Re: Question about DHCP and Ping Packets

I believe the correct answer is no and this would be in the RFC for
DHCP. Due to the entire process and the reasoning behind the DHCP
Client's method of renewing its lease. Remember it does not wait it
expire the lease. It will constantly seek to renew the same ip address
for resiliency purposes at set intervals during the life of the lease.
Imagine if it did not work like this and you were downloading
something and suddenly you received a new IP in the middlle of the
download. That is bad. The only way to not let a client get the IP
that it has been leased would be to clear the database or bindings.
However, if you do this, then you will have more problems since the IP
is now available for lease and it may still exist on the client.

On Sat, Oct 17, 2009 at 3:31 PM, Nanda, Manoj (DS-1)
<Manoj.Nanda_at_drs-defensesolutions.com> wrote:
> Any one know if there Is there is a way NOT to let the client renew the leased
> IP addess after the expiry of the initial lease period !
>
> ________________________________
>
> From: nobody_at_groupstudy.com on behalf of Nataliya Samoylenko
> Sent: Sat 10/17/2009 3:53 AM
> To: ALL From_NJ
> Cc: ccielab_at_groupstudy.com
> Subject: Re: Question about DHCP and Ping Packets
>
>
>
> Thank you Andrew
>
> ---
> Nataliya Samoylenko
>
>
> On Oct 16, 2009, at 5:48 PM, ALL From_NJ wrote:
>
>> Yes, it sends ping out first to see if the IP address is already
>> taken. What if an admin statically assigned an IP and did not put
>> an excluded address?
>>
>> Kind of makes sense.
>>
>> Why doesn't it just use ARP? Humm ... good question. Although, I
>> would think this would be more complicated since the HSRP process
>> would have to monitor the arp table for a response and not simply a
>> response.
>>
>> In my limited knowledge, ping sounds easier.
>>
>> Do not turn off the ping feature ... some site admins are clueless
>> and will blame you for DHCP problems when they messed up. They may
>> not have access to the routers, or even understand what you have
>> configured and so they might mis-configure a second windows DHCP
>> scope, statically assign something, etc ...
>>
>> I have also increased the number of pings to 4 in the installations
>> I did. I thought 2 pings was too little ... also, it is not
>> uncommon to lose one ping due to timeout. I 'feel' more comfy with
>> 4 pings ...
>>
>> HTH,
>>
>> Andrew
>>
>>
>>
>> On Fri, Oct 16, 2009 at 1:58 AM, Nataliya Samoylenko
>> <nataliya.samoylenko_at_gmail.com> wrote:
>> Hi,
>>
>> I have some problems with understanding what Cisco means when said:
>>
>> ==========
>> Configuring the Number of Ping Packets
>> By default, the DHCP Server pings a pool address twice before
>> assigning a particular address to a requesting client. If the ping
>> is unanswered, the DHCP Server assumes (with a high probability)
>> that the address is not in use and assigns the address to the
>> requesting client. To change the number of ping packets the DHCP
>> Server should send to the pool address before assigning the
>> address, use the following command in global configuration mode:
>>
>> Router(config)# ip dhcp ping packets number
>> ==========
>>
>> How can the router send icmp packet if he doesn't know host MAC
>> address?
>> So, the router just will send ARP requests?
>> When the router receive ARP replay he already knows, that a
>> particular address is in use. For knowing that he don't need to
>> send ICMP.
>>
>> Or, maybe, this for situation when client try to release address.
>> Then the router knows the MAC address and can try to send icmp
>> request.
>> But, guide states "assigning a particular address", not reassigning.
>>
>> Why don't just say ARP?
>>
>> And, as I understand, this also means that I should permit icmp
>> requests from and replay to my DHCP server (to router or switch).
>> Or, if I don't, the clients will loose their addresses.
>> (I know that I can disable this feature ( ip dhcp ping packets
>> number 0))
>>
>> Thanks
>>
>> ---
>> Nataliya Samoylenko
>>
>>
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>>
>>
>>
>>
>>
>>
>> --
>> Andrew Lee Lissitz
>> all.from.nj_at_gmail.com
>
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Received on Sun Oct 18 2009 - 02:21:12 ART