User`s guide

IES-612-51 User’s Guide
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CHAPTER 25
DHCP Relay
This chapter shows you how to set up DHCP relays for each VLAN.
25.1 DHCP Relay
DHCP (Dynamic Host Configuration Protocol, RFC 2131 and RFC 2132) allows individual
clients to obtain TCP/IP configuration at start-up from a DHCP server. You can configure the
IES-612-51 to relay DHCP requests to one or more DHCP servers and the server’s responses
back to the clients. You can specify default DHCP servers for all VLAN, and you can specify
DHCP servers for each VLAN.
25.2 DHCP Relay Agent Information Option (Option 82)
The IES-612-51 can add information to DHCP requests that it relays to a DHCP server. This
helps provide authentication about the source of the requests. You can also specify additional
information for the IES-612-51 to add to the DHCP requests that it relays to the DHCP server.
Please see RFC 3046 for more details.
25.2.1 DHCP Relay Agent Circuit ID and Remote ID Sub-option
Formats
The DHCP relay agent information feature adds an Agent Information field to the option 82
field of the DHCP headers of DHCP request frames that the IES-612-51 relays to a DHCP
server. The Agent Information field that the IES-612-51 adds contains an “Agent Circuit-ID
sub-option” that includes the port number, VLAN ID and optional information about the port
where the DHCP request was received.
The following figure shows the format of the Agent Circuit ID sub-option. The 1 in the first
field identifies this as an Agent Circuit ID sub-option. The length N gives the total number of
octets in the Agent Information Field. If the configuration request was received on a DSL port,
a 2-byte Port No field specifies the ingress port number (the first byte is always 0, the second
byte is in hexadecimal format). The next field is 2 bytes and displays the DHCP request
packet’s VLAN ID. The last field (A) can range from 0 to 24 bytes and is optional information
(that you specify) about this relay agent.
Figure 92 DHCP Relay Agent Circuit ID Sub-option Format