SG500 IP helper/DHCP between VLANs

[paulmpitt]

We recently put our SG500 switch in layer 3 mode.  Its set up with a few vlans (1,25,175,1000, etc.).  It can talk out and between vlans, no problems there.  The major problem I'm have is that I cannot get DHCP between the vlans.

From all that I have read online, in order to accomplish this is basically turning on dhcp relay and configuring ip helper.  But when I do, I get something like this:

console
switchb2c527(config)#ip helper-address 192.168.100.1 192.168.0.7 67 68 Ports 67-68 are reserved for BOOTP/DHCP

192.168.100.1 is the address of the vlan 1000, 192.168.0.7 is the where DHCP server which is on vlan 1.  I know its not best practice, I'm just trying to get it to work currently.  I get the same response whether I use 'all' or any of the other VLAN IPs.

Does this mean that this model will not do UDP relaying for DHCP?  Or am I going about this entirely wrong?  I'll post my config if it will help.

[Tom Watts]

Hi Paul, since the switch has built in DHCP RELAY in to the configuration IP HELPER is not applicable for this UDP service. DHCP RELAY is UDP RELAY for those ports therefore it is redundant.

-Tom
Please mark answered for helpful posts

[paulmpitt]

That does make sense now.  Perhaps I've just set the DHCP relay portion up wrong.  I'll re-read some of the documentation on this, and try to get it running as it should be.

[paulmpitt]

Ok I found the issue.  The DHCP server was located on VLAN 1, which incidentally is also the management VLAN (i.e. a non-routable VLAN).  After moving the DHCP server to another VLAN and pointing it to the IP address of the server, it worked like a charm.

So, the lesson here is that you shouldn't put your DHCP server on the management VLAN or you're gonna have a bad time.

[Alex Ribas]

The Cisco SMB switches operate differently.

The IP Helper command on these units is just for UDP forwarding of odd services.

There are dedicated commands for DHCP relay:

ip dhcp relay address XXX.XX.XX.XX
ip dhcp relay enable
ip dhcp information option
ipv6 dhcp relay destination XXXX:XXX:XXXX:XX:XX

Then, on each SVI, you enable DHCP Relay as needed (you do not enable DHCP relay on the SVI directly connected to the DHCP server):

interface vlan XX
 ip dhcp relay enable