Re: Question about DHCP and Ping Packets

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
Blogs and organic groups at http://www.ccie.net