3-3Figure 3-2 Padding contents for sub-option 1 of Option 82Figure 3-3 Padding contents for sub-option 2 of Option 82Mechanism of Option 82 supported on DHCP relay agentThe procedure for a DHCP client to obtain an IP address from a DHCP server through a DHCP relayagent is similar to that for the client to obtain an IP address from a DHCP server directly. The followingare 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 Option82 and processes the packet accordingly.z If the request packet contains Option 82, the DHCP relay agent processes the packet dependingon the configured strategy (that is, discards the packet, replaces the original Option 82 in thepacket with its own, or leaves the original Option 82 unchanged in the packet), and forwards thepacket (if not discarded) to the DHCP server.z If the request packet does not contain Option 82, the DHCP relay agent adds Option 82 to thepacket and forwards the packet to the DHCP server.2) Upon receiving the packet returned from the DHCP server, the DHCP relay agent strips Option 82from the packet and forwards the packet with the DHCP configuration information to the DHCPclient.Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVER packets andDHCP-REQUEST packets. As DHCP servers coming from different manufacturers process DHCPrequest packets in different ways (that is, some DHCP servers process Option 82 in DHCP-DISCOVERpackets, whereas the rest process Option 82 in DHCP-REQUEST packets), a DHCP relay agent addsOption 82 to both types of packets to accommodate to DHCP servers of different manufacturers.DHCP Inform Message Handling Feature Used in IRF SystemSome DHCP clients, after obtaining IP addresses, send DHCP Inform messages to the DHCP server torequest other configuration parameters (such as DNS server and default gateway addresses).
