3Com WX3000 operation manual Mechanism of Option 82 supported on Dhcp relay agent

Figure 2-2Padding contents for sub-option 1 of Option 82

Figure 2-3Padding contents for sub-option 2 of Option 82

Mechanism of Option 82 supported 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 support on DHCP relay agent.

1)Upon receiving a DHCP request, the DHCP relay agent checks whether the packet contains Option

82 and processes the packet accordingly.

zIf the request packet contains Option 82, the DHCP relay agent processes the packet depending on the configured strategy (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.

zIf the request packet does not contain Option 82, the DHCP relay agent adds Option 82 to the packet and forwards the packet to the DHCP server.

2)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.

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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