I had some issues while configuring some VRF’s on a Cisco router and using that router as a DHCP server. First of all the router wasn’t binding any DHCP request. The DHCP server configuration is defined below.
ip dhcp pool guest
network 10.10.0.0 255.255.252.0
dns-server 184.108.40.206 220.127.116.11
The configuration of the DHCP server is very straightforward. Exception is the use of the VRF interface to bind the DHCP server to. With this configuration the DHCP server isn’t working, because no IP addresses are bind to clients.
The second issue is about configuring some IP address exclusions for the configured pool. This can be done via the command ip dhcp excluded-address vrf <vrf-name> <first ip-address> <last ip-address> (info). Depending on the IOS version used, this command isn’t available in CLI. I had this issue with the CIsco 2811 I was using, so I tried to ip dhcp class command. I added the following to the configuration of the DHCP server.
ip dhcp class dhcp_class_unsecure
remark limit IP addresses
ip dhcp pool unsecure
network 172.16.252.0 255.255.252.0
dns-server 18.104.22.168 22.214.171.124
address range 172.16.253.1 172.16.253.255
This isn’t exactly the same as configuring IP exclusions, because the ip dhcp class command is used to group clients on specific characteristics. Clients that match these characteristics are assigned an IP address from the specific class. In my situation the use of the ip dhcp class command fixed the problem.