Problems with VLANs / STATIC ROUTE and ip helper (DHCP SERVER)

[gciria]

Hi,

I try to create an enviroment to practice,. This scenario is the following.

One DHCP SERVER directly connect to a router with addres 172.20.100.10/24

This router is directly connect to a SW with addres 172.20.250.2/24 (this is my default vlan 1)

And the SWITCH is connected with this router and 3 PCs with differents vlans.

The idea is that these pcs obtains IP with DHCP from DHCP SERVER, but it's impossible to do!.

The DHCP server can't ping to SW and SW can't do ping to dHCP server, i put static route from dhcp lan to SW but don't arribe

the static route in ROUTER is

172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks
C 172.20.100.0/24 is directly connected, GigabitEthernet0/0
S 172.20.100.1/32 [1/0] via 172.20.250.1
L 172.20.100.2/32 is directly connected, GigabitEthernet0/0
C 172.20.250.0/30 is directly connected, GigabitEthernet0/1
L 172.20.250.2/32 is directly connected, GigabitEthernet0/1

And the port of SW to ROUTER is configured as a trunk

interface FastEthernet0/1
description LAN_ROUTER
switchport trunk allowed vlan 1,10,20,100
switchport mode trunk

and the vlans implicit are.

interface Vlan1
description Networking
ip address 172.20.250.1 255.255.255.0
!
interface Vlan10
description LAN_10
ip address 172.20.10.1 255.255.255.0
ip helper-address 172.20.100.10
!
interface Vlan20
description LAN_20
ip address 172.20.20.1 255.255.255.0
ip helper-address 172.20.100.10
!
interface Vlan100
description SERVERS
ip address 172.20.100.1 255.255.255.0

can any help me? why DHCP SERVER can't ping to SW, and why PCs doesn't ping to dhcp to obtain IP? thanks a lots

[balsamovan]

Have you configured sub-interfaces for the trunk port on the Router?

e.g. interface gig 0/1.20, interface gig0/1.30

If not, you will need to do this because the Router doesn't know how to respond/recognise different VLANs, as such, packet for devices on VLAN 20 or VLAN 30 are not able to get to VLAN 100 (location of DHCP server)

[Jacopo Belcredi]

Hi

Did you configure subinterfaces on router's g0/1?

You need a subinterface for each vlan you configured.

You have also to configure the ip helper-address on the router's subinterfaces, not on the switch ports.

Jacopo