[c-nsp] Redundant DHCP Server

[Mohammad Khalil]

Thanks all , clearer for me now

BR,
Mohammad

> From: ncutting at edgetg.co.uk
> To: ncutting at edgetg.co.uk; eng_mssk at hotmail.com; peter at rathlev.dk
> CC: cisco-nsp at puck.nether.net
> Subject: RE: [c-nsp] Redundant DHCP Server
> Date: Tue, 2 Jun 2015 10:06:47 +0000
> 
> That should be, that the client receives both offers.
> 
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Nick Cutting
> Sent: 02 June 2015 10:15
> To: Mohammad Khalil; Peter Rathlev
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] Redundant DHCP Server
> 
> DHCP always sends the broadcast / relay to both servers.
> 
> Whoever answers first will send back to the client, then it is up to the client to ACK the address that it wants.
> 
> IF the MLS adds clients without receiving their ACK, "yes I want that address" then that is the bad implementation of DHCP on IOS.
> I find that the DHCP conflict database gets full - when there are no conflicts - and we have to periodically remove addresses from here.
> Also reservations are a massive pain - I think DHCP on IOS is about the worst implementation there is.
> 
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Mohammad Khalil
> Sent: 02 June 2015 09:05
> To: Peter Rathlev
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] Redundant DHCP Server
> 
> Hi peter , thanks for the great reply :) Usually I rely on DHCPD package on Linux distributions to configure my DHCP server , but the issue is that my client wants his MLS to do that job 
> 
> Is there a way that I can remove the unused releases from the database like my case where I am using two hosts only ?
> Is there a way that I can make the DHCP assign addresses for the clients in a round-robin fashion?
> 
> Thanks again
> 
> BR,
> Mohammad
> 
> > Subject: Re: [c-nsp] Redundant DHCP Server
> > From: peter at rathlev.dk
> > To: eng_mssk at hotmail.com
> > CC: cisco-nsp at puck.nether.net
> > Date: Mon, 1 Jun 2015 21:07:02 +0200
> > 
> > Hi Mohammad,
> > 
> > On Mon, 2015-06-01 at 13:44 +0300, Mohammad Khalil wrote:
> > > Sorry for the bad format
> > 
> > You did an okay-ish job of making it better. :-) But you should 
> > probably consider using another mail client. Then you don't have to 
> > spend all that time making your emails readable.
> > 
> > ...
> > > Now , when the clients requested IP address
> > > PC1> show
> > > NAME  IP/MASK            GATEWAY       MAC                LPORT  [...]
> > >       192.168.10.6/24    192.168.10.3  00:50:79:66:68:01  20501  [...]
> > > 
> > > PC2> show
> > > NAME  IP/MASK            GATEWAY       MAC                LPORT  [...]
> > >       192.168.10.133/24  192.168.10.3  00:50:79:66:68:02  20502 
> > > [...]
> > > 
> > > R1#sh ip dhcp binding
> > > Bindings from all pools not associated with VRF:
> > > IP address       Client-ID/           Lease expiration       Type
> > >                  Hardware address/
> > >                  Username
> > > 192.168.10.7     0100.5079.6668.01    Mar 01 2002 12:23 AM   Automatic
> > > 192.168.10.6     0100.5079.6668.02    Mar 01 2002 12:23 AM   Automatic
> > > 
> > > R2#sh ip dhcp binding
> > > Bindings from all pools not associated with VRF:
> > > IP address       Client-ID/           Lease expiration       Type
> > >                  Hardware address/
> > >                  User name
> > > 192.168.10.133   0100.5079.6668.01    Mar 02 2002 12:18 AM   Automatic
> > > 192.168.10.132   0100.5079.6668.02    Mar 02 2002 12:18 AM   Automatic
> > >  
> > > I do not understand why the two servers assigned IP addresses?
> > 
> > This is probably "normal" for IOS. Each of the two servers offer an 
> > address to the client, but the client only actually ACKs one of these.
> > Since the client doesn't NAK the other lease (the one it didn't take) 
> > the server doesn't know for certain that the lease isn't taken. (DHCP, 
> > being a UDP based protocol, does have some weaknesses concerning 
> > packet
> > loss.)
> > 
> > I think "real" DHCP servers (no offense towards IOS) start by handing 
> > the client a shorter-than-normal lease and then hands out a 
> > full-length lease at the first "renew" request.
> > 
> > This shouldn't be a problem. If the DHCP service is supposed to be 
> > redundant you would have to have addresses enough for every device on 
> > each of the two routers anyway. Otherwise you will not have enough 
> > addresses if one of the fails.
> > 
> > More dedicated DHCP servers can coordinate their leases and avoid this 
> > 50% waste. But I don't think IOS is that advanced.
> > 
> > --
> > Peter
> > 
> > 
>  		 	   		  
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