Getting DHCP to work on my Vlan

azobiora

Hi guys,

Please i need suggestions to why i can't seem to get DHCP to work very well on my vlans.

I configured router on a stick senerio having 3 vlans. Vlans 20, 30 n 40

I could route btw my vlans very well, everything seems good and dandy with the routing, then i configured ip helper-address on the subinterface that repesents each vlan for the router to recieve broadcast for the nodes plugged into my switch which has these vlans configured on, to recieve ip address from a windows box that's configured for DHCP in my network. I was joyed when i plugged a pc into the ports allocated to vlan 30 and i got an ip from the dhcp server and could browse the internet! then took the cable off from vlan 30 took it to vlan 20 and 40 no joys at all. I couldn't recieve an IP address from the server.

Any suggestions as to what the problem could be please anyone help.... Well the good news is that i have this on lab network i configured trying to do a full deployment, as of yet i haven't done it on my full network cos i want to fully get this sorted out! Don't want to get calls from Users that they couldn't pick an ip address.

Would really appreciate your help please.

dtlokee

A couple of things to remember when you are using DHCP from a remote subnet:

1. You must add a ip helper-address with the valid ip address of the DHCP server
2. The DHCP server must have a valid scope matching the subnet of the interface on the router that you put the ip helper address command on.
3. Make sure the scope is active on the DHCP server

mikej412

azobiora wrote: »

I was joyed when i plugged a pc into the ports allocated to vlan 30 and i got an ip from the dhcp server and could browse the internet! then took the cable off from vlan 30 took it to vlan 20 and 40 no joys at all. I couldn't recieve an IP address from the server.

What IP Address do you see when you do an ipconfig command? The old vlan 20 IP? Or an IP Address beginning with 169?

What did you try to receive a new IP Address? Did you do an ipconfig /release or ipconfig /renew? Did you reboot the PC?

azobiora

dtlokee wrote: »

A couple of things to remember when you are using DHCP from a remote subnet:

1. You must add a ip helper-address with the valid ip address of the DHCP server
2. The DHCP server must have a valid scope matching the subnet of the interface on the router that you put the ip helper address command on.
3. Make sure the scope is active on the DHCP server

Hi Dtlokee,

I would say option 1 and 3 you sent i have done just that! I do have a valid ip address on the subif. Ok here's the config

subif)#0.20 (this is the ip address of the subif 172.16.20.1)
subif)#ip helper-address 192.168.1.250 (this is the address of the dhcp server)

subif)#0.30 (this is the ip address of the subif 172.16.30.1)
subif)#ip helper-address 192.168.1.250

subif)#0.40 (this is the ip address of the subif 172.16.40.1)
subif)#ip helper-address 192.168.1.250

A funny thing again is that.... I have the cable coming from my corporate network that doles out the ip address from the DHCP server plugged into a switchport on vlan 20 while my machin is on a switchport on vlan 30. But if i move it to a switchport to vlan 20 or 40 i can't seem to be getting an ip address. But if i have same cable coming from my corp into swxport of vlan 40 same as my pc cable into swxport of vlan40 i get an ip. Both cables into vlan 20 i don't get an ip address.

I really don't know what could be the problem!

Thanks

dtlokee

did you also make a scope on the DHCP server matching each of the subnet addresses - 172.16.20.0/nn, 172.16.30.0/nn and 172.16.40.0/nn (I put /nn because you didn't post the subnet mask)


You also show the helper address as 192.168.1.250 but you say you plugged this server into a switchport in vlan 20 which has a subnet address of 172.16.20.0, this is a mismatch. Can you ping the helper address from your router?

azobiora

mikej412 wrote: »

What IP Address do you see when you do an ipconfig command? The old vlan 20 IP? Or an IP Address beginning with 169?

What did you try to receive a new IP Address? Did you do an ipconfig /release or ipconfig /renew? Did you reboot the PC?

Hi Mike,

Good to see and hear from you. I have done all that you can think of.... The truth is i am not so new to the world of networking. I have done all that there is to trouble shooting. Just begining to get use to the world of Cisco. I must say that it's fun though trying out things and they work! if you read the last post i have b4 this now you'd see all that i have done!

The good thing is that it's just some testnetwork that i have this on, just trying out my hands on these things b4 i go ahead and do a full deployment! So i am happy this is coming right now! But i configured more of what i am looking at deploying this at my Corp Network. Acutally doing this on a Cisco 2811 and 2950, but in ma real Corp network i have 2960 switches.

I'd need your advice on this! Thanx man

azobiora

dtlokee wrote: »

did you also make a scope on the DHCP server matching each of the subnet addresses - 172.16.20.0/nn, 172.16.30.0/nn and 172.16.40.0/nn (I put /nn because you didn't post the subnet mask)


You also show the helper address as 192.168.1.250 but you say you plugged this server into a switchport in vlan 20 which has a subnet address of 172.16.20.0, this is a mismatch. Can you ping the helper address from your router?

Hey Dee!!

it's a /24 subnet mask. And no i can't reach the 192.168.1.250 ip helper address from the router! and i didn't add the 172.16.20./nn subnet address in the dhcp server!

do i need to do all these to achieve the full resolving?

adam-b

Anyway you could do a packet capture on the dhcp server to see if it is getting dhcp requests from all three of your vlans?