DHCP Leasing issue in Catalyst 4500 switch

[vincent david]

Dear All,

Have configured DHCP in Cisco Catalyst Switch 4500 for wireless users and IP leasing is working good. However, recent days i've noticed that once all the IP's got assigned and users moved to different network/disconnected from wireless network, the IP's are still bonded in the DHCP binding table.

Once the lease period of 9 hours expires, it is releasing the IP from DHCP pool. But that is not how it should work, it should ve checked the lease during 50% of lease time and if its not reachable, it should release the bindings.

Please let me know your findings,

Currenct Configuration and Logs:

*************************************

ip dhcp excluded-address 10.64.237.1 10.64.237.20

ip dhcp pool vlan103
utilization mark high 80 log
network 10.64.236.0 255.255.254.0
default-router 10.64.237.1
dns-server 10.64.239.32
lease 0 9

Pool vlan103 :
Utilization mark (high/low) : 80 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 510
Leased addresses : 490
Excluded addresses : 20
Pending event : none
1 subnet is currently in the pool :
Current index IP address range Leased/Excluded/Total
0.0.0.0 10.64.236.1 - 10.64.237.254 490 / 20 / 510

%DHCPD-6-HIGH_UTIL: Pool "vlan103" is in high utilization state (490 addresses used out of 510). Threshold set at 80%.

[Reza Sharifi]

Hi,

Can you try testing it with a shorter lease time (maybe 5 or 6 hours)?

HTH

[vincent david]

Dear Reza,

Thanks for your reply. Shorter Lease time would work obviously. 

However, my concern is about why the IPs which are not assigned and not used is not getting cleaned up before it reaches the Lease time.

[andrewswanson]

Hi

Not much you can do if the DHCP client doesn't send a release. Most wireless clients just disappear from the network. Most DHCP clients will send a renewal half way through the lease but if they don't the lease is left to expire on the DHCP server:

https://technet.microsoft.com/en-us/library/cc958919.aspx

I use 1 hour lease time on busy wireless networks with no impact to users.

hth
Andy

[vincent david]

Thanks Andy,

The same article they have mentioned the below clause which says "address becomes available for reassignment"

But that is not happening in my scenario. Manually clearing  the stale entries is pain.

"clear ip dhcp binding"

"If a computer stops using its assigned IP address (for example, if a computer is moved to another network segment or is removed), the lease expires and the address becomes available for reassignment."

[andrewswanson]

Hi Vincent - is the command in the thread below available on the 4500?

hth

Andy

https://supportforums.cisco.com/discussion/11145176/dhcp-how-use-ip-dhcp-binding-cleanup-interval-command

[vincent david]

Thanks for the commands Andy.

Have configured it.. Lets see how it works ... Fingers crossed ..

[vincent david]

Hi Andy,

ip dhcp binding cleanup interval (x)

Have tested the above command and it works like this,

it waits (x) time interval to removes the Bindings after the lease time expires. However, in the article which you mentioned says

"This would really depend on whether the PC releases the IP before getting disconnected.If the DHCP release message is sent the binding should be removed from the table allowing another device to use the IP. Otherwise until the lease time expires the binding is present."

Hence, it is not possible to release the unused IP bindings before the lease time. 

[andrewswanson]

Yes, that's the case. It would be interesting if Cisco could do something with IP Device Tracking to release DHCP bindings for client's that are no longer connected. Maybe one day!

Cheers

Andy