An interesting DHCP issue with WIFI

I had been noticing that sometimes wifi clients would not connect to
some of my several wifi APs.

Especially the Iphone or Ipad...

I may have found out why.

The main router is a Draytek 2900, with wifi. That had DHCP enabled,
as one usually has.

Then I had several Linksys (etc) APs coming off the LAN. These did not
have DHCP enabled, so the DHCP was done by the router.

So far so good; worked fine.

Until you try to move a particular client from one end of the house to
the other.

The Draytek router sees the same named device (not sure what that
means...) appearing via different APs, so each time it needs a new
routing.

With the Ipad/Iphone, the problem got extra bad because they are moved
around more.

Does this make sense?

I have changed all the APs to do DHCP, and it already works better.

The WAG354G cannot do DHCP (if used as an AP) so that will go on Ebay.

Peter wrote:
> I had been noticing that sometimes wifi clients would not connect to
> some of my several wifi APs.
>
> Especially the Iphone or Ipad...
>
> I may have found out why.
>
> The main router is a Draytek 2900, with wifi. That had DHCP enabled,
> as one usually has.
>
> Then I had several Linksys (etc) APs coming off the LAN. These did not
> have DHCP enabled, so the DHCP was done by the router.
>
> So far so good; worked fine.
>
> Until you try to move a particular client from one end of the house to
> the other.
>
> The Draytek router sees the same named device (not sure what that
> means...) appearing via different APs, so each time it needs a new
> routing.
>
> With the Ipad/Iphone, the problem got extra bad because they are moved
> around more.
>
> Does this make sense?
>
> I have changed all the APs to do DHCP, and it already works better.
>
> The WAG354G cannot do DHCP (if used as an AP) so that will go on Ebay.

More likely it has to do with stale values in the ARP caches of each of
the wireless devices.

New AP sees MAC address of device, but all the ARP caches say **that**
MAC address is accessed via a different AP.

Your solution might work reliably if each DHCP server issues a unique
range of addresses within the subnet. If two DHCP servers issue
addresses from the same range you risk two different devices getting the
same IP address - a recipe for disaster.

Migh be better to use WDS between all the different APs

--
Graham J

"Graham J" wrote in message news:4ea09a83$0$2543$(E-Mail Removed)...

Peter wrote:
> I had been noticing that sometimes wifi clients would not connect to
> some of my several wifi APs.
>
> Especially the Iphone or Ipad...
>
> I may have found out why.
>
> The main router is a Draytek 2900, with wifi. That had DHCP enabled,
> as one usually has.
>
> Then I had several Linksys (etc) APs coming off the LAN. These did not
> have DHCP enabled, so the DHCP was done by the router.
>
> So far so good; worked fine.
>
> Until you try to move a particular client from one end of the house to
> the other.
>
> The Draytek router sees the same named device (not sure what that
> means...) appearing via different APs, so each time it needs a new
> routing.
>
> With the Ipad/Iphone, the problem got extra bad because they are moved
> around more.
>
> Does this make sense?
>
> I have changed all the APs to do DHCP, and it already works better.
>
> The WAG354G cannot do DHCP (if used as an AP) so that will go on Ebay.

More likely it has to do with stale values in the ARP caches of each of
the wireless devices.

New AP sees MAC address of device, but all the ARP caches say **that**
MAC address is accessed via a different AP.

Your solution might work reliably if each DHCP server issues a unique
range of addresses within the subnet. If two DHCP servers issue
addresses from the same range you risk two different devices getting the
same IP address - a recipe for disaster.

Migh be better to use WDS between all the different APs

--
Graham J
>
>
Or move to a smaller house ;-)

Graham.
%Profound_observation%

"Graham." <(E-Mail Removed)> wrote

>More likely it has to do with stale values in the ARP caches of each of
>the wireless devices.
>
>New AP sees MAC address of device, but all the ARP caches say **that**
>MAC address is accessed via a different AP.
>
>Your solution might work reliably if each DHCP server issues a unique
>range of addresses within the subnet.

The internal LAN is 192.168.5.x.

On the DHCP on the wifi APs I am issuing 192.168.y.50 onwards where y
is more than 5 and is different for each AP. Is that OK?

Or should I set each AP to issue say 192.168.10.z where z is a
different range for each AP?

>If two DHCP servers issue
>addresses from the same range you risk two different devices getting the
>same IP address - a recipe for disaster.
>
>Migh be better to use WDS between all the different APs

Peter wrote:
>
> "Graham."<(E-Mail Removed)> wrote
>
>> More likely it has to do with stale values in the ARP caches of each of
>> the wireless devices.
>>
>> New AP sees MAC address of device, but all the ARP caches say **that**
>> MAC address is accessed via a different AP.
>>
>> Your solution might work reliably if each DHCP server issues a unique
>> range of addresses within the subnet.
>
> The internal LAN is 192.168.5.x.
>
> On the DHCP on the wifi APs I am issuing 192.168.y.50 onwards where y
> is more than 5 and is different for each AP. Is that OK?
>
> Or should I set each AP to issue say 192.168.10.z where z is a
> different range for each AP?
>
>> If two DHCP servers issue
>> addresses from the same range you risk two different devices getting the
>> same IP address - a recipe for disaster.
>>
>> Migh be better to use WDS between all the different APs

That implies that each AP is routing between the 192.168.5.0/24 network
and the respective 192.168.y.0/24 network. If so, then that should
work. Presumably your intention is that machines connected to one AP
shall not see machines conencted to another AP?

If you want them all on the same network then set (for example):

ADSL router DHCP 192.168.5.0 - 192.168.5.49
AP1 DHCP 192.168.5.50 - 192.168.5.99
AP1 DHCP 192.168.5.100 - 192.168.5.149

.... etc

But I would expect all the APs to behave sensibly with their respective
DHCP services turned off.

I think you need to give us a better network diagram ...

--
Graham J

alexd <(E-Mail Removed)> wrote

>OK, so how does the Draytek *know* that you're on a different AP, and why
>would it care?

A good Q; I don't know.

There might be an issue with just the Apple devices. These have a
tendency to behave strangely. The Ipad wifi is capable of crashing the
Draytek 2900 wifi, so the Draytek needs a reboot. It also fails to
connect to a WPA network if there is a WEP network in range, for which
it has the login. None of these things have been investigated
thoroughly however - there is only so much time I want to sink into
useless IT hardware

>From what you've described, the APs are bridged wireless to
>wired so they're all in the same subnet. Is the Draytek wireless bridged
>with its LAN, or does the wireless interface on the Draytek have a different
>subnet? Are the APs really APs [bridged] or are they doing something clever
>IP-wise as well?

They should be just bridged.

Since I went back to the DHCP config in each one, it seems to work OK
for now.

On Fri, 21 Oct 2011 23:10:31 +0100, Graham J wrote:

> That implies that each AP is routing between the 192.168.5.0/24 network
> and the respective 192.168.y.0/24 network. If so, then that should
> work. Presumably your intention is that machines connected to one AP
> shall not see machines conencted to another AP?
>
> If you want them all on the same network then set (for example):
>
> ADSL router DHCP 192.168.5.0 - 192.168.5.49 AP1 DHCP 192.168.5.50 -
> 192.168.5.99
> AP1 DHCP 192.168.5.100 - 192.168.5.149
>
> ... etc
>
> But I would expect all the APs to behave sensibly with their respective
> DHCP services turned off.
>
> I think you need to give us a better network diagram ...

If he sets the netmask to /16 instead of /24, then he can have each ap on
a different x 192.168.x.0 and still all be in 192.168/16

Rgds

Denis McMahon

Denis McMahon wrote:
> On Fri, 21 Oct 2011 23:10:31 +0100, Graham J wrote:
>
>> That implies that each AP is routing between the 192.168.5.0/24 network
>> and the respective 192.168.y.0/24 network. If so, then that should
>> work. Presumably your intention is that machines connected to one AP
>> shall not see machines conencted to another AP?
>>
>> If you want them all on the same network then set (for example):
>>
>> ADSL router DHCP 192.168.5.0 - 192.168.5.49 AP1 DHCP 192.168.5.50 -
>> 192.168.5.99
>> AP1 DHCP 192.168.5.100 - 192.168.5.149
>>
>> ... etc
>>
>> But I would expect all the APs to behave sensibly with their respective
>> DHCP services turned off.
>>
>> I think you need to give us a better network diagram ...
>
> If he sets the netmask to /16 instead of /24, then he can have each ap on
> a different x 192.168.x.0 and still all be in 192.168/16

Of course you are correct, but given the difficulty that the OP seems to
have in understanding what he is doing, I thought it better not to
confuse him further.

--
Graham J