1841 router does not forward dhcp relay information through sub-interfaces

[abdulkromah]

Hello all,

I configured sub-interfaces(F0/0.10, F0/0.20)  to relay DHCP information to my VLANs,  but it does not work. hosts do not obtain DHCP addresses. if such config is set on the physical interface(F0/0) for a single subnet, all works well. [RIP seems to work well. if a static IP address is set, I can ping all network addresses even loopbacks learned through RIP).  I really need help!

Configuration

(Config)# int F0/0.10

               encapsulation dot1q 10

               ip address 192.168.1.254  255.255.255.0

(Config)# int F0/0.20

               encapsulation dot1q 20

               ip address 10.231.0.254 255.255.255.0

[Georg Pauwen]

Hello,

what do you mean with 'relay DHCP' ? Where is the DHCP server (or is the router the DHCP server) ?

Post the full config of your router...

[Deepak Kumar]

Hi,

Where is your DHCP server? 

Example: If your server is in the VLAN 10 and suppose DHCP server IP address is 192.168.1.10 then you must define DHCP helper IP address on the VLAN 20 for getting IP address from the DHCP.

If your DHCP is in another VLAN or network then you must define the IP Helper address command on both VLANs with DHCP server IP address: 

If your server IP address is 192.168.1.10 then configuration will be:

Config)# int F0/0.10

               encapsulation dot1q 10

               ip address 192.168.1.254  255.255.255.0

(Config)# int F0/0.20

               encapsulation dot1q 20

               ip address 10.231.0.254 255.255.255.0

              IP helper-address 192.168.1.10

and If your DHCP server is in another network as 192.168.2.10 is DHCP server which is connected on another router or switch then you have a valid route toward the DCHP server and configuration will be:

Config)# int F0/0.10

               encapsulation dot1q 10

               ip address 192.168.1.254  255.255.255.0

             IP helper-address 192.168.2.10

(Config)# int F0/0.20

               encapsulation dot1q 20

               ip address 10.231.0.254 255.255.255.0

              IP helper-address 192.168.2.10

Regards,

Deepak kumar