Solved: Cisco dhcp server question

glebpe185 · ‎08-05-2013

Hi guys,

Just a quick one please.

I have a Cisco 887 to be replaced.

On the old one there is DHCP pool configured and on the new one I am going to copy that Pool configuration as is.

If I swap the routers will the SHCP server on the new router check if the IP address already exist in the network before it offers/ack current DHCP lease from a host? Just wanted to make sure there won't be any dunlicated ip issues ...

Thanks a lot!

Gleb.

cadet alain · ‎08-06-2013

Hi,

if you leave dhcp conflict logging which is the default with an empty binding table then the DHCP server will try to allocate IPs from the beginning of the scope and it will ping to verify the IP it wants to offer is not already used.

If this is the case then it will put this IP in its conflict database and won't reoffer it to anyone including the primary owner and when the host tries to renew this IP it won't be able to and so it will have to rebind and it will get the first available IP in the pool.Now with conflict logging disabled, the DHCP server still pings before allocating an address so you won't allocate this IP to another host which would lead to duplicate adresses but this IP will not be made unavailable and another host may have it later if the ping test receives no reply.

So disable conflict loging with the command no ip dhcp conflict logging and you should be good to go.

Regards

Alain

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View solution in original post

cadet alain · ‎08-05-2013

Hi,

The database should have been saved on an external server on old DHCP server so that new one can use it too and in this case you could have migrated without anything else to do.

Now if this is not the case you can disable conflict logging on new server and it shouldn't complain about duplicate addresses but the oldest leases won't be in the binding table so they'll probably change IP after lease is expired.

Regards

Alain

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glebpe185 · ‎08-06-2013

Hi Thanks.

Yep the new router's bionding table is empty right now and some pc owns the first ip address in the pool right now. Just wanted to make sure the new router will not assign this ip address to some pc wih expired lease or just a new one. As far as I understood dhcp uses r-arp and icmp to check is has the ipaddress in the netwoek before it offres it to a pc.

cadet alain · ‎08-06-2013

Hi,

if you leave dhcp conflict logging which is the default with an empty binding table then the DHCP server will try to allocate IPs from the beginning of the scope and it will ping to verify the IP it wants to offer is not already used.

If this is the case then it will put this IP in its conflict database and won't reoffer it to anyone including the primary owner and when the host tries to renew this IP it won't be able to and so it will have to rebind and it will get the first available IP in the pool.Now with conflict logging disabled, the DHCP server still pings before allocating an address so you won't allocate this IP to another host which would lead to duplicate adresses but this IP will not be made unavailable and another host may have it later if the ping test receives no reply.

So disable conflict loging with the command no ip dhcp conflict logging and you should be good to go.

Regards

Alain

Don't forget to rate helpful posts.

glebpe185 · ‎08-06-2013

Thanks, it makes sense