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why occurs DHCP errors in a CIsco switch

Posted on 2010-11-24
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Last Modified: 2013-11-22
Hi everybody.

Its possible that we can have 2 DHCP Servers in a single network? how is the method to avoid conflicts? when the leased time expires could be any kind of problem? how can I avoid it?

thanks
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Question by:pdiazq
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by:moon_blue69
ID: 34209326
What devices you use for DHCP?
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jfrizzell earned 336 total points
ID: 34210520
Having two DHCP servers has no adverse affects on the network.  What happens is that when a client first comes online, it sends out a discover message looking for a DHCP server.  One, if not both will receive the Discover message and respond with an Offer message.  The first offer message to be received is usually accepted by the client.  The client will then send a Request message telling the first responding DHCP server that it accepts the offer and it also lets the other DHCP server know that its offer will not be used.  At this point, the final message seen would be the Acknowledgment message from the DHCP server to the client.

After about 50 percent of the lease time, the PC will send a renewal message to the DHCP server to extend the lease again.  If no response from the DHCP server, then the PC will send yet another message at about 40 percent of the time remaining on the lease.  At some point, if no response is acknowledge, the client will initiate the DHCP process over again and the first to respond wins the lease.  If the client initiates the lease again, obviously the first DHCP server has failed and the second one will assign a new address.
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by:pdiazq
ID: 34212557
Hi. The devices we are using as DHCP are cisco switches 6500. They are sending a conflict message:

Sep 24 08:38:28.249 COL: %DHCPD-4-PING_CONFLICT: DHCP address conflict:  
server pinged 172.22.3.125.

This message had appeared with a lot of addresses..
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by:jfrizzell
jfrizzell earned 336 total points
ID: 34212889
%DHCPD-4-PING_CONFLICT means an address in the pool is being used somewhere else.  This typically occurs when you do not include an excluded range to prevent the allocation of statically configured addresses.  On the switches, did you configure an excluded range?  You can try issuing the command clear ip dhcp conflict command.  Maybe one of your internal users has configured a static address to their PC.

If you could, paste your DHCP configurations and try the command clear ip dhcp conflict.
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by:ampranti
ampranti earned 164 total points
ID: 34217821
You can do it but is not recommended!
Why?
When client request an IP, will get one answer from the fatser DHCP.
If both DHCP servers provide IP addresses from the same range, many conflict will occur. Example:

2 dhcp servers, both serving 172.22.3.125.0/24 , exclusions for both: 172.22.3.125.1-10
PC1 ask for an IP << DHCP1 provide 172.22.3.125.11 (1st available)
PC2 ask for an IP << DHCP1 provide 172.22.3.125.12 (1st available now)
PC3 ask for an IP << DHCP2 provide 172.22.3.125.11 (1st available) --> CONFLICT!!!!

To solve this problem you can (and provide high availability DHCP service):

- DHCP server different ranges (first DHCP server provide 172.22.3.125.10-127 , second DHCP server 172.22.3.125.128-254)
or
- configure two DHCP servers, using the same storage for saving data
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Expert Comment

by:mahrens007
ID: 34223942
can I ask why you have two DHCP servers for the same subnet?
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Author Comment

by:pdiazq
ID: 34243078
we configured two DHCP to have redundancy..thanks
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Author Comment

by:pdiazq
ID: 34243247
If we have two DHCP servers, how the servers knows which IP was assigned by the other server to avoid these conflicts?

thanks

regards
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Expert Comment

by:mahrens007
ID: 34243349
how many nodes are we talking about?  could you have two different DHCP pools.  For example

Pool 1: .100-.150
Pool 2: .151-.200
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by:ampranti
ID: 34246046
pdiazq:You have to split the the ranges that re being assigned by the servers.
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