Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 1714)

Dynamic Host Configuration Protocol (DHCP) Commands
Software Reference for x310 Series Switches
64.16 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
ip dhcp-relay max-message-length
This command applies when the switch is acting as a DHCP Relay Agent and DHCP Relay
Agent Option 82 insertion is enabled. It sets the maximum DHCP message length (in
bytes) for the DHCP packet with its DHCP Relay Agent Option 82 data inserted. From this
value it calculates the maximum packet size that it will accept at its input. Packets that
arrive greater than this value will be dropped.
The no variant of this command sets the maximum message length to its default of 1400
bytes.
For DHCP Relay Agent introductory information, see “DHCP Relay Agent Introduction”
on page 63.7 .
Syntax ip dhcp-relay max-message-length <548-1472>
no ip dhcp-relay max-message-length
Default The default is 1400 bytes.
Mode Interface Configuration for a VLAN interface.
Usage When a DHCP Relay Agent (that has DHCP Relay Agent Option 82 insertion enabled)
receives a request packet from a DHCP client, it will append the DHCP Relay Agent 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 DHCP Relay Agent Option
82 data, the DHCP Relay Agent will increase the packet size to accommodate the DHCP
Relay Agent Option 82 data. If the new (increased) packet size exceeds that defined by the
maximum-message-length parameter, then the DHCP Relay Agent will drop the packet.
Example To set the maximum DHCP message length to 1200 bytes for packets arriving in interface
vlan7
, use the commands:
Parameter Description
<548-1472>
The maximum DHCP message length (this is the message
header plus the inserted DHCP option fields in bytes).
Note Before setting this command, you must first run the ip dhcp-relay
agent-option command on page 64.12 . This will allow the DHCP Relay Agent
Option 82 fields to be appended.
awplus# configure terminal
awplus(config)# interface vlan7
awplus(config-if)# ip dhcp-relay max-message-length 1200