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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