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Webcast-Catalyst9k
New Member

Does the "ip dhcp-server" cmd replace the "ip helper-address" cmd?

I noticed the "ip dhcp-server" command on my 3524 switches while troubleshooting dhcp. Currently I use "ip helper-address" for dhcp forwarding. While this works fine, I worry about the Cisco explanation that it forwards UDP packets to an address.

Years ago I thought I read a different explanation that said when you use the "ip helper-address" command on an interface, all IP broadcasts seen by that interface will be converted to unicast and forwarded to the helper-address.

Which explanation is closer to the truth, and does this mean ALL UDP packets (in the case of the first explanation), or ALL IP broadcasts (in the case of the second explanation), are forwarded??

I would think it would be easy enough to limit forwarding based on the destination port number (67?) to identify a dhcp request. When I saw the "ip dhcp-server" command I was left hoping this ability has been provided...

11 REPLIES
New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

3524 is a layer 2 switch. not sure how ip helper is working which can't used as a dhcp server which is what ip dhcp-server is for. I am surprised the ip helper-address is even working but anyway, check this URL for further info.

http://www.cisco.com/warp/public/473/100.html#18

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

I use the ip-helper on the router subinterfaces that the 3524 (really a gigastack of 3524's) is trunked to.

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

Great link but it still begs the question. It states that:

"The IP helper-address can be configured to forward any UDP broadcast based on UDP port number. By default, the IP helper-address will forward the following UDP broadcasts: "

tftp, dns, netbios nameserver, netbios datagram server, dhcp/bootp, ien-116 name service, tacacs.

By stating that "by default" it can be configured to forward all these broadcasts, it implies that there is a set of options to limit which types of UDP broadcasts are forwarded. However, when I type the command I see no other such options.

I'm right back where I started here...

Cisco Employee

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

XL being a layer 2 only switch, I dont think these commands apply to an XL switch. The interface VLAN is only for management purpose and you cannot have multiple VLAN interfaces up. You might see the command in the parser since it run IOS. The 3550's which is a layer 3 switch will support this

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

I totally understand this. Please excuse the lacking description of my setup. I was really more interested in two things:

-Does the "ip dhcp-server" command on the 3524's relace the "ip helper-address" command on the router interface connected to the switch.

-How does the "ip helper-address" really work.

now let go me check that url... :)

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

both of the commands are not designed to work on this switch. You may have the command but it won't work.

anway,

ip helper on a router turns on DHCP relay agent feature

Ip dhcp server makes the router a DHCP server.

Reading that URL is a good idea :)

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

(sigh) I understand and regularly configure ip helper-address's. I know where to configure them, and they work in my customers network like they are supposed to. I have them configured on 4908GL3 fiber sub-interfaces that act as an ISL trunk to a group of 3524's configured as gigastack. This stuff is pretty basic, I've been doing cisco since '90.

The "ip dhcp-helper" command I am asking about is listed as a global command on the 3524XL switches. One of my questions was if this command on the switch can replace or negate the need for the ip helper-address on the router.

The url is a good one although it taught me nothing new. The "ip dhcp-server" command is not even mentioned in it.

I know how to troubleshoot dhcp, and thoroughly understand how it works. I only mentioned that I was troubleshooting dhcp when I noticed the "ip dhcp-server" command on the switches.

Just so you understand, I am not having a problem with dhcp. the problem I was having was due to SMS pushing the MSfirewall client to my customers dhcp server, causing pc's without the MSfirewall client installed to fail dhcp requests.

I am just trying to find out:

-Does the "ip dhcp-server" command on the 3524's replace the "ip helper-address" command on the router interface connected to the switch.

-How does the "ip helper-address" really work.

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

oops, typo..

[The "ip dhcp-helper" command I am asking about is listed as a global command on the 3524XL switches. One of my questions was if this command on the switch can replace or negate the need for the ip helper-address on the router. ]

should read:

[The "ip dhcp-server" command I am asking about is listed as a global command on the 3524XL switches. One of my questions was if this command on the switch can replace or negate the need for the ip helper-address on the router. ]

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

Are you sure that this is not setting up the ios as a dhcp server where it would hand out IP addresses, mask and default gateway from a pool of addresses that you define in the configuration?

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

I figured I would bypass all of the posts about the 3524 being a Layer 2 device and on which devices the commands are present on... and directly answer your ORIGINAL question:

ip dhcp-server is used to configure the ip address of a network DHCP server (like a Win2000 server-based DHCP server) for dynamically handing out IP address to remote clients; ie on a network access server.

ip helper-address converts udp broadcasts of a selected range into a unicast/directed bradcast for a particular destination IP address/IP network. By default, a router does not forward ANY broadcasts + effectively ends a broadcast domain at a device port/interface... which is one of the many reasons we use routers.

Using ip helper-address + ip directed broadcasts together enables that broadcast to unicast translation from one port/interface's network to another. By default, the ip helper-address forwards these main UDP port types/services:

TFTP port 69

DNS port 53

Time port 37

NetBIOS name service port 137

NetBIOS datagram server port 138

BOOTP server port 67

BOOTP client port 68

TACACS port 49

To limit the default "group" of UDP port forwarding/translation from broadcast to unicast addresses, use:

ip forward-protocol {udp [port]}, which is the global config command that specifies which type of broadcast packets are forwarded.

Or, if you love creating ACLs, use:

ip directed-broadcast [access-list-number] to specify an extended ACL that can control which broadcasts are forwarded... only IP packets permitted by your configured ACL are able to be translated.

So there are your 2 commands you were interested in...

New Member

Re: Does the "ip dhcp-server" cmd replace the "ip helper-address

Thanks for the helpful response.

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