DHCP Requests over Nexus Issue

[Michael Morrison]

Hello all, this is my first post so please be gentle.  I am having an issue with DHCP requests on our new server network.

First some background:

The new network consists of 2x Nexus 5548UP runing NX-OS 6.0.  Behind the N5ks are multiple FEX types, but for the purpose of this issue I am using 2232TM-E FEXs.  Each FEX is setup in an active-active scenario, therefore each individual FEX port belongs to each N5k which are configured identically.  The N5k have a vPC peer-link setup between them and each FEX is also setup with vPC.  The N5ks are uplinked to a pair of Catalyst 4500X switches that function as our "core" switches.  From there, we have our old network which consists of a pair of Catalyst 4507R switches.  We use tansfer networks between the N5k and 4500X and between the 4500X and 4507R runing EIGRP.

I have "Server A" on VLAN 100 connected to FEX 2232TM-E... "DHCP Server B" is on VLAN 11 connected to 4507R.  I have "ip dhcp relay address x.x.x.x" configured on VLAN 100 to point to DHCP Server B on the old network.  VLAN 100 is configured with HSRP (see running-config output below).

Onto the issue:

Server A is not getting a DHCP addesses assigned to him.  The DHCP Server B gets the Discover packet from Server A and sends back the Offer... but the Offer either never makes it to Server A, or Server A Request never makes it back the DHCP Server B.  I am not sure which is the case exactly.

I know it may be hard to picture the whole scenario I described above, but any pointers would be greatly appreciated.  I'll be glad to dig up a good diagram of the network or provide more configs if necessary.

Additional info:

version 6.0(2)N1(2)

feature dhcp

service dhcp

ip dhcp relay

N5k #1:

interface Vlan100

   description server vlan

   no shutdown

   no ip redirects

   ip address 10.112.100.2/23

   hsrp 100

     authentication text hsrp-100

     preempt delay minimum 180

     priority 200

     ip 10.112.100.1

   ip dhcp relay address 172.26.20.40

N5k #2:

interface Vlan100

   description server vlan

   no shutdown

   no ip redirects

   ip address 10.112.100.3/23

   hsrp 100

     authentication text hsrp-100

     preempt delay minimum 180

     priority 150

     ip 10.112.100.1

   ip dhcp relay address 172.26.20.40

Thanks for any information!

[Michael Morrison]

Yes, it appears proxy ARP is disabled.  I issued the command "no ip proxy-arp" which did not have any affect to the interface.  Should this be enabled?

[paul driver]

Hello

Not sure the command on a NX-OS

sh ip int xx | in Proxy

int xx

ip proxy-arp

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[Michael Morrison]

ip proxy-arp enabled and no change to the behavior, still no DHCP offer received.

[paul driver]

Hello Michael,

Just to confirm where have you enabled proxy-arp?

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[Michael Morrison]

I had configured proxy-arp on "interface vlan 100".  Should I have enabled this someplace else?

[Michael Morrison]

I think we found it:

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

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.

Going to update to the latest NX-OS tomorrow morning.  Will update this post accordingly with the results.

[Michael Morrison]

Fixed!  Was a bug that seems to have been found and fixed just recently.

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

1st Found-In

6.0(2)N1(2)

Fixed-In

6.0(2)N2(1)

6.0(2)N1(2a)

Thanks for the help guys and for sticking with me through this issue!

[Richard Burts]

I am glad that you finally got it resolved. It has been a very interesting discussion. Thank you for using the rating system to mark the question as answered and thanks for the ratings. This indicates to other readers in the forum that useful information is found in this thread and that is helpful.

HTH

Rick

[Roman Polyachkov]

Hi, there!

I've got the same issue on N5K (version NX-OS - 7.0(1)N1(1) ) I've fixed it upgrading to 7.0(2)N1(1)

[horizons6]

This was a very helpful thred to read. Thank you!

TW

[Marvin Rhoads]

Addendum - we ran into the same problem with Wireless APs going via a Nexus 5k L3 core. Upgrading to a fixed NX-OS release resolved the issue.