Option 82 maximum message length – Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) User Manual

Page 1696

Dynamic Host Configuration Protocol (DHCP) Introduction

Software Reference for x310 Series Switches

63.10

AlliedWare Plus

Operating System - Version 5.4.4C

C613-50046-01 REV A

The action taken on packets with an Option 82 field is configurable. The command to
configure this action is shown below:

This command sets the action that the DHCP relay takes when a received DHCP client
request contains Option 82 information. Required parameters configure the switch to:

■

Leave the existing Option 82 field untouched (keep parameter)

■

Append its own Option 82 field after the existing field (append parameter - use this
when there is a trusted DHCP Snooping switch or another relay device between the
clients and the DHCP Relay)

■

Drop the packet (drop parameter)

■

Replace the existing Option 82 information with its own (the default - replace
parameter).

See the

ip dhcp-relay information policy command on page 64.13

for a command

description and command examples.

Checking Option 82 information in DHCP Server Responses

To configure the switch to check for the presence of Option 82 information in response
packets returned from DHCP servers, configure DHCP-relay agent-option checking with
the Interface Configuration command:

This command enables the DHCP Relay Agent to check Option 82 information in response
packets returned from DHCP servers. If the information does not match the information it
has configured for its own client (downstream) interface then the DHCP relay agent drops
the packet.

See the

ip dhcp-relay agent-option checking command on page 64.11

for a command

description and command examples.

Option 82 maximum message length

When a DHCP Relay (that has Option 82 insertion enabled) receives a request packet from
a DHCP client, it will append the Option 82 component data, and forward the packet to
the DHCP server. The DHCP client will sometimes issue packets containing pad option
fields that can be overwritten with Option 82 data.

Where there are insufficient pad option fields to contain all the Option 82 data, the DHCP
relay will increase the packet size to accommodate the Option 82 data. If the new
(increased) packet size exceeds that defined by the maximum-message-length
parameter, of the

ip dhcp-relay max-message-length

command then the DHCP relay

will drop the packet.

awplus(config)#

interface

awplus(config-if)#

ip dhcp-relay information policy
{append|drop|keep|replace}

awplus(config)#

interface

awplus(config-if)#

ip dhcp-relay agent-option checking

awplus(config)#