DHCP Relay 7k - Decommissioning DC

Bobby Mazzotti · ‎02-15-2013

Trouble in the water -

We currently have our nexus 7k's to allow DHCP relay down to the distrobution network. Our windows team is decomissiong the two DC's that we use for DHCP relay on our user access data vlans. We are doing some tests to see the affects of turning these devices down.

Yesterday we turned down example DC03 for 1-2hours, and noticed no issues with DHCP release renews for the VLAN it provides for, as DC04 is the secondary DHCP helper in the same VLAN. However, we turned DC03 back on after a few hours, and shutdown DC04 today and started seeing issues in most of the VLANS that DC03 and 04 provided addressing for.

My theroy is that since we shutdown DC03 yesterday that our Nexus OS used DC04 as the primary instead of the seconday. Thus, never knew to fail back to DC03 when we shutdown DC04 today...

VLAN Config -

interface Vlan31

no ip redirects

ip address 10.X.X.3/24

ip ospf passive-interface

ip router ospf 100 area 0.0.0.0

hsrp version 2

hsrp 31

preempt delay minimum 180

priority 80

timers 1 3

ip 10.X.X.1

ip dhcp relay address 10.X.X.23 DC03

ip dhcp relay address 10.X.X.24 DC04

no shutdown

I thought that VLANS with DHCP relay would just send out a broadcast to the two DHCP servers and see who responds first.

-Bobby

mgalazka · ‎02-15-2013

Bobby,

You are correct that when you have multiple relay addresses, the request will be unicast forwarded to each of the servers by the N7K.

Can you provide a bit more information please, so as to help understand the issue a bit further?

1. What version of NX-OS are you running?

2. Are you using true dhcp snooping, or just have the nx-os feature set enabled to do dhcp relay?

3. What is the SVI you posted, is that an example of one client VLAN that experienced issues?

4. Are the only DHCP clients that experienced issues ones that have their L3 gateway on the N7K?

5. Are there clients where the dhcp relay either happens on another N7K or on an IOS device via helper-address? If so, were they impacted too?

Thanks,

Matt

Peter Koltl · ‎02-27-2013

Perform a packet capture on a problematic client.