[ale] Cisco 2600 DHCP config question

[Paul Borghese]

Jim,

There is something called proxy arp that is enabled by default on Cisco
routers and may not show up in a traceroute.  

After a client grabs the "wrong" address, ping an IP in the testbed network.
Take a look at the arp table of the client that has pulled the wrong address
and see if the mac address is of the router or the end device.

If you have both the production network and test network going through the
same switch (or two switches that are trunked), I am willing to bet they are
on the same vlan.  So my vote is the switch is setup incorrect.  

If you are interested, send me a private e-mail with the router
configuration and I will take a look.  I am Cisco certified with CCIEs in
R&S and Security.

Take care,

Paul Borghese


-----Original Message-----
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Jim
To: ale at ale.org
Popovitch
Sent: Thursday, April 26, 2007 11:31 PM
To: Atlanta Linux Enthusiasts
Subject: Re: [ale] Cisco 2600 DHCP config question

On Thu, 2007-04-26 at 23:23 -0400, David Tomaschik wrote:
> Jim Popovitch wrote:
> > On Thu, 2007-04-26 at 22:48 -0400, Tim Meanor wrote:
> >   
> >> Yeah, the 2600 should be ignoring dhcp requests altogether.  I  
> >> admittedly haven't configured a router in years.  I do remember that  
> >> ip helper is used to direct dhcp broadcasts to a specific IP address  
> >> that is not on the local subnet.  I was thinking that maybe the 2600  
> >> had this enabled on its dev network interface.  If you don't get this  
> >> figured out, maybe you could post the config (with the names changed  
> >> to protect the innocent).
> >>     
> >
> > Will do.  It is strange, several folks with more experience are trying
> > to figure it out but coming up with blanks too.
> >
> > -Jim P. 
> >   
> I know this is unlikely, but can you be certain there is no other
> connection between the networks?  I've seen a similar problem where
> someone had put two networks on the same vlan on a switch.  I guess I'm
> asking: are you certain the 2600 is routing the packets?

No, I'm not 100% certain.  Unfortunately I don't have access to the
switch, which was setup by some well-intended folks during a hurried
weekend IP change over. ;-)

Thanks for the thought, I'll ask someone local to check into that.  What
you suggests does sound possible because once I pull a DHCP address on
to a client in the dev network, traceroutes don't show the router
between the client and hosts in the testbed network.  Hmmmm.

-Jim P. 

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