Cisco Systems OL-6240-02 manual 22-20

Chapter 22 Advanced DHCP Server Properties

Setting DHCP Forwarding

As the DHCP server processes these scripts, it checks the environment dictionary for this string:

cnr-forward-dhcp-request

3.When it finds that string and it has the value true (enabled), the server calls its forwarding code.

4.The forwarding code checks the environment dictionary for a string with this key:

cnr-request-forward-address-list

It expects a list of comma-separated IP addresses with an optional colon-delimited port number, as in this example:

192.168.168.15:1025,192.168.169.20:1027

By default, the server forwards to port 67. It sends a copy of the entire client request to each IP address and port in turn. If any element in the list is invalid, the server stops trying to parse the list.

5.After the forwarding code returns, the server stops processing the request. In the post-client-lookup extension point script, however, this action might create an optional log message with client-entry details.

The following example of a portion of a TCL extension script tells the DHCP server to forward a request to another server based on the information in the request. You can use such a script if there are multiple device provisioning systems in the same environment. In this case, you would run the extension script on the DHCP server to which routers forward broadcast requests. The script would determine which (if any) other server or servers should handle the request, and tell the original server to forward the request.

The sample script uses a static mapping of MAC address prefix to send modems from a specific vendor to a specific system:

proc postPktDecode {req resp env} { set mac [$req get chaddr]

set addrs ""

;# Very simple, static classifier that forwards all requests from devices

;# with a vendor-id of 01:0c:10 to the DHCP servers at 10.1.2.3 and 10.2.2.3: switch -glob -- $mac {

01:0c:10* {

set addrs "10.1.2.3,10.2.2.3"

}

}

;# If we decide to forward the packet, the $addrs var will have the IP addresses ;# where to forward the packet:

if {$addrs != ""} {

;# Tell the DHCP server to forward the packet...

$env put cnr-forward-dhcp-request true ;# ...and where to forward it:

$env put cnr-request-forward-address-list $addrs ;# No more processing is required.

return

}

}

A more flexible script could use a per-client configuration object, such as the Network Registrar client entry, to indicate which DHCP server should get the request.

	Cisco CNS Network Registrar User’s Guide
22-20	OL-6240-02