Cisco 6500 DHCP issue

[gandio420]

guys,

I am having this strange issue with one of the 6500 switches we have. We are running DHCP with DHCP pool set. There are about 40 devices on there which have addresses from DHCP. When i plugged in an new device into that switch it did not get any IP address. First thing i checked was DHCP conflict.When i ran the DHCP conflict commmand I saw many ip addresses they were in the conflict list. I cleared one of the address and rebooted the device plugged in. Device picked up that particualar ip that I released. However when I plugged another device into that switch it did the same thing no IP.

any clue?

here is the dhcp pool

dhcp pool poolname
network 10.20.21.0 255.255.255.128
default-router 10.20.21.1
option 66 ascii "171.200.3.10"
option 253 ascii "EST5EDT"
domain-name xxx.xxx
option 252 ip 171.200.3.15 171.200.3.16
option 42 ip 171.200.3.15
lease 30

any help would be great..ofcourse i replaced the name and domian name

 

[baddos]

Do the client computers have a firewall that is blocking icmp pings? Cisco dhcp uses a ping to see is an IP is in use before assigning that address out.

 

[gandio420]

no firewall between the devices. they are not computers they are ip phones.

 

[VinceWhirlwind]

I wonder if you look at the full phone config file, if you don't see any options related to this - from my experience with phones, when you boot it up (off the network) you find it will have retained a memory of its last IP address.
Therefore, on the network, when it then sends a DHCP request, the server will try to ping that address before agreeing to assign it, and get a response to its ping, giving the server a false "conflict".

A packet capture on the phone and server ports would be useful in this instance to see the full conversation as the phone boots up.

 

[ifoam]

dhcp conflicts happen when there is another DHCP server. Is that a possibility?

Also, your subnet is pretty small: network 10.20.21.0 255.255.255.128

Do you intend for the DHCP server to only have 126 usable addresses?

 

[VinceWhirlwind]

He only has 40 phones. And from what he is saying, the entire available range is "conflicting", which is clearly the artifact of some unexpected behaviour.

 

[ifoam]

sorry missed that.

what does:

"show ip dhcp bindings" show?

There are many possibilities including a DHCP DoS attack. Make sure one of your clients isn't taking up multiple addresses.

 

[gandio420]

show ip dhcp bindings

shows me the ip addresses and devices that are connected no dos attacks type of stuff