DHCP request across multiple vlans?

[miguel.desantiago]

Hello all,

A server on vlan 51 will only issue IP addresses on vlan 51.  The subinterfaces on the router have the command ip helper-addess with the IP address of the DHCP server.  From the switch I can ping the DHCP server and ping all subnets under the subinterfaces.  I got OSPF routing and have all subnets being advertised to allow intervlan routing.  Why can't the DHCP server assign an IP address to another Vlan when a host plugs into the switch?  The router and switch are trunked.

Router#

interface fa0/0.51

ip address 192.168.1.1 255.255.255.0

encap dot1Q 51

ip helper-address 192.168.1.201

interface fa0/0.52

ip address 192.168.2.1 255.255.255.0

encap dot1Q 52

ip helper-address 192.168.1.201

Switch#

interface fa0/0

switchport mode trunk

switchport trunk encap dot1Q

interface fa0/1

switchport access vlan 51

interface fa0/2

switchport access vlan 52

interface fa0/3

description DHCP Server

switchport access vlan 51

[Peter Paluch]

Hello Miguel,

I do not believe that the configuration you have posted here is realistic. The Fa0/0.51 and Fa0/0.52 interfaces are configured in the same IP network. However, a real router would never allow that.

Can you post the true configuration of your router? Also, if the DHCP server is a Cisco IOS device, can you post its configuration, too?

Best regards,

Peter

[miguel.desantiago]

Sorry it was a mistype by me.  I corrected it! 192.168.2.1 it should of been.

[Peter Paluch]

Hello Miguel,

Thanks for the correction.

First of all, you should remove the ip helper-address 192.168.1.201 from your Fa0/0.51 subinterface. The DHCP server itself is in the VLAN 51 and therefore, clients can talk to it directly. This command should be used only on (sub)interfaces in different VLANs.

Second, does the DHCP server have a default route configured so that it is capable of reaching the 192.168.2.0/24 network? This is very important because DHCP responses for clients in the network 192.168.2.0/24 will be sent to the IP address 192.168.2.1.

Third, is the DHCP server actually configured to provide addresses in the 192.168.2.0/24 network? What kind of DHCP server is that, anyway? Is that a Cisco device, or a Windows or GNU/Linux server?

Best regards,

Peter