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Tuesday, October 18, 2011

IP DHCP Help while configuring 2921

Our current configuration on a cisco 2921 (Verson 15.0(1r)M9 Release (fc1)):

ip dhcp pool Voice
network 10.21.179.0 255.255.255.128
default-router 10.21.179.1
option 150 ip 10.21.179.196 10.21.179.200

Issue is when a device looks to renew their lease no IP's are available. Debug ip dhcp events tells us the dhcp pool is exhausted. It appears that when the devices, Cisco phones in this case, goes to renew their lease, they do not renew with the current IP they already have. This causes the pool to become exhausted of ip addresses. For some reason, the router holds the IP's in reserved status and will not reassign them back out for a specific amount of time.

There are two choices to explore
  1. Decrease your dhcp timer.
  2. Increase your dhcp scope.
If you have more devices than your scope allows, you're going to see this problem over and over.
If you don't, then just decreasing the time the router keeps the mac address assigned to the ip will do the trick.

The DHCP command below is listed below - default is 24 hours or 1 day.

Router(dhcp-config)#lease

Also if you haven't got thousands of phones then assign them static bindings for their specific mac-addresses

ip dhcp pool PHONE1
host x.x.x.x
hardware-address xxxx.xxxx.xxxx

and so on for each phone keeping the parent pool with all the settings then each child pool will inherit these: default-router, tftp server,...


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