Based on your comments, I think you misunderstand DHCP relay agents and DHCP Option 82. I will explain this below.
You need to understand the difference between layer-2 (MAC) addresses and layer-3 (IP) addresses, and how they are used.
Layer-2 addresses are used on a LAN to deliver frames from one host on the LAN directly to another host on the LAN. Layer-2 frames have source and destination layer-2 (MAC) addresses. A host sending to a host on the same LAN will use the layer-2 (MAC) address of the destination host as the destination layer-2 (MAC) address, and the layer-2 protocol (ethernet) will deliver the frames directly to the destination hosts. A host sending frames from one LAN to another LAN will use the layer-2 (MAC) address of its configured gateway (router) as the destination layer-2 (MAC) address. The sending host knows nothing about how ethernet does this (via direct connection, switch, hub, or anything else); it only knows that it sends frames out its interface. That means the host is ignorant of any switches or switch ports on the LAN.
Layer-2 (MAC) has a special broadcast address (ff-ff-ff-ff-ff-ff) that means the frames with that address are delivered to all the hosts on the LAN. a DHCP Request uses that broadcast address as the destination layer-2 (MAC) address, so every host on a LAN, including the DHCP server will receive it. Layer-2 frames only exist on the LAN on which they were originated (explained below), so this presents a problem for broadcasts, including DHCP requests, because they cannot be sent from on LAN to another LAN. That is why DHCP relay agents and DHCP Option 82 were created.
Layer-3 addresses are used to deliver packets from one LAN to another LAN, and they have layer-3 (IP) source and destination addresses. When a router receives a layer-2 frame with it layer-2 (MAC) address, it will strip off and discard the layer-2 frame, including the layer-2 (MAC) addresses, to expose the layer-3 packet. The router will look at the layer-3 (IP) destination address and compare it to the destination networks in its routing table. If if finds that the layer-3 (IP) address matches one of the networks in its routing table, it will send the packet to the next interface, and it will build a new layer-2 frame for the protocol of the new interface, which may, or may not, use MAC addresses, but the layer-2 addresses for the new layer-2 frame will be for the new interface; all the previous layer-2 frame information was discarded.
When a host sends a DHCP Request, it is sent to the layer-2 (MAC) broadcast address in the frame, and the layer-3 (IP) broadcast address (255.255.255.255, which is not allowed to be routed) in the packet. If the DHCP server is on the same LAN as the requesting host, this works well because the DHCP server gets the request. The DHCP server will then send a DHCP Offer back to to the requesting host. Depending on how the DHCP server is configured, it can send it to the layer-2 (MAC) broadcast address, or the layer-2 (MAC) host address because it learned that from the layer-2 DHCP Request frame. This also works well if the DHCP server and requesting host are on the same LAN. No special DHCP options are needed.
The problem arises when the DHCP server and the requesting host are on different LANs. DHCP is broken because they cannot broadcast to each other, and the requesting host's layer-2 (MAC) address is no good on the DHCP server's LAN. In fact the requesting host's layer-2 (MAC) address would be stripped at the first router.
To solve this problem, DHCP relay agents were created. The DHCP relay agent is configured on the router's LAN interface. The router will receive the DHCP Request because it is sent to the layer-2 (MAC) broadcast address. The DHCP relay agent is configured with layer-3 (IP) addresses of one or more DHCP servers. That is because the DHCP servers are on other LANs. The DHCP relay agent will take the broadcast DHCP Request and package it into a layer-3 packet and send it to the LANs where the DHCP servers are.
The next problem arises when the DHCP server needs to send back a DHCP offer. It would normally send it to the layer-2 (MAC) broadcast address, or the layer-2 (MAC) source address seen in the DHCP request frame on the DHCP server's LAN, but that is not where the requesting host is. That will not work because the requesting host is on a different LAN. This is where DHCP Option 82 comes in.
DHCP Option 82 allows the DHCP relay agent to tell the DHCP server to send the DHCP Offer back to it, instead of trying to deliver it on the LAN where the DHCP server is as it normally would. That is the circuit to which you refer in your comments.
As you can see, DHCP works well when both the DHCP server and the requesting host are on the same LAN, and DHCP doesn't require a DHCP relay agent or DHCP Option 82 in that case. DHCP relay agents and DHCP Option 82 were later added to DHCP as a way to centralize DHCP servers on a LAN that is different than the LAN where the requesting hosts are. Using a DHCP relay agent and DHCP Option 82 when both the DHCP server and the requesting host are on the same LAN is pointless, and they actually add unnecessary complexity to the DHCP process, but they are essential for the situation when the DHCP server and the requesting host are on different LANs.
Edit:
Consider this scenario:
I would like my DHCP server to assign IP addresses to clients based on
what port the client is connected. If the client is on port 1 I would
like it's IP to be 192.168.0.1, the client connected to port 2 should
get 192.168.0.2, and so on. Now we need information from the switch.
This can be done by SNMP, but that would probably be slow. So what if
the DHCP DISCOVER message contained the port? Yes, that's where Option
82 comes to play. The Circuit ID could contain the port. And that is
what switches do. The switch inserts Option 82 in the DHCP messages
and forwards the messages to the DHCP server.
As I explained in my comments, a switch is a transparent device, and both the DHCP server and the hosts connected to the switch are unaware of the switch or the switch ports. That is not how DHCP server work, and DHCP is not assigned as you would like, and if you were to move a host from one switch port to another during its DHCP lease time, it will still get the same IP address because it is based on its MAC address.
DHCP uses scopes as pools from which to choose IP addresses to assign to MAC addresses. Since the DHCP server and hosts are ignorant of even whether or not a switch is used, you cannot have DHCP assign addresses using the method you would like.
If your concern is knowing which IP address is assigned to which switch port, you can find that information by comparing the DHCP MAC/IP address assignments (tells you which IP address is assigned to which MAC address) and the switch's MAC address table (tells you to which switch port a MAC address is connected).
That is a layer-2 switch. You should be configuring the DHCP relay agent on a layer-3 device (your router). DHCP will work correctly on layer-2 because broadcasts work across the layer-2 broadcast domain. It is layer-3 that needs the help because broadcasts do not cross a layer-3 device. – Ron Maupin – 2016-11-12T16:30:03.057
Ron, the point is I want to know what port on a switch a client is connected to. Only the switch knows that. Hence the switch (even if it is a Layer2-switch) should act as a relay agent. And indeed it does. The manual says: "the relay agent will insert and remove DHCP relay information (option 82 field) in messages between DHCP servers and clients" . – Jeff Merlin – 2016-11-12T16:42:30.560
That is not where you would put a DHCP relay agent. You would put it on the router interface for the LAN. It is the router that needs the help to send a DHCP request to another LAN. – Ron Maupin – 2016-11-12T16:44:11.093
Ron, no one need help (except me) ;-) Could you please explain how the router would know the port number on another switch? – Jeff Merlin – 2016-11-12T16:52:50.900
The router, or any host on the LAN, doesn't need to know anything about switch ports. The switch builds a MAC address table that relates MAC addresses to the ports where the MAC addresses came in. The hosts (a router is a host on the LAN) simply address frames with MAC addresses, and the switch sends the frames to the correct port. If the MAC address is not in the MAC address table, the switch floods the frame to every other port. Hosts will send to the router MAC address the when the IP address is on a different LAN. DHCP requests are broadcast to every host on the LAN, including the router. – Ron Maupin – 2016-11-12T16:59:10.817
Option 82 was designed to allow a DHCP Relay Agent to insert circuit specific information into a request that is being forwarded to a DHCP server. The Circuit ID sub-option is supposed to include information specific to which circuit the request came in on. It's an identifier that is specific to the relay agent, so what kind of circuit is described will vary depending on the relay agent. In an Ethernet-based network this is probably a port on a switch. That's what I want. And the DGS-1210 is capable of doing that, even if it's just a Layer2-switch. – Jeff Merlin – 2016-11-12T17:01:05.720
Option 82 is used by a router to package a broadcast DHCP request and send it to a DHCP server on a different LAN. The router acts as a DHCP relay agent, but this must be configured on the LAN interface of the router. DHCP on a LAN where the requester and the DHCP server both are doesn't need Option 82 because the DHCP server will receive the broadcast DHCP request. If both are on the same LAN, DHCP work without Option 82. Hosts on a LAN, including the DHCP server know nothing about switches or switch ports, and they don't need to. – Ron Maupin – 2016-11-12T17:04:44.697
Ron, I don't think you understand. Let's say this is a switch, some clients a DHCP server AND NOTHING ELSE. No router at all. The DHCP server still wants to know what port on a switch a client is connected to. There is no router to run a DHCP Relay Agen on. What do we do? Run the agent on the switch. See? – Jeff Merlin – 2016-11-12T17:04:46.760
No, you are incorrect. No host on a LAN is even aware of a switch or the switch ports. A switch is a transparent device. Option 82 is not needed on a LAN. – Ron Maupin – 2016-11-12T17:05:43.197
So why does the DGS-1210 have a DHCP Relay agent???? – Jeff Merlin – 2016-11-12T17:07:44.173
Probably because it share a common code with layer-3 switches by the same manufacturer. I will add a detailed answer about how this works. – Ron Maupin – 2016-11-12T17:10:21.943
Hehe... Well, it works on a DES-3010G... Just as expected. :-) – Jeff Merlin – 2016-11-12T17:11:59.240
It will work on the switch without Option 82 if both the DHCP server and requesting host are on the same LAN. I will explain in my answer. – Ron Maupin – 2016-11-12T17:13:01.103
When I say "it works" I mean the Circuit ID is inserted in the Option 82 info. That has nothing to to with subnet, DHCP server or router. It's done by the switch and no one else! ;-) – Jeff Merlin – 2016-11-12T17:20:08.263
My point is that it is unnecessary on a LAN. Option 82 was designed to help routers package a broadcast DHCP request to send it to a different LAN because broadcast cannot leave the LAN on which they were originated. This tells the DHCP server that this was done, and it should send the DHCP Offer back to the router, rather than sending it back on its own LAN. – Ron Maupin – 2016-11-12T17:23:11.437
By the way, when you edit a question, especially when it changes the question, you should MARK the edit in the question. – Ron Maupin – 2016-11-12T18:25:17.810
The question is not changed. I still want to know if it is possible to configure the DGS-1210 to add Option 82 information to DHCP messages like the DES-3010 does. The scenario I added was to give an example of why I need a relay agen on a Layer 2 switch. You said "You should be configuring the DHCP relay agent on a layer-3 device" and that doesn't work (or is pretty pointless). – Jeff Merlin – 2016-11-12T22:20:31.110