DHCP/BOOTP Broadcast Forwarding

[rsommer]

Would like to forward DHCP and BOOTP broadcasts (not turn the broadcast into a unicast) to a specific subnet on our WAN. This gives us the flexibility to change our DHCP servers without having to change router configs (in our case anyway). Example: Server resides on subnet 1.1/16, my server address is 1.1.0.50, and I have clients on subnet 1.2/16 making DHCP requests.

Using command ip helper-address 1.1.0.50 works.

Using command ip helper-address 1.1.0.0 does not work. According to some

the broadcasts should be forwarded to subnet 1.1.0.0. Doesn't appear to be.

I haven't done any traces yet. Figured this is an easy question for experts out

there.

In the Nortel world that I'm leaving, I would have configured DHCP relay agents on each interface along the 'way' to the servers and back. Cisco implementation appears to be less cumbersome....so far.

Appreciate any help.

[MickPhelps]

ip helper 1.1.0.255 should do the trick (if you're using a /24 subnet mask).

Be sure that the interface connecting to the DHCP servers has enabled directed broadcasts (IP DIRECTED-BROADCAST).

Mick.

[mfaust]

I'm not sure of your reasons for doing this, but I will try to help. If you are looking for redundency for your DHCP, I would suggest splitting your scope between two DHCP servers instead of forwarding broadcasts. If that is not possible try this: "ip helper-address 1.1.255.255". That should be the broadcast address on your subnet. I don't know if that will work or not. If that doesn't work look at : "ip forward protocol ...". If you use two DHCP servers, be sure to have both helper addresses configured. Let me know how you come out with this. It looks like a fun little experiment!!!

[rsommer]

thanks for the replies....helper-address 1.1.255.255 works

now part 2 (sorry)

does this forward all udp broadcasts (i think so)

what is the best way to limit it to bootp/dhcp?

[nuno.morais]

Hi.

To be more selective you should do the following:

ip forward-protocol udp 67

ip forward-protocol udp 68

no ip forward-protocol XXX

where XXX is any protocol that is forwarded by default when you use "ip helper-address", namely DNS, TFTP, NTP, NetBIOS name and datagram service.

Hope this helps.

NM