How many DHCP conflicts before switch stops trying to assign address

[Jonnyballgame34]

Have what I think is a simple question but cant' find the answer.  I've got a /25 with about 70 active dhcp clients, and about 20 rogue static IP address assignment.  Even though there are still 20+ IP's available, new devices won't pick up an DHCP address.  

Is there a standard to how many attempts a server will after running into a conflict(s)---dont' know if that makes sense, but

-first try--finds dhcp conflict, does it quit, or does it move onto the next IP it thinks should be available? 

I'm wondering if I put exclusions on the ones that are tied up with statics, then having them move those devices to dhcp, it will force them toward the DHCP addresses that are legitimately available.  Then I can go through and remove the exclusions and return them to the pool. Thanks

[Julio E. Moisa · Accepted solution]

Hi

Is very difficult to have DHCP conflicts. Once the DHCP server received a request it will try to ping the IP and get response to discard any conflict. If the DHCP is configured on a router try to use the command ip dhcp conflict resolution, to keep the server verifying. 

http://www.cisco.com/c/en/us/td/docs/ios/12_2/ipaddr/command/reference/fipras_r/1rfdhcp.pdf

If you are going to use static IPs, I recommend excluded a range of IP addresses from the DHCP server. On Cisco devices it could be done executing:

ip dhcp excluded-address <first IP to be excluded> <last IP address to be excluded>. On server you can select the first IP to be leased. 

You also should verify the leasing time, I have seen sysadmins configuring it for 24 hours.

Please rate the comment if it is useful.

:-)

[Jonnyballgame34]

Reza, Julio,

Thanks very much for taking the time to look at my question and answer, (and thanks for the command reference).

Did a poor job of explaining it. This is a remote site, and the local guy likes to go rogue, which is to say anytime a phone doesn't come up on DHCP, he just throws a static on it.  So I was guessing having all those addresses tied to statics, was causing the conflicts leading to DHCP failures---or in short, having the statics caused dhcp failures leading to him putting in more statics and causing more dhcp failures.  I went through comparing the DHCP and ARP tables and excluded all the addresses that were in use, so that it would only try to allocate form legitimately available addresses.  

Worked Ok on the first few, hoping that is a solution.  

[Reza Sharifi]

Hi Jon,

All your static IPs should be excluded from the scope.  As a rule of thump, I usually exclude the fist 5 IPs for admin use which is usually for router interfaces, SVIs, VRRP, HSRP, etc.. Also, exclude static IPs for any servers, printers and any other device that needs static address.  The rest for DHCP to assign IPs.

HTH