DHCP Relay - PXE - Nexus 7000

[yuribank415]

Recently deployed Nexus 7010 in the core of our network.

DHCP Relay is configured on all SVIs, but certain hosts cannot PXE Boot or obtain DHCP leases.

Various servers set the "broadcast flag" in the DHCP header when they send DHCP-Discover packets during PXE. When the DHCP server sends back a DHCP-Offer (with the Broadcast flag set) , the Nexus 7000 fails to forward the packet out the SVI. However, if the DHCP Client were to clear the Broadcast flag (Unicast) in its DHCP-Discover packet, the subsequent DHCP-Offer will also have the Broadcast flag cleared (Unicast), and will be sucessfully forwarded out the SVI.

This seems to be the exact issue described here:

https://supportforums.cisco.com/thread/2224341
&

https://lists.isc.org/pipermail/dhcp-hackers/2012-April/001977.html    (This user patched Linux ISC DHCPD to always clear the Broadcast flag)

CSCug46504 illustrates the problem I am seeing. Apaprently it was fixed for Nexus 5000. I do not see any acknowledgement of this being an issue on the Nexus 7000.

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCug46504

Notes:

Nexus 7010

NX-OS 6.2(2a)

[yuribank415]

After doing some packet captures & debugs I believe I've isolated this issue to be a side effected of FabricPath FTAG selection & the DHCP Relay implementation in nx-os.

CORE1 (root for FTAG1) was sending the broadcast dhcp offer via the path for FTAG2 (CORE2 being root for FTAG2). CORE2 would receive the packet but not forward it along the multi destination tree ( due to DHCP Relay being configured). This is why DHCP in Unicast mode didn't have any issues, since DHCP Relay would ignore the Unicast packet being sent to the dhcp client.

https://tools.cisco.com/bugsearch/bug/CSCug57001

Swapping the root switches for ftag1 & ftag2 solved the issue. However DHCP Relay should be fixed so as to operate correctly no matter which FTAG is used for the broadcast pacet.

EDIT:

BUG-ID for DHCP-Relay issue dropping the broadcast packet (dhcp offer)

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCuj22757