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Option Number and Description
● 8: DHCPINFORM
Parameter
Request List
Option 55
Clients use this option to tell the server which parameters it requires. It is a series of octets where each
octet is DHCP option code.
Renewal Time Option 58
Specifies the amount of time after the IP address is granted that the client attempts to renew its lease
with the original server.
Rebinding Time Option 59
Specifies the amount of time after the IP address is granted that the client attempts to renew its lease
with any server, if the original server does not respond.
End Option 255
Signals the last option in the DHCP packet.
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.
To insert Option 82 into DHCP packets, follow this step.
● Insert Option 82 into DHCP packets.
CONFIGURATION mode
int ma 0/0
ip add dhcp relay information-option remote-id
For routers between the relay agent and the DHCP server, enter the trust-downstream option.
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.
258 Dynamic Host Configuration Protocol (DHCP)