Zyxel VES-1624FT-55A [173/312] Dhcp relay

VES-1624FT-55A User’s Guide

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CHAPTER 29

DHCP Relay

This chapter shows you how to set up DHCP relays for each VLAN.

29.1 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

IP DSLAM 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.

29.2 DHCP Relay Agent Information Option (Option 82)

The IP DSLAM 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 IP DSLAM to add to the DHCP requests that it relays to the DHCP server.

Please see RFC 3046 for more details.

The DHCP relay agent information feature adds an Agent Information field to the option 82

field of the DHCP headers of client TCP/IP configuration request frames that the IP DSLAM

relays to a DHCP server. The IP DSLAM supports two formats for the DHCP relay agent

information: Private and TR-101.

29.2.1 TR-101 Format

The Agent Information field that the IP DSLAM adds contains an “Agent Circuit-ID sub-

option” that includes the system name or IP address, slot ID, port number, VPI, and VCI on

which the TCP/IP configuration request was received.

The following figure shows the format of the TR-101 Agent Circuit ID sub-option. The 1 in

the first field identifies this as an Agent Circuit ID sub-option. The next field specifies the

length of the field. The hostname field displays the system name, if it has been configured, the

extra information field (A) if the hostname was not configured, or the IP address in dotted

decimal notation (w.x.y.z), if neither the system name nor the extra information field was been

configured. In either case, the hostname is truncated to 23 characters, and trailing spaces are

discarded. The hostname field is followed by a space, the string “atm”, and another space.

Then, a 1-byte Slot ID field specifies the ingress slot number (the IP DSLAM’s slot ID is

always 0), and a 1-byte Port No field specifies the ingress port number. Next, the VPI and VCI

denote the virtual circuit that received the DHCP request message from the subscriber.