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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 19 Configuring DHCP Features
Configuring DHCP Features
Configuring the Reforwarding Policy
By default, the reforwarding policy of the switch is to replace existing relay information in packets
received from DHCP clients with switch DHCP relay information. If the default action is not suitable
for your network configuration, you can use the ip dhcp relay information policy {drop | keep |
replace} global configuration command to change it. (This feature is not available when DHCP
snooping is enabled on the switch.)
Note
To ensure the correct operation of the reforwarding policy, make sure to disable the relay agent
information check by using the no ip dhcp relay information check global configuration command.
Beginning in privileged EXEC mode, follow these steps to change the action of the reforwarding policy.
To return to the default reforwarding policy, use the no ip dhcp relay information policy global
configuration command.
Specifying the Packet Forwarding Address
A DHCP relay agent is any device that forwards DHCP packets between clients and servers when they
are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of
an IP router, where IP datagrams are transparently switched between networks. By contrast, relay agents
receive DHCP messages and then generate a new DHCP message to send on another interface.
If the DHCP server and the DHCP clients are on different networks or subnets, you must configure the
switch with the ip helper-address address interface configuration command. The general rule is to
configure the command on the Layer 3 interface closest to the client. The address used in the ip
helper-address command can be a specific DHCP server IP address, or it can be the network address if
other DHCP servers are on the destination network segment. Using the network address enables any
DHCP server to respond to requests.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip dhcp relay information policy {drop
| keep | replace}
Configure the reforwarding policy. The default is to replace (overwrite)
existing information with switch DHCP relay information.
•
Use the drop keyword if you want the switch to discard messages
with existing relay information if the option-82 information is also
present.
•
Use the keep keyword if you want the switch to retain the existing
relay information.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entries.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.