Use of BROADCAST (B) flag in dhcp...

teamdhcp — Wed, 08 Nov 2006 18:56:00 GMT

DHCP server or relay agent sending or relaying a DHCP message directly to a DHCP client should examine the BROADCAST bit in the ’flags’ field. If this bit is set to 1, the DHCP message SHOULD be sent as an IP broadcast using an IP broadcast address as the IP destination address and the link-layer broadcast address as the link-layer destination address. If the BROADCAST bit is cleared to 0, the message SHOULD be sent as an IP unicast to the IP address specified in the ’yiaddr’ field and the link-layer address specified in the ’chaddr’ field.

By default, Windows Vista will start with BROADCAST(B) flag set to 1 which means DHCP Server or relay agent has to send reply to broadcast address. User can change this setting by defining REG_DWORD value DhcpConnForceBroadcastFlag under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID} registry key.

As a new enhancement, DHCP client in Windows Vista has incorporated intelligence to toggle between the two values of the broadcast flag to interop with routers that do not support the broadcast flag. Once the client is able to acquire an IP address from the DHCP server, it retains the state of flag throughout the lease time.

The toggle behavior will be controlled by a registry value “DhcpConnEnableBcastFlagToggle” under HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID}

In case the registry value is set to 1, the DHCP client will do the toggle and only one DORA sequence will be done during the acquisition phase.

The DHCP client will start afresh and send the broadcast flag in the DHCP DISCOVER packet with a value as controlled by the registry key. If it fails to get a response from the DHCP server within the allotted 1 min and the toggle flag is not disabled through registry, it will restart the DISCOVER process, this time toggling the broadcast bit sent in the packet. If the server is reachable this time, it will update the interface specific value of the flag in its cache to 1. This will be used to determine whether or not the flag needs to be set in every subsequent DHCP packet till the next DORA cycle. In the next DORA cycle, the value of the broadcast bit will again be determined by the registry value configured by the admin (or 0 by default).

Update (November 05, 2008)

If you run into issues with your Vista installation being unable to get an IP Address via DHCP, please send us the following information for further investigation:

Router Vendor, Router/Gateway Model Number, Firmware Version, Your Location (Country & State), ISP. You may send this information via email to msnetworkteam_AT_live_DOT_com.

Thanks,

Ajay

Team Networking

Microsoft Windows DHCP Team Blog : broadcast bit

Use of BROADCAST (B) flag in dhcp...