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

frnkblk at iname.com frnkblk at iname.com
Thu Aug 27 23:19:13 EDT 2015

Reminds me of this recent thread:


-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
Markus Hauschild
Sent: Thursday, August 27, 2015 6:07 AM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] DHCP Relay breaks after some time on Cat 3750X


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?

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