SRW224G4P VLAN Hell

Unanswered Question
Jan 15th, 2010

I am confused on why my setup is failing: my network is quite simple and it is comprised of 3 network elements:

1- A VDSL Modem (which acts as the DHCP server of the network)

2- A linksys  SRW224G4P switch (with a static IP)

3- A AIRONET 1140 (Cisco) (with a static IP) (I wouldn't have used VLANS if the AIRONET didn't make it a must for multiple SSID - actually I didn't know that VLANs existed before that :smileysad: )

So the AIRONET has two SSIDs one associated with VLAN 1 (default) and the other to VLAN 2. I have connected the AIRONET to my linksys switch over a line which I have setup as a "Trunk" which carries both VLAN 1 packets and VLAN 2 packets. Then I connected my switch in 2 different ways to the Modem, none of them worked:

  • I set up a "General" port on port 1 with VLAN 1 packets UNTAGGED and VLAN 2 packets UNTAGGED and connected my modem to port 1 of the switch (i understand that this setup is not correct but I had tried it anyway.)
  • I set up an "Access" port on port 1 with VLAN 1 packets UNTAGGED and another "Access" port on port 6 with VLAN 2 packets UNTAGGED and connected both port 1 and port 6 to my modem.

The end result is this, in both case, when I over wireless to the VLAN 1 associated SSID I have absolutely no problem,  but when I connect to the VLAN 2 associated SSID i don't get an IP address.

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Joachim Kern Fri, 01/15/2010 - 05:38

have you added the vlans to the trunk port with the gui ?

Althouh you configured it to be a trun k, yoy are required to configure which VLANs are b eing supported on the port.

burcsahinoglu Fri, 01/15/2010 - 05:55

if you mean the SRW224G4P switch yes.

I set up the port where the AIRONET is connected to be trunk and handle VLAN1 UNTAGGED (default) and VLAN2 TAGGED.

Joachim Kern Fri, 01/15/2010 - 06:37

yes, that is what i meant.

try to configure a static ip address on the wlan client when you connect to ssid (vlan 2) and seeif you have connectivity

burcsahinoglu Fri, 01/15/2010 - 06:58

We thought it might be a DHCP issue so we setup a machine with a static ip. But we got the same result: the machine is not able to access the internet over the modem. (and of course we if connect the machine to the SSID associated with the default VLAN we have no issue.)

Joachim Kern Fri, 01/15/2010 - 07:21

the switch looks ok to me.

i guess you need to trace (wireshark) the links to see which device blocks the packets.

- does the aironet send the tagged packets towards the switch ?

- does the vdsl modem return the packets on the port where the vlan 2 is supported

alissitz Fri, 01/15/2010 - 16:23

Hello, good evening,

If I understand correctly, you have the following:

  • One 1140 series AP set up for multiple vlans
  • One switch that is set to trunking for this port and has these additional vlans configured on it
  • The switch trunk port has the additional vlans (besides vlan 1) set for tagged and not excluded

This is a layer 2 switch

  • The switch port to the modem is a trunk port with the native vlan as 1, and the other vlans are set to tagged
  • The modem is vlan aware, and has multiple interfaces / virtual interfaces; one for each vlan
  • The modem has multiple dhcp scopes set up, one for each vlan

Do I have all of this right? Do please let me know,

Andrew

burcsahinoglu Sat, 01/16/2010 - 00:05

Hello Andrew,

I don't know if you are asking me or saying that the SRW224G4P is a layer 2 switch.

All i know is that is is vlan aware and that I can set up the port to be access, trunk or general and I can associate different VLAN to each port.

As far as connecting to the modem.

No, the switch has 2 ports going to the modem, both of them are set up for UNTAGGED traffic, one carries VLAN 1 UNTAGGED traffic the other VLAN 2 UNTAGGED. The modem is not VLAN aware and his DHCP cannot be set for different scope and I don't need to have different scope.

I am attaching a picture with the layout of the network I hope the setup will be clear.

As I said I have no interest in using virtual lans but using the aironet with multiple SSID forced me to do so.

Attachment: 
alissitz Tue, 01/19/2010 - 08:54

Appreciate the update.

Yes, understood the switch is L2 and VLAN aware ... it is a nice switch with a lot of flexibility.

I need to understand a little more of what is happening on the LAN and what you are trying to do. The modem has two interfaces connected to the LAN, but is servicing only one interface?

It sounds a little like the modem is running in an active / standby mode.

Does it respond to DHCP requests on both interfaces, whichever one it received the packets on, or only on the primary interface?

You have created two networks with having two VLANs, but your DHCP server can only serve one network with one scope.  This is not the suggested design when using multiple VLANs.

So ... if the modem does not understand 802.1q tags / VLANs, then the ports are configured to send only untagged traffic only.  This is fine.  Each port is in it's own VLAN, and the modem is unaware.

Can you verify the following?

1) DHCP requests are being sent and recieved to and from the modem on either interface, so you can have DHCP effectively run on either interface / VLAN.

2) If the modem can work in a multi vlan network - you might need to check with the modem vendor

Also, it might be good to run a sniffer on the computer for each interface.  You can see requests, responses and any errors.  HTH,

Andrew Lee Lissitz

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