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-612-51A 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-612-51A 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-612-51A to add to the DHCP requests that it relays to the DHCP server. Please see RFC 3046 for more details.

25.2.1DHCP 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-612-51A relays to a DHCP server. The Agent Information field that the IES-612-51A 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 90 DHCP Relay Agent Circuit ID Sub-option Format

 

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IES-612-51A User’s Guide