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My setup looks like this:

     Switch (T1600G-28TS)
 -------------------------------
 | 1 |   |   | 6 | 5 |   | ... |
 -------------------------------
   |           |   |                          * SSID: NW1 | VLANID: 2
   |           |   |            AP (AP500)   /
   |           |   |         ---------------/
   |           |   +---------|  Multi SSID |--* SSID: NW2 | VLANID: 3
   |           |             ---------------\
   |           |                             \ 
   |           |                              * SSID: Guest | VLANID: 4
   |           +----- Client PC
   |              
   |              
   +----- DHCP Server

Port config on the switch:

 --------------------------
 | Port | PVID |   VLANs  |
 |  1   |  99  | 2,3,4,99 |
 |  5   |  99  | 2,3,4,99 |
 |  6   |   2  |        2 |
 --------------------------

The DHCP server is an ubuntu linux 16.x LTS; dhcpd listens on these interfaces:

 eth0.2 192.168.0.0/24 "NW1"
 eth0.3 192.168.1.0/24 "NW2"
 eth0.4 192.168.2.0/24 "Guest"

when I connect the Client PC to the switch I expect to get an IP Adress from the NW1 (subnet 0) but sometimes I get an IP form the Guest (subnet 2) range.

The dhcpd log looks like this:

 dhcpd[2909]: DHCPDISCOVER from b8:27:eb:5e:bc:ee (twis) via eth0.2
 dhcpd[2909]: DHCPDISCOVER from b8:27:eb:5e:bc:ee via eth0.4
 dhcpd[2909]: DHCPDISCOVER from b8:27:eb:5e:bc:ee (twis) via eth0.3
 dhcpd[2909]: DHCPOFFER on 192.168.0.126 to b8:27:eb:5e:bc:ee (twis) via eth0.2
 dhcpd[2909]: DHCPOFFER on 192.168.2.104 to b8:27:eb:5e:bc:ee (twis) via eth0.4
 dhcpd[2909]: DHCPOFFER on 192.168.1.108 to b8:27:eb:5e:bc:ee (twis) via eth0.3

not what I would expect! here's the switches ARP table: switch ARP table

I looks to me like the AP is forwarding DHCP broadcasts to all VLANs. If I disconnect it everything works as expected:

 dhcpd[2909]: DHCPDISCOVER from b8:27:eb:5e:bc:ee (twis) via eth0.2
 dhcpd[2909]: DHCPOFFER on 192.168.0.126 to b8:27:eb:5e:bc:ee (twis) via eth0.2

The AP has it's DHCP setting set to off and there is no menu to configure anything like DHCP snooping or multicast.

Am I missing something?

Update: The AP apparently broadcasts the DHCPREQUESTs to all VLANs he's in (according to the switch config). Changed NW2 & Guest VLANID to 93 and 94 on the AP but the behavior did not change...

  • What is the port and VLAN configuration of the AP port that connects to switch port 5? – Tero Kilkanen Aug 11 '16 at 9:06
  • @TeroKilkanen: the PVID of Port 5 is 99 and the VLANS 2,3,4 & 99 are marked Tagged on this one – user1859022 Aug 11 '16 at 9:43
  • And there are no untagged VLANs on that port? – Tero Kilkanen Aug 11 '16 at 9:47
  • @TeroKilkanen: I'll double check as soon as I get back - port 5 might be an Untagged member of 99... Could you explain why this could be a problem (since port 4 definitly is only an Untagged member of VLAN 2) – user1859022 Aug 11 '16 at 10:06
  • I'm assuming b8:27:eb:5e:bc:ee is the MAC address of the client PC's wired interface - confirm that. Does the client PC have a wireless connection also? Can you suggest why the switch has learnt that MAC on port 6 - your diagram doesn't show anything connected there? – Mintra Aug 12 '16 at 15:28
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I can't comment before I have 50 reputation unfortunately. I don't think your AP's has anything to do with your problem, because your PC is connected over Ethernet and not wireless.

From what you're saying I would suggest that you check your configuration on port 4, it should be configured as an access port and you should also assign a VLAN to it. You should also check if your DHCP server is configured to give out wrong ip addresses.

  • the thing is: if I disconnect the AP everything is fine... – user1859022 Aug 11 '16 at 9:46
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Turns out that this is a BUG in the APs firmware. The TP-Link support provided a beta version of the firmware: 1.0.0 Build 20160607 Rel. 48929 Beta that addresses this specific issue.

Anyways thank you guys for trying to figure this out!

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