Problem with Nexus 5596 and DHCP for Cisco IP Phones (Phones can't get address but computers can)

[kasper123]

We are in the process of installing CUCM and IP Phones but we are seeing a strange problem related to DHCP on Nexus 5596.

We are trying to use DHCP relay on the Nexus 5K to a server in a different VLAN. The computers are getting IP addresses from DHCP without problem but the IP phones are not able to get an address.

Is it possible that we have hit this bug: CSCug46504 

The condition for the bug is "...the broadcast/bootp flag needs to be set in the dhcp discover from the client, which means the server would send a broadcast offer."

Not sure if this applies to DHCP requests made by the IP Phones.

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

CSCug46504

Symptom:
On a Nexus 5000 series switch a dhcp offer might not be forwarded to the client through dhcp relay when the server/client are on different vlans.

Conditions:
For this to happen the broadcast/bootp flag needs to be set in the dhcp discover from the client, which means the server would send a broadcast offer.

Workaround:
None

The version of the software on the nexus matches the one mentioned in the bug report. 

[Marvin Rhoads]

It's quite possible. I hit that bug myself with Cisco wireless APs. A packet capture of the traffic would verify the flag setting.

The currently recommended NX-OS release for Nexus 5500 series is 7.0(5)N1(1). I'd upgrade and see if it fixes the issue.

[kasper123]

Can we go straight to version 7.0(5)N1(1) from our current 6.0(2)N1(2).

Is there some upgrade you would recommend?

Regards.

[Marvin Rhoads]

Sure - you can do a direct upgrade. See the instructions in the Nexus 5500 Upgrade / Downgrade Guide.

When you have Layer 3 services enabled on a Nexus 5k, the upgrade is disruptive (i.e., there will be an outage while the device reloads).