3Com 3C13636 Configuration Manual page 461

3Com Router 3000 Ethernet Family
Configuration Guide
Option 82 provides many sub-options. Among them, only sub-option 1 and sub-option
2 are available on the DHCP relay. Option 82 allows the address information of the
DHCP client and the DHCP relay such as MAC address and VLAN ID, to be recorded
on the DHCP server. In conjunction with other software, it may implement DHCP
address assignment restriction and accounting.
I. Operating mechanism of option 82 support on the DHCP relay
When obtaining an IP address through a DHCP relay, the DHCP client goes through
four phases, discovery, offer, selecting, and final check, as it would directly through a
DHCP server. This section however, describes only the operating mechanism of option
82 support on the DHCP relay.
The following is how option 82 support is operating on the DHCP relay:
1) The DHCP client broadcasts a request when it is initialized.
2) If no DHCP server is present on the local network, the DHCP relay connected to
the network checks the packet for the option 82 field. If the packet does not carry
the option 82 field, the DHCP relay inserts the option 82 field into the packet and
forwards it with the MAC address and VLAN ID of the switch port connected to the
DHCP client, and the MAC address of the DHCP relay itself. If the packet carries
option 82 information, the DHCP relay does one of the following depending on the
adopted strategy:
Drop the packet.
Forward the packet with its option 82 information intact.
Forward the packet with its option 82 information being replaced with that of the
DHCP relay.
When the DHCP server receives the request, it records the option 82 information
and then sends a response carrying DHCP configuration and option 82
information back to the DHCP relay.
The DHCP relay sends the received response to the DHCP client with option 82
information removed.
Note:
To accommodate the DHCP request processing mechanisms of different DHCP relay
vendors, the DHCP relay with option 82 support enabled inserts option 82 information
into all DHCP requests, whether they are DHCP_DISCOVER or DHCP_Request.
II. Protocols and standards
The DHCP relay supports option 82 in compliance with the following protocols:
RFC 2131 Dynamic Host Configuration Protocol (DHCP)
RFC 3046 DHCP Relay Agent Information Option
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Chapter 9 DHCP Configuration