Restricting ONT's Ethernet port to singe active IP address

Frank Bulk fbulk at
Wed Apr 22 21:05:31 UTC 2009


This is great real world information, thanks.  

How do you go about releasing an IP address if a customer had two active
leases for old equipment and you need to get a third going?  With a cable
modem we reboot it, with DSL it doesn't really happen with PPPoA/E.


-----Original Message-----
From: Jerimiah Cole [mailto:jcole at] 
Sent: Wednesday, April 22, 2009 3:49 PM
To: frnkblk at
Cc: Users of ISC DHCP
Subject: Re: Restricting ONT's Ethernet port to singe active IP address

Frank Bulk wrote:
> I don't think "mac limit" is an ISC dhcp directive, sounds more like
> something from an access vendor.  Unfortunately our access vendor doesn't
> have a MAC address limit we can configure, but the software/hardware
> currently supports 16.

You're right.  I thought this came from a different mailing list :)

The config I pasted works on all kinds of gear though.  We do the same 
thing on ATM based DSL by spawning with the PVC information included in 
the Option 82 data:

class "qwest-dsl" {
   match if binary-to-ascii(10,8,".",substring(option 
agent.remote-id,4,4)) = "<";
   spawn with option agent.remote-id;
   lease limit 2;

> It sounds like you have some practical experience -- does 2 leases do the
> trick, and how often do you have someone call in to say they can't get an
> (because they're playing musical chairs with their broadband routers)?

It's been a few years since I worked the support desk, but as I recall, 
it was quite rare.  It was rarer still that somebody was actually trying 
to use more than two.

> Have
> you measured how many of your customers have two IP addresses?

Right now on one server, 3 out of 1889 active leases.

> Ideally if the second lease was issued the DHCP server would flush the
> first, but that doesn't help because the client still thinks it's a valid
> lease and would try to communicate with that IP.

Ideally we could uniquely identify clients based on things other than 
'client-id' or 'hardware'.  Thats a different conversation though :)

In my experience, 'lease limit 1' created too many support calls. 
'lease limit 2' is good enough.  Most people who have more than one 
device also have more than two.


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