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We've been working on a wireless DHCP oddity and thought I'd mention it to the group and see if others may have seen such behavior.  I'm working with our Apple contacts to investigate further.

The symptom is a MacBook Air (OSX 10.9) , awakening from sleep, using it's 802.11 interface and taking a 'long' time to acquire an address via DHCP.  The client may then show the infamous "!" over the wifi icon.
We found that the first few DHCP responses seem to be lost (by either wireless or the client) and after ~20 seconds the next OFFER is finally seen & used.

When awake, a manual renewal always works fine.
When wired, the new MacBook does not exhibit the problem.
New iPads nor older MacBooks do not exhibit the problem.

It seems that when new MacBook Air lives on the same subnet with a Windows DHCP server, the hosts appear to ignore/drop broadcasted DHCP OFFERS initially.  When located on the same subnet, Windows DHCP server will L2 broadcast the OFFERs.  

Using a DHCP server that L2 unicasts while on the same subnet, or moving the server to a different subnet and relaying DHCP, the client always sees the first OFFER. 

The wireless environment is Cisco WLC7.5, 3602i, in FlexC mode (DHCP_required on/of makes no difference).
The Windows DHCP server is Windows Server 2008 R2 SP1.

Given bcast bit = 0 and giaddr and ciaddr are 0.0.0.0, it seems the server should be L2 unicasting OFFERS anyhow.
RFC2131:
 If the broadcast bit is not set and 'giaddr' is zero and
   'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK
   messages to the client's hardware address and 'yiaddr' address.


--
Garry Peirce
Network Architect
Networkmaine, University of Maine System
1-207-561-3539
********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/.

Comments

We've been working on a wireless DHCP oddity and thought I'd mention it to the group and see if others may have seen such behavior.  I'm working with our Apple contacts to investigate further.

The symptom is a MacBook Air (OSX 10.9) , awakening from sleep, using it's 802.11 interface and taking a 'long' time to acquire an address via DHCP.  The client may then show the infamous "!" over the wifi icon.
We found that the first few DHCP responses seem to be lost (by either wireless or the client) and after ~20 seconds the next OFFER is finally seen & used.

When awake, a manual renewal always works fine.
When wired, the new MacBook does not exhibit the problem.
New iPads nor older MacBooks do not exhibit the problem.

It seems that when new MacBook Air lives on the same subnet with a Windows DHCP server, the hosts appear to ignore/drop broadcasted DHCP OFFERS initially.  When located on the same subnet, Windows DHCP server will L2 broadcast the OFFERs.  

Using a DHCP server that L2 unicasts while on the same subnet, or moving the server to a different subnet and relaying DHCP, the client always sees the first OFFER. 

The wireless environment is Cisco WLC7.5, 3602i, in FlexC mode (DHCP_required on/of makes no difference).
The Windows DHCP server is Windows Server 2008 R2 SP1.

Given bcast bit = 0 and giaddr and ciaddr are 0.0.0.0, it seems the server should be L2 unicasting OFFERS anyhow.
RFC2131:
 If the broadcast bit is not set and 'giaddr' is zero and
   'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK
   messages to the client's hardware address and 'yiaddr' address.


--
Garry Peirce
Network Architect
Networkmaine, University of Maine System
1-207-561-3539
********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/.

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