DHCP Relay & PXE (Nexus 5500)

[lekeosi11]

I'm having trouble with DHCP relay for PXE clients on a Cisco Nexus 5596 (with Layer 3 module) running version 6.0(2)N1(1).

The DHCP relay function works for normal clients, but PXE clients fail to get a DHCP offer.

The PXE clients work fine with DHCP relay on IOS. The boot server is not on the same network as the PXE clients.

I realise that there are differences in the DHCP relay functions on IOS & NX-OS, [ http://packetlife.net/blog/2012/jun/28/dhcp-relay-nx-os/ ] but I haven't seen any links that explicitly suggest PXE will not work with NX-OS DHCP relay.

Has anyone encountered this before?

Thanks

L

[Steve Fuller]

Hi,

Are you only using a single Nexus switch or are there a pair? If they're a pair, are you using vPC and are the clients that you're trying to PXE boot attached to the Nexus switches via vPC?

I've had issues when PXE booting servers that are connected to the Nexus via vPC because the server is only PXE booting via one of its NICs, but the Nexus sees both NICs operational and sends the DHCP response to the server NIC that isn't being used for PXE boot.

Regards

[lekeosi11]

Hi Steve,

I've got a pair of Nexus 5596 using VPC.

The PXE clients are not connected to the Nexus via VPC. They only have single NICs.

Thanks

[lekeosi11]

Hmm. I wonder if it is related to this bug.

http://www.cisco.com/en/US/docs/switches/datacenter/nexus5500/sw/release/notes/Rel_6_0_2_N1_1/Nexus5500_Release_Notes_6_0_2_N1.html

Resolved Caveats in Cisco NX-OS Release 6.0(2)N1(2a)

CSCug46504

DHCP relay does not work when the Bootp flag is set (Broadcast.)

[Steve Fuller · Accepted solution]

Quite possible. I guess your setup matches the point that the PXE client and DHCP server are not on the same subnet.

When the client doesn't get the DHCP offer, do you know if the DHCP server has sent one i.e., the DHCP request has got the DHCP server, it's sent an offer, but it never makes it back to the client?

Also are you able to get a capture of the BOOTP response from the DHCP server and check if the broadcast flag is set?

Regards

[lekeosi11]

I ran a debug dhcp on the Nexus and I can see that a DHCP offer was received from the DHCP server with the Broadcast flag set.

2013 Jun 21 13:01:06.875836 dhcp_snoop: Got pkt via socket

2013 Jun 21 13:01:06.875859 dhcp_snoop: Received ip pkt 0x8193c60 of len 393

2013 Jun 21 13:01:06.875870 dhcp_snoop: ip hdr len is 20 pkt len 69

2013 Jun 21 13:01:06.875918 dhcp_snoop: iod value is 0x211

2013 Jun 21 13:01:06.875931 dhcp_snoop: (fsrv_is_shared_intf_vdc) srv_type: 2 vdc_id: 1

2013 Jun 21 13:01:06.875950 dhcp_snoop: Pkt Transaction id : 0xa788431c

2013 Jun 21 13:01:06.875962 dhcp_snoop: Got the DHCP msg type option.

2013 Jun 21 13:01:06.875972 dhcp_snoop: Val of dhcp msg type is 2

2013 Jun 21 13:01:06.875982 dhcp_snoop: DHCPOFFER msg

2013 Jun 21 13:01:06.875991 dhcp_snoop: ctx id is 0x1

2013 Jun 21 13:01:06.876016 dhcp_snoop: yiaddr: xxx.xxx.xxx.xxx

2013 Jun 21 13:01:06.876026 dhcp_snoop: broadcast flag is set

2013 Jun 21 13:01:06.876037 dhcp_snoop: Relay Option 82 is disabled

2013 Jun 21 13:01:06.876047 dhcp_snoop:  Client and Server in Same VRF

.....

2013 Jun 21 13:01:06.876500 dhcp_snoop: DHCP Broadcast flag is set. Setting NET_L2_FLOOD flag and changing destination mac to broadcast

[Steve Fuller]

So I think you answered your own question. +5 to you

Out of interest are you able to get the DHCP debug for a normal client. I would presume the DHCP offer for them does not have the broadcast flag set and that's why it works.

Regards

[lekeosi11]

Hi Steve,

I ran a debug for normal clients and confirmed that the broadcast flag was not set for them.

Time for another upgrade (or downgrade to 5.2.1)

Thanks for your help.

[Steve Fuller]

Thanks for confirming the setting of the broadcast flag on your non PXE clients.

Would you be so kind as to mark the question as answered as it helps others find solutions if they come across the same problem.

Regards