[c-nsp] Redundant DHCP Server

[Peter Rathlev]

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