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23.1.3 Forwarding DHCP packets with Option 82
In a configuration with a DHCP relay agent, such as a Layer 3 switch, placed between the
Switch and a DHCP client, the DHCP relay agent might add Option 82 information to DHCP
packets sent from the DHCP client.
Figure 23-6 Example configuration when Option 82 information is added to the packets
Packets with Option 82 contain information used by a DHCP relay agent to transfer
extended information about the DHCP client, such as the MAC address of the terminal,
connection port number, and host name.
When DHCP snooping is enabled, packets with Option 82 received on untrusted ports are
discarded. Therefore, if the Switch is placed between a DHCP server and a DHCP relay
agent, and the DHCP relay agent adds Option 82 information to the packets, DHCP
snooping on the Switch cannot operate correctly.
In this case, execute the
configuration command to set the Switch to permit communication using packets with
Option 82.
Also, DHCP snooping checks if the source MAC address of a DHCP packet received on an
untrusted port matches the client hardware address in the DHCP packet (MAC address
matching). If a DHCP relay agent is connected to an untrusted port, the source MAC
address of a DHCP packet is rewritten by the DHCP relay agent, and the Switch judges the
DHCP packet as illegal and discards it.
Therefore, in addition to the setting of permitting communication using packets with Option
82, you need to execute the
command to cancel MAC address matching.
23.1.4 Limiting the rate of DHCP packet reception
If DHCP snooping is enabled to monitor the receipt of DHCP packets and the number of
ip dhcp snooping information option allow-untrusted
no ip dhcp snooping verify mac-address
23 DHCP Snooping
configuration
411
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