Administrator Guide

● Dynamic ARP Inspection
● Source Address Validation
Option 82 (DHCPv4 relay options)
RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment.
The code for the relay agent information option is 82, and includes two suboptions, circuit ID and remote ID.
Circuit ID This is the interface on which the client-originated message is received.
Remote ID This identifies the host from which the message is received. The value of this suboption is the MAC
address of the relay agent that adds Option 82.
In DHCPv4 relay , the Option 82 is not added by default. When the ip dhcp relay information-option is configured,
the Option 82 with suboptions 1 (Agent Circuit ID) and 2 ( Agent Remote ID) are added to the relayed DHCP packet. The
default values of the suboptions are as follows:
● Default Agent Circuit ID is constructed in the format VLANID:LagID:SlotID:PortStr. When the port is fanned-out,
the PortStr is represented as mainPort:subPort (all in ASCII format).
● Default Agent Remote ID is the system MAC address (in binary format).
The following example shows the format of the Circuit ID - 723:0:1:1.
Table 28. Circuit ID Format
VLAN ID LAG ID Slot ID Port Str
723 0 1 1
The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to:
● Track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can
harden a server against address exhaustion attacks.
● Associate client MAC addresses with a relay agent to prevent offering an IP address to a client spoofing the same MAC
address on a different relay agent.
● Assign IP addresses according to the relay agent. This prevents generating DHCP offers in response to requests from an
unauthorized relay agent.
The server echoes the option back to the relay agent in its response, and the relay agent uses the information in the option to
forward a reply out the interface on which the request was received, rather than flooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
By default, Option 82 is not inserted in DHCP packets.
To insert Option 82 into DHCP packets, follow this step.
● Insert Option 82 into DHCP packets.
CONFIGURATION mode
ip dhcp relay information-option [trust-downstream]
For routers between the relay agent and the DHCP server, enter the trust-downstream option.
● Manually reset the remote ID for Option 82.
CONFIGURATION mode
ip dhcp relay information-option remote-id
DHCPv6 relay agent options
The DHCPv6 relay agent inserts Options 18 and 37 before forwarding DHCPv6 packets to the server. These DHCPv6 options
are enabled by default and are not configurable.
Interface ID
(Option 18)
Interface on which the client-originated message is received.
The interface-ID is 12 bytes long and is constructed using three ifindexes: Logical, Received, and Physical.
Each of the ifindex is 4 bytes long.
Dynamic Host Configuration Protocol (DHCP) 301