Qtech QSW-2900-24T-AC [112/209] Request packets sent by a dhcp client fall into two categories dhcp discover packets and dhcp request packets as dhcp servers coming from different manufacturers process dhcp request packets in different ways that is some dhcp servers process option 82 in dhcp discover packets whereas the rest process option 82 in dhcp request packets a dhcp relay agent adds option 82 to both types of packets to accommodate to dhcp servers of different manufacturers

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· Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent circuit ID, namely Circuit

ID. It holds the port number and VLAN-ID of the switch port connected to the DHCP client, and is

usually configured on the DHCP relay agent. Generally, sub-option 1 and sub-option 2 must be used

together to identify information about a DHCP source.

· Sub-option 2: A sub-option of option 82. Sub-option 2 represents the remote agent ID, namely Remote

ID. It holds the MAC address of the DHCP relay agent, and is usually configured on the DHCP relay

agent. Generally, sub-option 1 and sub-option 2 must be used together to identify information about a

DHCP source.

c) Related specification

The specifications concerning option 82 supporting are as follows:

RFC2131 Dynamic Host Configuration Protocol

RFC3046 DHCP Relay Agent Information Option

d) Mechanism of option 82 supporting on DHCP relay agent

The procedure for a DHCP client to obtain an IP address from a DHCP server through a DHCP relay agent is

similar to that for the client to obtain an IP address from a DHCP server directly. The following are the mechanism of

option 82 supporting on DHCP relay agent.

1) A DHCP client broadcasts a request packet when it initiates.

2) The DHCP relay agent on the local network receives the request packet, and then checks whether the packet

contains option 82 and processes the packet accordingly.

3) If the packet contains option 82, the DHCP relay agent processes the packet depending on the configured

policy (that is, discards the packet, replaces the original option 82 in the packet with its own, or leaves the original

option 82 unchanged in the packet), and forwards the packet (if not discarded) to the DHCP server.

4) If the packet does not contain option 82, the DHCP relay agent adds option 82 to the packet and forwards

the packet to the DHCP server. The forwarded packet contains the port number of the switch to which the DHCP

client is connected, the VLAN to which the DHCP client belongs, and the MAC address of the DHCP relay agent.

5) Upon receiving the DHCP request packet forwarded by the DHCP relay agent, the DHCP server stores the

information contained in the option field and sends a packet that contains DHCP configuration information and

option 82 to the DHCP relay agent.

6) Upon receiving the packet returned from the DHCP server, the DHCP relay agent strips option 82 from the

packet and forwards the packet with the DHCP configuration information to the DHCP client.

& Note:

Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVER

packets and DHCP-REQUEST packets. As DHCP servers coming from different

manufacturers process DHCP request packets in different ways (that is, some DHCP

servers process option 82 in DHCP-DISCOVER packets, whereas the rest process option

82 in DHCP-REQUEST packets), a DHCP relay agent adds option 82 to both types of

packets to accommodate to DHCP servers of different manufacturers.