Sunday, September 19, 2010

DHCP IP Exclusion Command


As I was reading one of the CCNA:Voice chapters I noticed that my phones were picking up the wrong VLAN IP addresses. I spent hours and hours today working the issue until I realized what it was. My DHCP router has two dhcp pools to pick from when handing out IP address, one named VOICE for my IP phones (172.16.1.0 /24) and one named DATA (172.16.2.0 /24) for PC's and everything else simply enough. I like to exclude a small range of IP's from being handing out becuase you'll see this in most real world environments. So I excluded 10 IP's on both dhcp pools, 172.16.x.1 - 10 which allows me 20 IP addresses for servers, future routers, etc. Well what I didn't pay attention too was that I accidentally excluded the range 172.16.1.1 to 172.16.2.10 instead of this:

172.16.1.1 to 172.16.1.10 (10 spare IP's on the VOICE VLAN)
172.16.2.1 to 172.16.2.10 (10 spare IP's on the DATA VLAN)

I didn't notice that this was causing the issues because it was still handing out IP's but only from 172.16.2.11 on up obviously. Once I caught the error and typed this command:

ip dhcp excluded-address 172.16.2.1 172.16.2.10
ip dhcp excluded-address 172.16.1.1 172.16.1.10


Sure enough both phones picked up the right IP addresses, well after I restarted the phones from the voice router that is.

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