WiFi AP Broadcasts DHCP Requests to all VLANs

Question

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:

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 09:06
@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 09:43
@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
@Mintra yes `b8:27:eb:5e:bc:ee` is the mac adress of the client PC; The switch learned the MAC on port 6 because in my desperation I tried to change the port - 4 & 6 are configured identically; Client PC does not have a wireless interface at all and was never connected to the AP directly. — user1859022, Aug 13 '16 at 16:26

score 0 · Answer 1 · answered Aug 11 '16 at 09:14

0

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.

answered Aug 11 '16 at 09:14

Bungicasse

145
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1
10

the thing is: if I disconnect the AP everything is fine... – user1859022 Aug 11 '16 at 09:46

score 0 · Accepted Answer · answered Aug 16 '16 at 18:41

0

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!

answered Aug 16 '16 at 18:41

user1859022

101
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WiFi AP Broadcasts DHCP Requests to all VLANs

2 Answers2