DHCP forwarding with Cisco and Fortigate
When configuring a network with a central Fortigate firewall and a Cisco 871 router placed on the site I had a problem getting the DHCP relay feature to work.
On the Cisco router I configured the interface as shown below:
interface Vlan1
ip address 10.10.10.10
255.255.255.0
ip helper-address 1.1.1.1
The Cisco router forwarded the DHCP packet but I did not get any IP address. When using a packet sniffer I found that the Cisco router sent out the packet with source address=0.0.0.0. The Fortigate firewall did not approve this.
To solve this I removed the helper-address and typed in these lines instead:
service dhcp
ip dhcp relay information policy keep
ip dhcp pool LAN
relay source 10.10.10.0 255.255.255.0
relay
destination 1.1.1.1
This changed the source address and solved my DHCP forwarding issue.
Labels: cisco

