Re: DHCP Leasing
- From: Ben Smith <online_bensmi@xxxxxxxxxxxxx>
- Date: Sun, 20 Nov 2005 18:35:37 -0800
In article <07500AAC-1D8C-4DD8-A306-0DE62086D322@xxxxxxxxxxxxx>,
fmsmcse@xxxxxxxxxxxxxxxxxxxxxxxxx says...
> Wonder the following situation, you've centralized your IP distribution in a
> main site and several sites, let's say 15 sites, all depend on the main one
> to get an IP.
>
> All clients in the 15 sites have gotten an IP early in the morning and then
> at midday the link goes down for all 15 sites to the main site and, as you
> are lucky enough, there's a power down in those 15 sites.
> By what you explained, all the clients would get an APIPA then...??
Exactly. And this is one of the exact scenarios APIPA was designed for,
because in this scenario all of the hosts would be able communicate with
each other in the APIPA address space.
>
> "Ben Smith" wrote:
>
> > In article <1FC4E5F9-6F61-48AD-BC80-1A736CDC4899@xxxxxxxxxxxxx>,
> > fmsmcse@xxxxxxxxxxxxxxxxxxxxxxxxx says...
> > > Client01 one leases an IP and so does Client02.
> > > One hour after their leasing, the DHCP server goes down.
> > > Client01 and Client02 have to reboot for any reason.
> > >
> > > After rebooting, Client01 maintains its IP address, as per the leasing
> > > expiration. Client02 releases the IP (remember they both have the same
> > > leasing period) for some reason.
> > >
> > > Both clients are Windows XP, and their behavior is expected to be the same,
> > > since they have the same TCP/IP configuration. They are in the same subnet
> > > and both can find the default gateway. No alternate IP is assigned.
> > >
> > > Any clue about why Client02 would release its IP before the leasing time
> > > expires?
> > > Remember Client01 has the same config and does not release the IP.
> >
> > This is not quite how DHCP leases work. DHCP leases are controlled by
> > the DHCP server, not the client.
> >
> > When a DHCP client reboots, after the IP stack is started, it issues a
> > DCHP Discovery packet, if the DHCP servers that receive the broadcast
> > respond with a Acknowledge packet. If the DHCP server that acknowledges
> > the client was the one that issued the address the client has cached,
> > the client will request it in the Request packet, if the lease is still
> > active, the DHCP server will respond with a Offer package for the
> > address and the lease time is reset.
> >
> > If the DHCP server is not online when the client reboots, the client
> > will use an APIPA address and continue to rebroadcast a Discovery packet
> > every 5 minutes (2000 and later).
> >
> > As for why the two XP clients are behaving differntly, my guess is that
> > the network IP properties on each client are configured differently.
> > Most likely, APIPA is turned off on Client01. Client02 is working as
> > expected.
> >
> > If you would like to learn more about this I suggest reading the RFC on
> > DHCP (while boring as heck, if there is no better source for learning
> > DHCP, which is a pretty simple protocol) and/or installing a network
> > monitor and watching DHCP traffic.
> >
>
.
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