BAD IP ADDRESS

[PIKEYMIKEY]

My DHCP is getting filled with BAD IP ADDRESSES and I am not sure why.

Looking at the VLAN config on the L3 switch the interface has both DHCP helper addresses set.. could this be causing it? how to get around it?

[luis_cordova]

Hi @PIKEYMIKE,

Could you show us the configuration of the DHCP pool and the interfaces related to that pool?

Regards

[johnd2310]

Hi,

There is nothing wrong with that message. The DHCP server is telling you the BAD Address is already being used on the network.Before your DHCP server assigns an ip address, it checks(via ping) that address is not already being used. If it finds the address is assigns, it marks the address as BAD and does not assign it to a client.

I am assuming you are using two dhcp servers and have two helpers address on your router. You can stop these messages  by configuring  your DHCP servers to assign addresses for half the pool e.g. one server assigns 192.168.1.1 - 128 and the other assigns 192.168.1.129 - 254

Thanks

John

[Alan Ng'ethe]

The other thing you could do is to either shorten or lengthen the lease duration for either of the subnets, depending on the usage patterns of the clients. For example, if you have a guest Wi-Fi, it would be helpful to have the DHCP reclaim addresses as soon as possible after clients exit the network.

[muralikrishnan]

I am Also having this same problem but for only one vlan , initally ip was relesed from DHCP server and after that i have configured the dhcp server for that perticular vlan in the switch but then also i,m facing the same issue can anyone help in this case 

[Jens Albrecht]

Hi @muralikrishnan,

as soon as you configure a DHCP pool on a Cisco device it will start sending ICMP requests (ping) to check whether or not the IP addresses in the pool are already in use. Whenever the device gets an ICMP reply the corresponding IP address will be marked as BAD so that it will not be handed out to DHCP clients. This is done automatically in the background so that after some time all IP addresses that are already in use will be listed as BAD.

This can easily be avoided if you tell the DHCP server exactly what IP addresses should be given to the clients.
Example:

You configure a DHCP pool for the network 10.15.60.0/24 and only want to assign the IP addresses .50 - .199 to DHCP clients because other addresses are configured statically to some devices. Then you need to exclude all IP addresses from this pool that you do not want to hand out:

ip dhcp excluded-address 10.15.60.1 10.15.60.49
ip dhcp excluded-address 10.15.60.200 10.15.60.254
ip dhcp pool VLAN60
    network 10.15.60.0 /24
    default-router 10.15.60.1
    <dns, domain, lease time, other settings>

Please note that the exclusions are configured in global config mode and NOT inside the pool.

HTH!