We have an occasional user's PC that is getting an IP from our DHCP server in the wrong subnet. The PC is in VLAN 2 with IP subnet range 172.16.2.0/24, and the DHCP server (172.16.2.10) is in the same VLAN. We also have a requirement for an application to forward broadcasts from 172.16.2.0/24 to 172.16.1.255 (some UDP ports above 11000 and configured with the "ip forward-protocol udp 11005" and "ip forward-protocol udp 11006"). For VLAN 2, we have "ip helper-address 172.16.1.255" configured. We have 8 other VLAN interfaces configured to forward DHCP requests to 172.16.2.10 via "ip helper-address 172.16.2.10".
The interesting part is that the occasional user's PC (in VLAN 2) will get an IP address for VLAN 1. It appears that the "ip helper-address 172.16.1.255" is causing DHCP requests in VLAN 2 to appear to be coming from VLAN 1 when the hosts get the incorrect IP. If I disable the "ip helper-address 172.16.255), the other application breaks. I do see the expected DHCP server in the "ip config /all" and the host appears to be in an island and cannot communicate with any other host. There is no loop in the topology. I need the DHCP for the ip helper commands in the configuration for the other VLANs.
The DHCP server is Win2008 R2, and the switch is a Catalyst is as follows:
Cisco IOS Software, Catalyst 4000 L3 Switch Software (cat4000-I5K91S-M), Version 12.2(25)EWA5, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by Cisco Systems, Inc.
Compiled Thu 17-Nov-05 15:53 by alnguyen
Image text-base: 0x10000000, data-base: 0x115F1BE0
ROM: 12.1(12r)EW
Dagobah Revision 55, Swamp Revision 16
Core_Switch uptime is 1 year, 47 weeks, 6 days, 6 hours, 30 minutes
System returned to ROM by power-on
System restarted at 16:12:08 UTC Mon Sep 14 2009
System image file is "bootflash:cat4000-i5k91s-mz.122-25.EWA5.bin"
Does anyone have a suggestion as to what I might try? Thanks - Allyn
