[c-nsp] DHCP Relay breaks after some time on Cat 3750X

Adam Greene maillist at webjogger.net
Thu Aug 27 09:01:24 EDT 2015

Hi Markus,

We have been doing DHCP relay on 3750's, 3560's, 3750G's and 3850's for years without any issues. The only thing that jumps out at me is that you have ipv6 and ospfv3 enabled, whereas we don't. 

Hope this feedback helps in a vague and general way ... :\


-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Markus Hauschild
Sent: Thursday, August 27, 2015 7:09 AM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] DHCP Relay breaks after some time on Cat 3750X

I forgot to attach the config file.

On Thu, Aug 27, 2015 at 1:06 PM, Markus Hauschild <hauschild.markus at gmail.com> wrote:
> Hi,
> I have a weird problem with DHCP relay on a Cat 3750X:
> Configuration is as follows (full config w/o passwords in attachment):
> Two "ip helper-address <ip>" statements (one for each server) are set 
> on each vlan interface where I want to have DHCP relay.
> After I boot the switch DHCPDISCOVER broadcasts from clients are 
> properly forwarded to both DHCP servers (example output taken from the 
> first one):
> Aug 26 17:49:57 aveta dhcpd: DHCPDISCOVER from 60:57:18:x:y:z via 
> Aug 26 17:49:58 aveta dhcpd: DHCPOFFER on to 
> 60:57:18:x:y:z ($hostname) via Aug 26 17:49:58 aveta 
> dhcpd: DHCPREQUEST for ( from 60:57:18:x:y:z 
> ($hostname) via Aug 26 17:49:58 aveta dhcpd: DHCPACK on 
> to 60:57:18:x:y:z
> ($hostname) via
> Yet after a while DHCPDISCOVER broadcasts are no longer forwarded to 
> any DHCP server, only unicast (e.g. DHCPREQUEST) still works.
> I had a similar config running for months (obviously a few times 
> port/vlan assignments changed, I migrated from OSPF+OSPFv3 to OSPFv3 
> only ...) until I had this problem occur for the first time.
> The current situation is that DHCP relay works for a certain period of 
> time (hours to days) after a cold boot until all DHCP broadcasts are 
> silently discarded again.
> I do see the broadcasts on wireshark in the same VLAN, I don't see any 
> DHCPDISCOVER on the server with tcpdump.
> I tried debug ip dhcp server packet but couldn't find anything 
> helpful. Any ideas what to further debug/look into? Anything wrong 
> with the config? Possible Hardware problem?
> Greetings,
> Markus

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