DHCP relay v. ip helper address on a 3750

[gerryobrien]

What is the difference between using a DHCP relay and ip helper address on a 3750? Does anybody have a a minimum configuration to get either of them working?

[pmajumder]

Hi,

There is no difference between the two. DHCP Relay essentially is the ability of a host to forward DHCP packets between clients and servers, when they reside on different subnet. ip helper-address is the cisco syntax for enabling DHCP relaying.

Attach the following to the appropriate interfaces:

ip helper-address

[Richard Burts]

ip helper-address and dhcp relay are not the same and there are significant differences.

ip helper-address does much more than just DHCP. By default ip helper-address will forward broadcasts for these protocols: UDP 69 (TFTP), UDP 53 (DNS), UDP 37 (time service), UDP 137 (NetBIOS Name Server), UDP 138 (NetBIOS Datagram Server), UDP 67 and 68 (BOOTP client and server/DHCP), UDP 49 (TACACS), and UDP 116 (Name Service). Additional protocols can be enabled for forwarding using the command ip forward-protocol.

for DHCP ip helper-address will take the DHCP request from the client and forward it (unchanged) to the DHCP server. DHCP relay the router receives the DHCP request from the client and builds a new request to send to the server (which gives the router an opportunity to add information in the request about the router).

[pmajumder]

I do agree that it does forward more protocols than just DHCP traffic, though you can disable the rest using the command no ip forward-protocol.

However, I still believe that the command ip helper-address is also used for enabling dhcp relay. If that is not the case then what is the command for enabling dhcp relay? - that is without having to configure the router as as dhcp server.

[Richard Burts]

see this URL:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a7ea5.html#wp1116732

for information about the ip dhcp relay commands. There are several commands relating to dhcp relay including:

ip dhcp relay forward spanning-tree

ip dhcp relay information check

ip dhcp relay information option

ip dhcp relay information policy

ip dhcp relay information trust-all

ip dhcp relay information trusted

ip dhcp smart-relay

[gerryobrien]

Hi. Is there an issue with using ip helper address from a VLAN on a 3750? We had a configuration that worked fine using Bridge Groups on a 2948G-L3 but stopped working when a similar configuration was moved to a 3750 and VLANs were setup instead of bridge groups. The DHCP requests are not seen outside of the VLAN. Cf attached config file

[pmajumder]

It does work with the 3750 (see document) - document also explains DHCP Relay and the command ip helper-address:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12218se/3750scg/swdhcp82.pdf

Make sure that you have the ip helper-address command on all the SVI interfaces bewteen your dhcp server and your clients. Also make sure your access list is not blocking any incoming or outgoing dhcp traffic to/from your clients.

Hope this helps!

[jobraswe@cisco.com]

Verifies and expands on my initial thoughts... thank you for clarifying... very helpful...

[dangreen12]

Well stated Rick. Thank you!!  That was the best explanation I have seen.

[Richard Burts]

You are welcome. Thanks for the kind words. I am always glad to know when my explanations have been helpful to participants in the community.

[ISSOintraining!]

Thank you so much for the information. This is an extremely helpful forum and greatly appreciated.

[ISSOintraining!]

Do you have to run 'feature dhcp' in order to create a dhcp relay address for a vlan? If so, what are the implications of initializing the feature on an established NX-OS switch?

[Georg Pauwen]

Hello,

here is the short answer: without 'feature dhcp' enabled, nothing related to DHCP on your NX-OS switch will work...

--> 'When the DHCP feature is disabled, you cannot configure the DHCP relay agent, DHCP snooping, or any of the features that depend on DHCP. In addition, all DHCP configuration is removed from the device.

[paul driver]

Hello
There is also a feature call dhcp smart relay which is a global command used when you are running secondary addressing on L3 interfaces, this feature allows the use of secondary addressing as the source gateway ip address in the dhcp packet when no offer/response is received from the dhcp server in relation to the primary ip address of the L3 interface.

Very useful when you have an extended L3 interface with multiple dhcp scopes related to that primary and secondary addressing and the primary dhcp scope has maxed out or you are in transition of ip address migration