Please read the disclaimer before continuing …
Once the basics of my router were up and running, I wanted to ensure that my wireless network could be served by a DHCP server. This is complicated by the fact that :-
- My wireless network is a separate network.
- I use a Unix-based DHCP server (partially because I’m very familiar with it).
The first thing to do is to configure a “helper address” on the wireless interface – in this case the vlan that incorporates the wireless network :-
interface Vlan102...ip helper-address 10.0.0.21...
You can configure more than one helper address in here, which may be useful for debugging purposes – or in a production environment you may wish to run a failover dhcp server.
In theory, this should send all broadcast DHCP packets onto the specified network address. However it also forwards other UDP broadcast traffic onto that host which may not be what we want. Specifically for a DHCP server we don’t want DNS, TFTP, etc. These other protocols can be excluded with (at the global level) :-
no ip forward-protocol udp tftp no ip forward-protocol udp domain no ip forward-protocol udp netbios-ns no ip forward-protocol udp netbios-dgm no ip forward-protocol udp tacacs
Using the ‘symbolic’ names for the protocols aids the readability of the configuration file.
At which point the instinct is to fire up the dhcp server and see if it works. Very natural of course, but I would suggest that sniffing the traffic to check that the relevant packets are being forwarded is wise at this point. Or you are risking testing two things at once – the packet forwarding and the dhcp server. Despite being very experienced with DHCP servers, it turned out that my main problem in setting this all up was a faulty DHCP server!
After fixing that problem, my newly configured wireless network worked fine.
Some additional configuration that may be worth trying :-
- ip dhcp relay prefer known-good-server – The explanation for this on the command line is somewhat mysterious, but the documentation indicates that with this turned on, the DHCP clients will need to renew their leases less frequently. How this works, I don’t know.
- ip dhcp relay information option – This inserts information about the relay agent (the router) into “option 82”. This additional information can be logged which can be useful.