Tuesday, May 6, 2008

Cisco Static DHCP Configuration

Assigning STATIC IP addresses on a Cisco Router via DHCP

Although it’s not something that is probably all that common, and it’s actually the first time I have had a need to use this on a router, I thought I would share this information with you anyway

NOTE: This particular config was done on a Cisco 877 ADSL / DSL router however its known to work on the Cisco 800 series routers in general including the Cisco 827 Cisco 837 Cisco 877W the Cisco 1720 Cisco 1721 Cisco 1750 series and the CIsco 1600 series


Normal DHCP config for a Cisco Router

ip dhcp excluded-address 192.168.1.1 192.168.1.10

We are excluding anything in the range between 192.168.1.1 to 1.10
In this example our servers are in this range and we don’t want any conflicts, also the router is .1

ip dhcp pool POOL-NAME
network 192.168.1.0 255.255.255.0
dns-server 192.168.1.7 192.168.1.8
default-router 192.168.1.1
lease 0 8


The pool name is exactly that a name it will mean more to you than to the router so make it something you will know.

Network indicated that this is the network we are using 192.168.1.1/24 or a class C
dns-servers well those are the DNS Server Addresses we are going to assign to the DHCP clients.
The default-router is the default gateway
The lease time is the amount of time that the client machine will keep the IP before trying to renew itself 0 days 8 hours.






Now if for some reason you need to assign a static ip to a user via DHCP its pretty simple.
There are probably a number of reasons I can think of that you would do this.
For instance, if you have certain access to things based on your IP address. For instance if IP 192.168.1.11 192.168.1.12 and 192.168.1.13 have access to browse without having to use a proxy server (important people like your boss who pays your salary and he needs to be able to surf copious amounts of porn without anyone knowing

Anyway the why is up to you the how is listed below

The first thing you want to do is find out what the MAC Address is
Now if you are already running a DHCP server on the router its pretty easy

router#sh ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.1.78 0012.7980.77b5 Aug 07 2006 08:36 PM Automatic
192.168.1.86 000e.7f32.37d7 Aug 07 2006 07:36 PM Automatic
192.168.1.23 0014.3863.4c56 Aug 07 2006 06:10 PM Automatic


Now the 3 MAC Addresses need to be reserved so that the next time those MAC addresses logon to the network they are assigned 192.168.1.11 1.12 and 1.13

So this is what you do
router#clear ip dhcp binding 192.168.1.78
router#clear ip dhcp binding 192.168.1.86
router#clear ip dhcp binding 192.168.1.23


If you want to clear the binding for all IP’s in the DHCP Pool you could just run
router#clear ip dhcp binding *

Now that the IPs are not bound

You can simple add something like this

ip dhcp pool STATIC-1
host 192.168.1.11 255.255.255.0
hardware-address 0012.7980.77b5
!
ip dhcp pool STATIC-2
host 192.168.1.12 255.255.255.0
hardware-address 000e.7f32.37d7
!
ip dhcp pool STATIC-3
host 192.168.1.13 255.255.255.0
hardware-address 0014.3863.4c56


These 3 addresses will now only be issued to the holders of the MAC Addresses

2 comments:

Stephen Reese said...

Just what I needed. Thanks.

alexey bass said...

Just what I need too, thanks!

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