Connectivity Guide

● Dynamic ARP Inspection
● Source Address Validation
Option 82
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 is comprised of two sub-options, 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 sub-option is the MAC
address of the relay agent that adds Option 82.
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 can use 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
By default, the DHCPv6 relay agent inserts Options 18 and 37 before forwarding DHCPv6 packets to the server.
Interface ID
(Option 18)
This is the interface on which the client-originated message is received.
Default values: The length of Interface ID is 12 bytes comprising of logical ifindex (VLAN, LAG,
or physical interface), received ifindex (LAG or physical interface), and physical ifindex. Each
ifindex value is 4 bytes long.
In the interface ID, each ifindex (4 bytes) is in hexadecimal. Convert hexadecimal values of each
ifindex separately to decimal and the derived decimal value can be used to get the actual interface
name. For more information about deriving the interface name from interface index, see the section
Example of deriving the interface index number.
Remote ID
(Option 37)
This identifies the host from which the message is received.
Default values: The default value of this option is the MAC address of the relay agent that adds Option
37.
294 Dynamic Host Configuration Protocol (DHCP)