dhcp excluded-address 192.168.2.254
--> ip dhcp excluded-address 192.168.10.254

!
ip dhcp pool round1
network 192.168.1.0 255.255.255.0
default-router 192.168.1.254
!
ip dhcp pool round2
network 192.168.2.0 255.255.255.0
default-router 192.168.2.254
!
ip dhcp pool vlan10
network 192.168.10.0 255.255.255.0
default-router 192.168.10.254
!
ip cef
no ipv6 cef
!
license udi pid CISCO1941/K9 sn FTX1524XTVU-
!
spanning-tree mode pvst
!
interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/0.1
encapsulation dot1Q 1 native
ip address 192.168.1.254 255.255.255.0
!
interface GigabitEthernet0/1
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/1.2
encapsulation dot1Q 2
ip address 192.168.2.254 255.255.255.0
!
interface GigabitEthernet0/1.10
encapsulation dot1Q 10
ip address 192.168.10.254 255.255.255.0
!
interface Vlan1
no ip address
shutdown
!
ip classless
!
ip flow-export version 9
!
line con 0
!
line aux 0
!
line vty 0 4
login
!
end

[helplesskheng]

I had changed a few things to make things simpler, but it is roughly still the same thing. 

Also is trucking actually possible across routers to switch?

[Georg Pauwen]

Hello,

as suggested earlier, you need to set all inter-switch links and the uplinks to the router as trunks. Find attached the revised, working project file (you need PT version 7.3 to open it)...

[Cristian Matei]

Hi,

     I understand that the DHCP server is the router, but the default gateway for the PC3 to PC7 will be switch1?

         -Make sure you configure on switch1 "ip helper address", on the SVI's corresponding to the vlan number for PC3 to PC7.

         -Make sure the DHCP router has a route back for all those subnets for which it's a DHCP server (the IP subnets of the PC's) towards SW1

         - Make sure the DHCP router has a separate DHCP pool for each of the VLAN's of PC3 to PC7

Regards,

Cristian Matei.

[Mlex1]

in you situation packet tracer has a lot of bag, e recommend for use gns3 or vmware, upper colleges gives for you advice