Ethernet Switches & Routers

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Occasional Contributor
Posts: 6
Registered: ‎07-28-2011

DHCP relay with private IP address

Hi,

We have a Brocade FCX switch/router and are setting an ip helper-address on a ve interface to use our DHCP server on another VLAN than the DHCP request comes from. This works as expected when the DHCP server replies with an IP for the client other than 192.168.X.X. As soon as the DHCP server wants to assign the asking DHCP client an IP from the 192.168.X.X network the reply is not forwarded by the FCX to the client (I see the answer going out of the server with tcpdump). What can we do to get this working?

Thanks a lot for your help!

Regards,

Tobias

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: DHCP relay with private IP address

Hi,

     What versions of code are you runing please (do a 'show version').

     If the subnet if anthing else apart fromt he 192.168.x.x e.g. 172.16.x.x then you say that works?

     Do you have anything else setup on the switch that is also using the 192.168.x.x range?

Thanks

Michael.

Occasional Contributor
Posts: 6
Registered: ‎07-28-2011

Re: DHCP relay with private IP address

Hi,

Here is the 'sh ver':

Copyright (c) 1996-2010 Brocade Communications Systems, Inc.
    UNIT 1: compiled on Feb 16 2011 at 05:39:19 labeled as FCXR07202a
                (6007397 bytes) from Primary fcxr07202a.bin
        SW: Version 07.2.02aT7f3
  Boot-Monitor Image size = 369292, Version:07.1.00T7f5 (grz07100)
  HW: Stackable FCX624
==========================================================================
UNIT 1: SL 1: FCX-24G 24-port Management Module
         Serial  #: BGF2231F06K
         License: FCX_FULL_ROUTER_SOFT_PACKAGE   (LID: dihHHIGhFLm)
         P-ENGINE  0: type DB10, rev 01
==========================================================================
UNIT 1: SL 2: FCX-4XG 4-port 10G Module (4-SFP+)
==========================================================================
  800 MHz Power PC processor 8544E (version 33/0022) 400 MHz bus
65536 KB flash memory
  512 MB DRAM
STACKID 1  system uptime is 144 days 20 hours 6 minutes 25 seconds
The system : started=cold start

We are normally using a public IP space and are distributing them through DHCP. But for some devices there are static entries in the DHCP server for distributing IPs from the private space (We are mixing several subnets in the same VLAN).

On the switch is an IP like 192.168.X.Y/24 configured, but it's not in the same subnet like the private IPs we'd like to distribute over DHCP (e.g. 192.168.a.b/24).

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: DHCP relay with private IP address

Hi,

     Ok in truth that is something that I have never tested, I assign a vlan per subnet when setting up DHCP helper.   Anyone else try this?

Thanks

Michael.

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: DHCP relay with private IP address

Hi

     With a bit more thought I do not think having a VLAN with multipul ip subnets will work.

IP Helper transforms the DHCP broadcast into a unicast request. Since the unicast address has the router IP address associated with the VE as its source, the DHCP server can take that address and use it to determine the proper scope for the lease but this would work for only one subnet per vlan.

Thanks

Michael.

Occasional Contributor
Posts: 6
Registered: ‎07-28-2011

Re: DHCP relay with private IP address

Hi

I'm pretty shure it should work, because you can add permanent leases to the dhcp server, even if it's on a different subnet.

And we had it working with a freebsd and linux router/dhcrelay. So there must be something on the brocade side which is different than on a linux/freebsd router... It stopped working as soon as we migrated this vlan to the brocade router.

Tobias

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