Wireless devices not receiving DHCP address

[thomasweening]

Hello,

This is my first time here and the first post I have done, so please let me know if I need to do something different in the future.

In short, my wired devices can receive an IP address and connect to the Internet. However, none of my wireless devices (smartphone, tablet, etc, etc.) will get one.

I have a router-on-a-stick topology with 4 VLANs (wrless_1, wrless_2, wired, and mgmt&native) (vlan_10, vlan_20, vlan_30, vlan_99 respectively). Why is that only the wired devices are getting an IP address? For additional context, I have two wireless routers that are acting as access points in VLAN10 and VLAN20. My DHCP is on an Ubuntu Server, as well as a primary and secondary DNS server that I created.

Any help would be appreciated.

Thank you!

[balaji.bandi]

Please clarify  Wirelss device ( AP or connected Client not getting IP address ?)

AP in what mode ?

[thomasweening]

So, I have 3 routers, 3 switches, and 2 wireless routers. The wireless routers are in AP mode (not being used as a wireless router). The router-on-a-stick topology does include the ip helper-address command already (among the rest of the topology and commands). The devices off of the APs (vlan_10 and vlan_20) are not getting the IP addresses, but the devices on vlan_30 are, which are the wired ones.

Does that make sense?

Thank you!

[Richard Burts]

We do not have enough information to really understand your problem or to suggest solutions. Am I correct in understanding that neither vlan 10 or vlan 20 receive IP addresses but vlan 30 does? What about vlan 99?

There are several aspects that might be problematic. As a starting point would you post the configuration for the router on a stick? As a next step can you confirm that the DHCP server has appropriate scopes configured for vlan 10 and vlan 20? What addresses does the DHCP server have for these vlans?

[thomasweening]

The router-on-a-stick is posted. Do you need the switch configs as well?

The DHCP server is not on the router, but on an external operating system (Linux). Does this matter? Would the DHCP server need to be configured on the router?

The router, as seen in the config has the ip helper-address command configured on it, but would that cause issues?

Thank you!

[Georg Pauwen]

Hello,

the router config looks good as far as I can tell. Make sure the port of the switch connected to the FastEthernet0/1 interface of the router is in trunk mode, and that all Vlans (10/20/30/99) actually exist on the switch (show vlan).

Also, what brand/type/model are the APs ?

Actually, post the configuration of the switch as well (and indicate on which ports of the switch the APs are connected)...

[balaji.bandi]

interface FastEthernet0/1
 no ip address
 duplex auto
 speed auto

what is the configuration on the switch side, where the above port connected, is this trunk and allowed all the vlan 10/20/30/99 ?

post the switch config also

[Richard Burts]

Thanks for posting the config of the router. As others have mentioned the router config seems appropriate. You asked a few questions that I would suggest these answers:

- "Do you need the switch configs as well?" At this point I do not think we need the switch configs. The output of show interface trunk might be useful to verify that the trunking is working and that all 3 vlans are active on the trunk.

- Does it matter that the server is an external device and not on the router? It should not matter that the server is external. Sometimes configuring DHCP on the router is convenient. Sometimes you might need some function in DHCP that an external server can provide that might not be available on the router. Or you might have a situation where there are multiple DHCP scopes that appear on multiple routers. It might be preferable to use a DHCP server where the DHCP configuration is centralized and is easier to manage than it would be spread over multiple routers.

- would the configured ip helper-address cause a problem? Assuming that the DHCP server is at 192.168.1.188 then the configured helper address should not cause a problem. And if the server is at this address then helper address is absolutely needed for vlan 10 and 20. Since the subnet for vlan 30 is the subnet that includes the server address then the helper address is not needed for vlan 30.

I wonder if there is significance that the one vlan that is successfully receiving IP addresses from DHCP is the vlan that includes the address of the server? Can you verify that the server address is 192.1468.1.188? Can you verify that the server does have separate DHCP scopes set up for subnets 192.168.1.0/26 and for 192.168.1.64.0/26

If you connect a PC to a port in vlan 10 and manually configure an IP address, mask, and gateway is the PC able to ping the server? And the same for vlan 20?

[thomasweening]

The strange thing is that when I go to ping my DNS server address from a wireless device, there is no response. The only way it seems to work is if I use 8.8.8.8 as the DNS server address. Also, as is probably the case, I can ping from the wired device to the DNS server, as they are on the same subnet. So, something seems to be off on the switch configs if nothing is missing on the routers. Maybe you can find something I am missing. I will post the switch configs.

The APs are Netgear wireless routers that are acting as wireless APs, but that wouldn't cause an issue would it?

I'm also going to re-post the router config because it is now being used as a DHCP server (the external DHCP is off now).

Thank you!

[Georg Pauwen]

Hello,

can you put a description on the switchports so we know which switchport is connected to what ?

E.g.:

s2

interface FastEthernet0/1

description Uplink to r3
switchport trunk native vlan 99
switchport mode trunk

[Richard Burts]

Thanks for the update. Interesting that the external DHCP server is now not being used. It does probably make it a bit easier to troubleshoot what is going on. 

In the original post the reported issue was that devices in vlan 10 and 20 were not getting IP addresses. Now that the external server is not used and the DHCP scopes are on the router would I be correct in assuming that devices in vlan 10 and 20 are now successful in getting IP addresses? And would I be correct in understanding that now the issue is that those devices are not successful in pinging to DNS at 192.168.1.186 and at 192.168.1.187?

To help in troubleshooting the issue would you post the output of these commands on the router

show cdp neighbor

show arp

show ip interface brief

and of these commands on the switches

show cdp neighbor

show interface trunk

show vlan

And from the servers at 186 and 187 could you post the output of ipconfig (or other appropriate command if not Windows machines)

[thomasweening]

Here's another clincher I forgot to mention: the DNS servers are on virtual machines, if that matters. Their host is connected to the Internet with the same IP address (which is why I can ping them). BUT, the DNS servers on the virtual machine(s) cannot ping the default gateway, which is probably why devices cannot communicate with it...

The DNS servers on the virtual machine are on a /26 subnet with the default gateway being set to vlan_30 (wired) subinterface (fa0/1.30 on the router), if that matters.

Anyway, here are the files requested (ifconfig (Ubuntu OS) is not installed for some reason, so had to resort to using the OS's network manager feature to get the information... Is that sufficient?).

Thank you!

[Georg Pauwen]

I would say the DNS stuff is certainly something to keep in mind, but first, we need to figure out why the wireless clients do not get an IP address.

Post a topology diagram showing how all devices are interconnected, and indicate which interfaces they use to connect to each other.

[thomasweening]

Folks, I was able to get this figured out by making the DNS server be put on a live device, as opposed to a virtual machine, among other settings needing to be configured, but thank you all for your assistance with this!

Thank you!

[Richard Burts]

Thanks for the update. Glad to know that you got it working.