DHCP server offering an IP addresses of a wrong subnetwork

DHCP server offering an IP addresses of a wrong subnetwork

Post by Gualber » Thu, 19 Jun 2003 05:34:00

Hi all!  Please could someone point me in the right direction...
I have a Cisco router with ip helper-address configured that is
routing between three VLANs (Workstations: VLANs A and B, servers:
VLAN C).  I have two DHCP servers VLAN C, (dhcpd 2.0p15) providing IP
addresses to both workstation VLANs.  Right now I have one DHCP server
providing addresses for VLAN A and the other one for VLAN B.  I need
to have redundancy in my design so I decided to devide the address in
two parts the space of each subnetwork (they are class C), so that one
DHCP server would take care of the lower half of both subnetworks (A
and B) and the other one would take care of the higher half (half of a
class C is enough to provide access to all the workstations on a
VLAN).  I followed some sample configurations and it seemed that
everything was working fine, but I found some problems.  Whenever a
workstation sends a DISCOVER packet requesting for an IP address that
is not in the scope of the servers (say a laptop that was previously
connected to a totally different network), the DHCP servers will
"OFFER" an IP address belonging to VLAN A address space, even if the
laptop is connected to a VLAN B port!!!  In this case, the router is
not forwarding this OFFER packet back to the laptop (I guess its
noticing that theres "something wrong" and "it knows" that address
wont work).  So the laptop never gets an IP address and times out.
Since there is no overlapping in the address spaces that both DHCP
servers handle in this new configuration, I have the same problem if a
laptop connects to VLAN A and then tries to connect to VLAN B).

My question is: why doesnt the DHCP server know that the request is
comming from VLAN B?  Shouldnt it "know" that the request came from
VLAN B, since it knows the IP address of the router that is proxying
the request to it (its part of the information included in the
protocol). Is there a way to force the DHCP server to look at the IP
address of the router that is sending the request to it to determine
from which range it shoult pick up an address?

For workstations this isnt usually a problem, since they are not
moving around and they "remember" the IP address they had before
rebooting and the DHCP servers offer them an address of the right

I had to roll back to my previous configuration.

There have got to be people out there moving happily around between
VLANs and getting their IP addresses automatically having redundant
DHCP servers (which makes them even happier)!!!

Any recommendation?

Thanks in advance!



1. 3com 905b gets Ip address by using DHCP from NT DHCP server


I have a 3com 905b, which FreeBSD make this device as xl0. I tried get the
IP address from a DHCP server on WinNT4 server(build 1381, sp4).  I can get
the IP address when booting FreeBSD up. But when expire inteval coming up,
FreeBSD cannot get IP from  NT4 again. I tried to ping each other, and ping
result is negative. Could some one give me a suggestion to deal with this
situation. Thank you

PS: WinNT 4 is working functionally and properly.


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